sailboat displacement hull

My Cruiser Life Magazine

Basics of Sailboat Hull Design – EXPLAINED For Owners

There are a lot of different sailboats in the world. In fact, they’ve been making sailboats for thousands of years. And over that time, mankind and naval architects (okay, mostly the naval architects!) have learned a thing or two.

If you’re wondering what makes one sailboat different from another, consider this article a primer. It certainly doesn’t contain everything you’d need to know to build a sailboat, but it gives the novice boater some ideas of what goes on behind the curtain. It will also provide some tips to help you compare different boats on the water, and hopefully, it will guide you towards the sort of boat you could call home one day.

Table of Contents

Displacement hulls, semi displacement hulls, planing hulls, history of sailboat hull design, greater waterline length, distinctive hull shape and fin keel designs, ratios in hull design, the hull truth and nothing but the truth, sail boat hull design faqs.

Basics of Hull Design

When you think about a sailboat hull and how it is built, you might start thinking about the shape of a keel. This has certainly spurred a lot of different designs over the years, but the hull of a sailboat today is designed almost independently of the keel. 

In fact, if you look at a particular make and model of sailboat, you’ll notice that the makers often offer it with a variety of keel options. For example, this new Jeanneau Sun Odyssey comes with either a full fin bulb keel, shallow draft bulb fin, or very shallow draft swing keel. Where older long keel designs had the keel included in the hull mold, today’s bolt-on fin keel designs allow the manufacturers more leeway in customizing a yacht to your specifications.

What you’re left with is a hull, and boat hulls take three basic forms.

• Displacement hull
• Semi-displacement hulls
• Planing hulls

Most times, the hull of a sailboat will be a displacement hull. To float, a boat must displace a volume of water equal in weight to that of the yacht. This is Archimedes Principle , and it’s how displacement hulled boats get their name.

The displacement hull sailboat has dominated the Maritimes for thousands of years. It has only been in the last century that other designs have caught on, thanks to advances in engine technologies. In short, sailboats and sail-powered ships are nearly always displacement cruisers because they lack the power to do anything else.

A displacement hull rides low in the water and continuously displaces its weight in water. That means that all of that water must be pushed out of the vessel’s way, and this creates some operating limitations. As it pushes the water, water is built up ahead of the boat in a bow wave. This wave creates a trough along the side of the boat, and the wave goes up again at the stern. The distance between the two waves is a limiting factor because the wave trough between them creates a suction. 

This suction pulls the boat down and creates drag as the vessel moves through the water. So in effect, no matter how much power is applied to a displacement hulled vessel, it cannot go faster than a certain speed. That speed is referred to as the hull speed, and it’s a factor of a boat’s length and width. 

For an average 38 foot sailboat, the hull speed is around 8.3 knots. This is why shipping companies competed to have the fastest ship for many years by building larger and larger ships.

While they might sound old-school and boring, displacement hulls are very efficient because they require very little power—and therefore very little fuel—to get them up to hull speed. This is one reason enormous container ships operate so efficiently. 

Of course, living in the 21st century, you undoubtedly have seen boats go faster than their hull speed. Going faster is simply a matter of defeating the bow wave in one way or another.

One way is to build the boat so that it can step up onto and ride the bow wave like a surfer. This is basically what a semi-displacement hull does. With enough power, this type of boat can surf its bow wave, break the suction it creates and beat its displacement hull speed.

With even more power, a boat can leave its bow wave in the dust and zoom past it. This requires the boat’s bottom to channel water away and sit on the surface. Once it is out of the water, any speed is achievable with enough power. 

But it takes enormous amounts of power to get a boat on plane, so planing hulls are hardly efficient. But they are fast. Speedboats are planing hulls, so if you require speed, go ahead and research the cost of a speedboat . 

The most stable and forgiving planing hull designs have a deep v hull. A very shallow draft, flat bottomed boat can plane too, but it provides an unforgiving and rough ride in any sort of chop.

If you compare the shapes of the sailboats of today with the cruising boat designs of the 1960s and 70s, you’ll notice that quite a lot has changed in the last 50-plus years. Of course, the old designs are still popular among sailors, but it’s not easy to find a boat like that being built today.

Today’s boats are sleeker. They have wide transoms and flat bottoms. They’re more likely to support fin keels and spade rudders. Rigs have also changed, with the fractional sloop being the preferred setup for most modern production boats.

Why have boats changed so much? And why did boats look so different back then?

One reason was the racing standards of the day. Boats in the 1960s were built to the IOR (International Offshore Rule). Since many owners raced their boats, the IOR handicaps standardized things to make fair play between different makes and models on the racecourse.

The IOR rule book was dense and complicated. But as manufacturers started building yachts, or as they looked at the competition and tried to do better, they all took a basic form. The IOR rule wasn’t the only one around . There were also the Universal Rule, International Rule, Yacht Racing Association Rul, Bermuda Rule, and a slew of others. 

Part of this similarity was the rule, and part of it was simply the collective knowledge and tradition of yacht building. But at that time, there was much less distance between the yachts you could buy from the manufacturers and those setting off on long-distance races.

Today, those wishing to compete in serious racing a building boat’s purpose-built for the task. As a result, one-design racing is now more popular. And similarly, pleasure boats designed for leisurely coastal and offshore hops are likewise built for the task at hand. No longer are the lines blurred between the two, and no longer are one set of sailors “making do” with the requirements set by the other set. 

Modern Features of Sailboat Hull Design

So, what exactly sets today’s cruising and liveaboard boats apart from those built-in decades past? 

Today’s designs usually feature plumb bows and the maximum beam carried to the aft end. The broad transom allows for a walk-through swim platform and sometimes even storage for the dinghy in a “garage.”

The other significant advantage of this layout is that it maximizes waterline length, which makes a faster boat. Unfortunately, while the boats of yesteryear might have had lovely graceful overhangs, their waterline lengths are generally no match for newer boats. 

The wide beam carried aft also provides an enormous amount of living space. The surface area of modern cockpits is nothing short of astounding when it comes to living and entertaining.

If you look at the hull lines or can catch a glimpse of these boats out of the water, you’ll notice their underwater profiles are radically different too. It’s hard to find a full keel design boat today. Instead, fin keels dominate, along with high aspect ratio spade rudders. 

The flat bottom boats of today mean a more stable boat that rides flatter. These boats can really move without heeling over like past designs. Additionally, their designs make it possible in some cases for these boats to surf their bow waves, meaning that with enough power, they can easily achieve and sometimes exceed—at least for short bursts—their hull speeds. Many of these features have been found on race boats for decades.

There are downsides to these designs, of course. The flat bottom boats often tend to pound when sailing upwind , but most sailors like the extra speed when heading downwind.

How Do You Make a Stable Hull

Ultimately, the job of a sailboat hull is to keep the boat afloat and create stability. These are the fundamentals of a seaworthy vessel. 

There are two types of stability that a design addresses . The first is the initial stability, which is how resistant to heeling the design is. For example, compare a classic, narrow-beamed monohull and a wide catamaran for a moment. The monohull has very little initial stability because it heels over in even light winds. That doesn’t mean it tips over, but it is relatively easy to make heel. 

A catamaran, on the other hand, has very high initial stability. It resists the heel and remains level. Designers call this type of stability form stability.

There is also secondary stability, or ultimate stability. This is how resistant the boat is to a total capsize. Monohull sailboats have an immense amount of ballast low in their keels, which means they have very high ultimate stability. A narrow monohull has low form stability but very high ultimate stability. A sailor would likely describe this boat as “tender,” but they would never doubt its ability to right itself after a knock-down or capsize.

On the other hand, the catamaran has extremely high form stability, but once the boat heels, it has little ultimate stability. In other words, beyond a certain point, there is nothing to prevent it from capsizing. 

Both catamarans and modern monohulls’ hull shapes use their beams to reduce the amount of ballast and weight . A lighter boat can sail fast, but to make it more stable, naval architects increase the beam to increase the form stability.

If you’d like to know more about how stable a hull is, you’ll want to learn about the Gz Curve , which is the mathematical calculation you can make based on a hull’s form and ultimate stabilities. 

How does a lowly sailor make heads or tails out of this? You don’t have to be a naval architect when comparing different designs to understand the basics. Two ratios can help you predict how stable a design will be .

The first is the displacement to length ratio . The formula to calculate it is D / (0.01L)^3 , where D is displacement in tons and L is waterline length in feet. But most sailboat specifications, like those found on  sailboatdata.com , list the D/L Ratio.

This ratio helps understand how heavy a boat is for its length. Heavier boats must move more water to make way, so a heavy boat is more likely to be slower. But, for the ocean-going cruiser, a heavy boat means a stable boat that requires much force to jostle or toss about. A light displacement boat might pound in a seaway, and a heavy one is likely to provide a softer ride.

The second ratio of interest is the sail area to displacement ratio. To calculate, take SA / (D)^0.67 , where SA is the sail area in square feet and D is displacement in cubic feet. Again, many online sites provide the ratio calculated for specific makes and models.

This ratio tells you how much power a boat has. A lower ratio means that the boat doesn’t have much power to move its weight, while a bigger number means it has more “get up and go.” Of course, if you really want to sail fast, you’d want the boat to have a low displacement/length and a high sail area/displacement. 

Multihull Sailboat Hulls

Multihull sailboats are more popular than ever before. While many people quote catamaran speed as their primary interest, the fact is that multihulls have a lot to offer cruising and traveling boaters. These vessels are not limited to coastal cruising, as was once believed. Most sizable cats and trimarans are ocean certified.

Both catamarans and trimaran hull designs allow for fast sailing. Their wide beam allows them to sail flat while having extreme form stability. 

Catamarans have two hulls connected by a large bridge deck. The best part for cruisers is that their big surface area is full of living space. The bridge deck usually features large, open cockpits with connecting salons. Wrap around windows let in tons of light and fresh air.

Trimarans are basically monohulls with an outrigger hull on each side. Their designs are generally less spacious than catamarans, but they sail even faster. In addition, the outer hulls eliminate the need for heavy ballast, significantly reducing the wetted area of the hulls. 

Boaters and cruising sailors don’t need to be experts in yacht design, but having a rough understanding of the basics can help you pick the right boat. Boat design is a series of compromises, and knowing the ones that designers and builders take will help you understand what the boat is for and how it should be used. 

What is the most efficient boat hull design?

The most efficient hull design is the displacement hull. This type of boat sits low in the water and pushes the water out of its way. It is limited to its designed hull speed, a factor of its length. But cruising at hull speed or less requires very little energy and can be done very efficiently. 

By way of example, most sailboats have very small engines. A typical 40-foot sailboat has a 50 horsepower motor that burns around one gallon of diesel every hour. In contrast, a 40-foot planing speedboat may have 1,000 horsepower (or more). Its multiple motors would likely be consuming more than 100 gallons per hour (or more). Using these rough numbers, the sailboat achieves about 8 miles per gallon, while the speedboat gets around 2 mpg.

What are sail boat hulls made of?

Nearly all modern sailboats are made of fiberglass. 

Traditionally, boats were made of wood, and many traditional vessels still are today. There are also metal boats made of steel or aluminum, but these designs are less common. Metal boats are more common in expedition yachts or those used in high-latitude sailing.

Matt has been boating around Florida for over 25 years in everything from small powerboats to large cruising catamarans. He currently lives aboard a 38-foot Cabo Rico sailboat with his wife Lucy and adventure dog Chelsea. Together, they cruise between winters in The Bahamas and summers in the Chesapeake Bay.

Yachting Monthly

• Digital edition

Busting the hull speed myth

• julianwolfram
• December 13, 2021

Waterline length is not the defining factor in maximum boat speed that we all think it is. Julian Wolfram busts the hull speed myth

Modern hull forms, like this Jeanneau SO440, use chines to create volume forward while keeping a narrow entrance at the waterline

Every sailor is delighted when the breeze picks up and the boat really starts to get going with a bone in her teeth.

Julian Wolfram is a physicist, naval architect, former professor of ocean engineering at Heriot-Watt in Edinburgh and a Yachtmaster Offshore who has cruised and raced for 45 years

The crew will want to know how fast she will go and perhaps surreptitiously race her against any similar sized boat in the vicinity.

Speculation may start about what allows one boat to go faster than another – is it the hull shape or the sails?

It is easy to spot good, well-trimmed sails but what about the hull ?

The important part is not visible below the water surface. However there is one key indicator that is often very apparent – the waves generated by the sailing yacht.

When a yacht picks up speed the wave pattern around it grows and the greater the speed the bigger the waves .

The energy in these waves is proportional to the square of their height – double the height and the energy goes up by a factor of four.

This energy comes from the wind , via the sails and rig , making the hull push water out of the way.

If less of this wind energy was wasted in producing waves the yacht would go faster.

When a typical displacement monohull reaches a speed-to-length ratio of around 1.1 to 1.2 (speed in knots divided by the square root of the waterline in feet) up to half the wind energy driving it is usually wasted in generating waves.

The hull speed myth: Half angle of entrance

So how can we tell if a yacht will sail efficiently, or have high wave resistance and waste a lot of energy generating waves?

The answer starts back in the 19th century with the Australian J H Michell.

In 1898 he wrote one of the most important papers in the history of naval architecture in which he developed a formula for calculating wave resistance of ships.

Light displacement cruising boat: The bow of this Feeling 44 is finer than older cruising boats

This showed that wave resistance depended critically on the angle of the waterlines to the centreline of the ship – the half angle of entrance.

The smaller the angle the smaller the height of the waves generated and the lower the wave-making drag.

A knife blade can slice through water with minimal disturbance – drag the knife’s handle through and you generate waves.

The big hull speed myth

For a displacement hull the so-called ‘hull speed’ occurs when the waves it generates are the same length as the hull.

This occurs when the speed-length ratio is 1.34.

It is claimed that hulls cannot go significantly faster than this without planing. It is called ‘the displacement trap’ but is a myth.

Heavy displacement cruising boat: An older design has a bow that is several degrees wider

As an example, consider a 25ft (7.6m) boat that goes at 10 knots in flat water.

This is a speed-length ratio of two. That is the average speed over 2,000m for a single sculls rower in a world record time.

The reason for this high speed is a half angle of entrance of less than 5º. Hobie Cats, Darts and many other catamarans have similarly low angles of entrance and reach even higher speed-length ratios with their V-shaped displacement hulls.

These hulls also have almost equally fine sterns, which is also critically important to their low wave resistance.

The monohull problem

Now a monohull sailing yacht needs reasonable beam to achieve stability and, unless waterline length is particularly long, the half angle of entrance will inevitably be much larger than those on rowing skulls and multihulls .

In his 1966 Sailing Yacht Design Douglas Phillips-Birt suggests values of 15º to 30º for cruising yachts.

Many older cruising yachts with long overhangs and short waterline lengths, for their overall length, have values around the top of this range.

Busting the hull speed myth: A Thames barge is a similar length and beam to a J-Class, but its bluff bow, built for volume, makes it much slower. Credit: Alamy Stock Photo

Newer sailing yachts, with plumb bows, have somewhat smaller half angles and a modern 12m-long fast cruiser may have a value around 20º and a racing yacht 17º or 18º.

Size matters here as, to achieve stability, a little yacht is likely to have a bigger half angle than a large one, such as the German Frers-designed 42m (138ft), Rebecca which has a half angle of entrance of under 13º.

Rebecca also has a fine, elegant stern which helps minimise the stern wave – I’ll come back to sterns and stern waves.

Interestingly the half angle of entrance is not mentioned in the otherwise excellent 2014 Principles of Yacht Design by Larsson et al, although it is currently used as one of the parameters in the preliminary estimation of wave resistance for ships.

While it is still particularly applicable to very slender hulls, naval architects are not generally familiar with Michell’s work.

His formula for wave resistance involves quadruple integrals of complex functions.

German-Frers’ designed Rebecca has a half angle of entrance of just 18°. Credit: Cory Silken

These are not ‘meat and drink’ for your average naval architect, and only a few mathematically inclined academics have much interest in theoretical wave resistance.

Michell’s work is rarely, if ever, covered in naval architecture courses now.

Nowadays the emphasis is much more on numerical methods, high-speed computers and computational fluid mechanics (CFD) using the so called Navier-Stokes equations.

Examining these equations, which apply to any fluid situation, does not give any insights into wave resistance, albeit they can model wave resistance very well when used in the piecewise manner of CFD.

It is very easy to measure the half angle of entrance at the design waterline when a yacht is out of the water.

Take a photograph directly upwards from the ground under the centreline at the bow.

Busting the hull speed myth: Multihulls achieve high speeds due to fine hulls, light displacement and ample stability. Credit: Joe McCarthy/Yachting Monthly

Now blow this up on a computer screen, or print it off at a large scale, and measure the angle with a protractor.

Alternatively, if you have a properly scaled accommodation plan drawn for a level close to the design waterline this will yield a reasonable approximation of the half angle of entrance.

Unfortunately there is not a simple relationship between the fineness of the bow and the wave drag.

But, all other things being equal, the smaller the half angle the better.

It is easy to measure and is a useful parameter to know when comparing yachts.

Stern shape and hull speed

The half angle of entrance cannot be taken alone as a measure of wave drag, and the fairness of the hull and in particular the run aft is also critical.

Just as the half angle of entrance dictates the height of the bow wave, so the fineness of the stern is a key influence on the height of the stern wave.

Consider the water flowing around both sides of the hull and meeting at the stern.

Modern race boats, like Pip Hare ‘s IMOCA 60, combine a fine angle of entrance with wide, flat hulls for maximum form stability and planing ability. Credit: Richard Langdon

If these streams meet at a large angle the water will pile up into a high stern wave.

On the other hand if they meet at a shallow angle there will be less piling up. A fine stern can maintain a streamline flow of water.

However if the sides of the hull meet at the stern at a large angle then the streamline flow will tend to separate from the hull, leaving a wide wake full of drag-inducing eddies.

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In many modern designs the hull sides are not far off parallel at the stern and it is then the upward slope of the buttock lines that are critical and, again, the shallower the slope the better from a hull drag perspective.

The slope of the buttocks can easily be measured if the lines plan is available and a good indication can be obtained from a profile drawing or a photo taken beam on with the boat out of the water.

Drawing a chalk line parallel to the centreline and half a metre out from it will provide a buttock line that can be checked visually for fairness when the boat is viewed from abeam.

A rowing scull easily exceeds its theoretical max hull speed. Credit: Alamy Stock Photo

Again, the smaller the angle the better – provided the transom is clear of the water.

An angle of more than 17º will lead to separated flow and eddy making. This also happens if the transom is immersed.

The greater the immersion the greater the drag, so weight in stern lockers on modern boats can be critical.

Modern hull design

The modern wedge shape attempts to resolve the conflicting demands of a small angle of entrance, good stability and a fine stern.

The plumb bow extends the waterline forward and, with the maximum beam taken well aft, the hull forward can be relatively narrow, providing a low half angle of entrance.

The stern is wide, which helps achieve good stability, but at the same time the buttocks rise slowly at a shallow angle to the water surface.

This gives a smooth and gradual change in the hull’s cross section area ensuring the water flow remains attached to the hull and that the stern wave is kept low.

A modern cruising boat gains stability from a wide stern, but needs twin rudders

This wide, flat stern also helps surfing down waves and possibly planing.

Some designs have chines just above the design waterline which increases usable internal volume and gives a little more form stability when heeled.

However, as soon as the chine is immersed there will be separation along the chine edge as water will not flow smoothly around a sharp edge.

It is just not possible to get the chine perfectly aligned with the streamlines of the water flow in all sailing conditions and there will be some extra drag at times.

There are two downsides to the wedge- shaped hull.

Overloading aft will create a large increase in drag

First the boat has to be sailed at a small angle of heel to keep the rudder properly immersed and to avoid broaching. This can be offset to some extent by using twin rudders .

The second is that the weight must be kept relatively low.

This is because a relatively small increase in weight causes a big increase in wetted surface area at the stern and hence in the frictional drag which makes the boat slower, particularly in light airs.

This is the downside of slowing rising buttocks and the reason why dinghy sailors get their weight forward in a light breeze .

Displacement Length Ratios

Traditionally for sailing yachts the displacement-length ratio has been used as a measure of speed potential, partly because it is easy to calculate from the yacht particulars.

It is waterline length (in metres) divided by the cube root of displacement (in cubic metres or tonnes).

A heavy boat, such as the Heard 35, will have a value of about 4 to 4.8.

A more moderate displacement boat, such as the Hallberg Rassy 342 or Dufour 32 Classic, will have a value in the range 5 to about 5.5; whilst a racing boat may a value of up to, and even over, 7.

A heavy displacement cruising boat with a fair run aft is less affected by additional weight

However the displacement length ratio can be misleading as making a hull 20% deeper and 20% narrower will keep the displacement the same but will significantly reduce the half angle of entrance and the wave drag.

It is interesting to note a Thames barge in racing trim has the same length-displacement ratio as a J class yacht, but their speed potential is vastly different.

Finally I should mention the older ‘length-displacement’ ratio, which is quoted in imperial units.

This is calculated by dividing a boat’s displacement in tons (2,240 pounds) by one one-hundredth of the waterline length (in feet) cubed.

Credit: Maxine Heath

It is still used in the USA and should be treated with caution.

The myth that your boat’s speed is only restricted by it waterline length does a disservice to its designers, and does little to help you understand how to get the best from her when the wind picks up.

Have a look at how the boat is loaded, how you sail on the wind, your boat handling and how much canvas you ask her to carry and you may discover more speed than you expect.

The remarkable John Henry Mitchell

Pioneer of wave theory

It’s worth saying a little more about the remarkable John Henry Michell.

He produced a series of ground-breaking papers including one that proved a wave would break when its height reached a seventh of its length.

He was the son of Devon miner who had emigrated to the gold mining area near Melbourne.

He showed such promise that he got a scholarship to Cambridge.

He was later elected a fellow of the Royal Society at the age of 35 – not bad for the son of a Devonshire miner.

His brother George was no slouch either – he invented and patented the thrust bearing that is named after him.

The half angle of entrance became the traditional factor for assessing the fineness of hulls.

It is defined as the angle the designed waterline makes with the centreline at the bow.It varies from less than 5º for very fine hull forms up to 60º or more for a full-form barge.

At higher speeds, modest increases in the half angle can give rise to substantial increases in wave resistance.

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The Definitive Guide to Sailboat Hull Types

If you’ve ever been on a sailboat or any kind of boat, one of the first parts of the boat you saw was its hull and you might not have even known it.

Simply put, the hull is the bottom part of a boat that rides in and on top of the water. When a sailboat is underwater, it’s accompanied by the keel and the rudder.

Just like knowing the different types of sails , knowing the hull type on your sailboat means you’ll have a better understanding of how your boat operates while it’s out on the water.

All in all, the hull of any boat is meant to keep the boat afloat and to ensure minimum resistance against the water while being propelled forward. Now let’s dive into the different sailboat hull types and even some other types of hulls in boats in general!

Main Sailboat Hull Types

There are two main hull types that we’ll be looking at that encompass the many other types of hulls that vary from these two main types.

Depending on the type of boat you have, you’ll be floating around with one or the other. We’ll take a look at what you can expect if your boat has either of these hull types.

Displacement Hulls

The most common sailboat hull type you’ll find out there is the displacement hull, which is very effective at pushing the water aside and powering through it during forward propulsion.

A displacement hull is often found not only on sailboats, but also fishing, freight, cruise, and other larger boats.

All boats that have a displacement hull will be limited in their speed based on the waterline length of the hull. Regardless of how much power you use, whether it’s from the wind or motor, the maximum speed can’t be increased.

This is why you’ll see people mention the waterline length of a boat’s hull when putting them on the market to sell.

The big advantage of having a displacement hull is that they require far less power to get moving across the water compared to the other main hull type; the planing hull.

What this means is that your boat will be able to cruise for a long time with the same amount of energy, which also allows you to carry more items on board.

Planing Hulls

It’s almost guaranteed that your sailboat won’t have a planing hull since they’re most commonly found on powerboats and personal watercrafts (PWCs), like jet skis.

Planing hulls allow the boat to lift itself out of the water, reducing drag and increasing the speed of the boat.

Almost any boat that’s equipped with a planing hull will be able to attain a speed much greater than a boat with a displacement boat.

The main reason for this is the lift that’s produced when traveling at high speeds which reduces drag on the water.

The maximum speed of a boat with a planing hull is dependent on the horsepower of the engine and how much of the hull can be removed from the water while still cruising.

The biggest advantage of having a planing hull is that your boat will be able to pick up speed quickly and reach a greater maximum speed.

This allows for shorter journey times. However, there needs to be a source of all that energy, which comes directly from a combustion engine. The faster a boat with a planing hull goes, the larger the cost of fuel will be.

How Planing Works

The way planing works is actually pretty interesting, so I thought I’d dive into it a bit. Even though a sailboat is virtually guaranteed not to have one, it’s always nice to know how other boats operate while out on the water.

1. Displacement

Before a boat with a planing hull actually planes, it starts out acting like a displacement hull.

As a matter of fact, a boat with a planing hull needs to reach a certain speed before it starts to produce lift. Before that happens, it’s essentially a displacement hull.

While a boat with a planing hull is picking up speed and lifting itself out of the water, it’s in a plowing mode.

You’ll know when a boat is in plowing mode when the bow of the boat is elevated and the boat is throwing a relatively large wake. The goal, however, is to move from plowing mode to planing mode, which requires further acceleration.

Once the boat with a planing hull reaches a certain speed, it’ll leave plowing mode and enter planing mode.

As I already described, planing is when the hull is gliding across the water with a smaller amount of the hull dragging in the water when compared to the previous modes. Different boats will start planing when reaching different speeds.

Common Sailboat Hull Styles

Now that we’ve gone over the two main types of hulls you’ll find in sailboats and other types of boats, we have a good foundation for the hull styles you’ll commonly see when out on the water.

There are three main hull styles that you’ll see quite often, so let’s take a look at those.

By far the most common hull style you’ll see on sailboats is the monohull, which is simply a single hull.

Traditionally, a sailboat will have a monohull and they can be found all over the place. It’s probably the style of hull that comes to most peoples’ mind when imagining a sailboat.

Monohulls on sailboats are virtually all displacement hulls. As we went over previously, this allows your sailboat to cruise for long stretches and has a greater efficiency compared to planing hulls.

However, most boats that exist on planet earth are monohulls, including powerboats, which can also be of the planing hull type.

When it comes to a monohull on a sailboat, the only way it can keep its stability is to have a proper keel attached to it.

A keel is a wing-like object that sticks out of the bottom of the hull in the water and provides a sailboat with ballast for stability. It’s important to understand how a keel works when operating a sailboat with a monohull since it’s one of the main reasons a sailboat can move forward without tipping.

There are certainly a lot of monohull sailboats out there, but there’s no doubt that you’ll also see your fair share of catamarans.

Catamarans are sailboats with two hulls and operate quite differently than their monohull cousin. Catamarans are known to be fast and are likely to outrun most monohull sailboats.

Unlike monohull sailboats, catamarans can be fitted with displacement hulls as well as planing hulls. However, even if they have a planing hull they can still produce a relatively good amount of cruising time and do so rather efficiently.

Catamarans are a bit different than monohulls in the sense that they can reach greater speeds. There are several reasons for this. For one, a catamaran doesn’t need a ballast for stability since the broad stance between the two hulls provides enough stability.

This means there’s no need for a large, heavy keel. Second, they’re often built out of lightweight materials that allow the boat to reach a higher maximum speed compared to heavier sailboats.

Also, if a catamaran has a planing hull, it’ll have the ability to produce lift resulting in reduced drag on the water and even greater speeds.

Unfortunately, catamarans do have the disadvantage of being more likely to capsize in unwanted high-wind situations.

Also, it’s very difficult for a catamaran to recover from capsizing as opposed to a monohull sailboat that has a good ballast from its keel.

You might have already guessed from the name, but I’ll state the obvious anyway. A trimaran is exactly like a catamaran but with three hulls instead of two.

Often times you’ll see a trimaran look like a monohull sailboat with a pair of hulls attached to its side.

Similar to a catamaran, trimarans can hit speeds much greater than your average monohull sailboat. As a matter of fact, they’re known to be “unsinkable” under the situation that the hulls on the port and starboard side of the central hull are completely filled up with water.

One of the coolest aspects of having a trimaran is that when it has a planing hull and/or a hydrofoil, the trimaran’s central hull will lift completely out of the water.

This gives it the effect that it’s floating across the air, which is the result of lift produced from the planing hull or a hydrofoil. It’s very cool to see this!

Sailboat Hull Bottoms

Apart from the main boat hull styles, like the monohull, catamaran, and trimaran, there are hull bottoms that pop up in the world of boating that can differ in style and function.

These hull bottoms are more of a deeper look at the hulls of a monohull, catamaran, or trimaran, so you can think of them more as a feature of any of the previously mentioned styles of hull.

Flat Bottom

A very common hull bottom for boats that are derived from the planing hull type is a flat bottom hull.

The flat bottom hull is considered to be one of the less stable styles of hulls, especially when confronted with rough waters.

However, you’ll often find them on boats that don’t necessarily ride in these situations, including fishing or taxi areas.

• Good for small lakes and rivers due to having a shallow draft.
• Able to hit relatively high speeds once entering planing mode.

Disadvantages

• Not good at handling choppy waters resulting in a rough ride.

Round Bottom

When it comes to sailboats, you’re most likely going to run into monohull sailboats that have a displacement style hull with a round bottom.

While these are the most common hull bottom for sailboats, they can also be found on smaller boats that are used for fishing, canoeing, and other similar kinds of boats.

• Easily moves through the water due to being a displacement hull type.
• When accompanied by a keel, it produces a great amount of stability from the ballast.
• Without a keel, it can roll when entering and exiting the boat as well as when waves are present.
• Less maneuverable compared to other hull styles.

Deep ‘V’ Bottom

If you’re operating a powerboat, then in all likeliness your boat has a planing hull with a deep ‘V’ bottom.

Since deep ‘V’ bottoms are found on planing hulls, these types of boats will be able to pick up speed quickly and at high maximums. This is the most common setup for powerboats out on the water.

This is the most common type of powerboat hull. This hull type allows boats to move through rough water at higher speeds and they provide a smoother ride than other hull types.

• Provides a smooth ride compared to its flat bottom rival.
• Good at handling rough water.
• Requires more power to plane compared to its flat bottom rival.
• Cannot handle sharp turns very well resulting in potential rolling or banking.

Multi-Chine Bottom

We took a good look at multi-hull styles like the catamaran and the trimaran earlier, which are the exact style of hulls that have a multi-chine bottom.

A multi-chine bottom is a great example of a displacement hull on either a catamaran or trimaran as it’s the most common bottom you’ll find.

• In a multi-hull boat, it has a great amount of stability due to its wide beam.
• In a multi-hull boat, it needs a large area when either tacking or jibing.

Main Parts of a Sailboat Hull

There’s some terminology I threw around while describing the many types of hulls a sailboat and other types of boats have.

As is the case with a lot of activities, learning the terminology is just something you have to do.

Thankfully, the terminology will eventually sink in overtime and eventually you’ll be able to ring off any hull terminology that comes up.

The bow is simply the most forward part of a sailboat and, thus, the very front of the hull.

The stern, conversely to the bow, is the most backward part of a sailboat and, thus, the very end of the hull.

The port side of a hull is the left side. I always remember this with the phrase “I left my port on the table”, with the port being wine.

This just so happens to also be the side where boats will have a red light turned on at night, which is the color of port wine.

The starboard side of a hull is on the right side.

Opposite the port side, in the evening boats will have a green light turned on and will be located on the starboard side of the boat.

Fore is a sailor’s way of saying “forward”.

Aft is a sailor’s way of saying “back”.

A transom is the aft-most (see what I did there?) section of the boat that connects the port and starboard sections of the boat.

The flare of a hull is where the hull starts to form a large angle the closer the hull gets to the deck.

The waterline is the line around the hull where the water touches when under a normal load.

Waterline Length

The waterline length, once referred to as the Load Waterline Length (LWL), is the length of the hull where the waterline is located.

This is not the entire length of the boat.

Length Overall (LOA)

The length overall (LOA) is, you guessed it, the overall length of the boat. This is measured from the tip of the bow to the end of the stern.

The freeboard is the space on the hull of a boat above the waterline and below the deck.

The draft is the length from the bottom-most part of a boat (the tip of the keel on a sailboat) and the waterline.

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Types of Sailboat Hulls

Sailboats come in numerous hull shapes. These include single-hull monohulls, along with double and triple-hull multihulls.

There are two main categories of sailboat hulls: monohulls and multihulls. Common monohull types include flat-bottom vessels, fin-keel racers, bulb and bilge keel cruisers, heavy semi-displacement sailboats, and dense full-keel displacement cruisers. Multihull designs include catamarans and trimarans.

In this article, we'll cover the most common types of sailboat hulls along with their best uses. We'll explain the difference between monohulls and multihulls, along with how keel shape influences sailboat performance.

We sourced the information for this article from sailing experts, hull shape guides, and the written wisdom of famous sailboat designers. Additionally, we researched sailboat sales figures to determine the most popular vessel configurations available today.

Table of contents

‍ Importance of Sailboat Hull Design

A sailboat is defined by its rig and hull shape. Sailboat hull shape is one of the deciding factors on how it will handle. Additionally, the shape (and displacement) of a sailboat hull can be used to determine its strengths and weaknesses. Learning about sailboat hull shape can help you understand what kind of boat you need and what your vessel is capable of.

You can easily categorize sailboats based on their hull shape. For example, a heavy deep-draft displacement hull is likely a slow, steady, and comfortable cruiser. In contrast, a sleek flat-bottomed sailboat or catamaran is likely built for speed and could easily outpace even the most nimble displacement cruisers.

The most common kind of sailboat is the monohull. When you think of a sailboat, probably think of a monohull. The term simply means that the vessel has one single hull and nothing more. This is in contrast to multihulls such as catamarans, which are easy to spot and differentiate from traditional designs.

Monohulls are popular because they work. They're easy to build and narrow enough to fit in most marina dock spaces. Monohulls are also generally easy to handle in a variety of conditions, both fair and foul.

One drawback of monohull designs is that they are not quite as stable as most multihulls, though monohulls can recover more easily from a serious roll or capsize. They also cost a lot less, as the vast majority of production sailboats ever constructed were of the same basic single-hull configuration.

Centerboards and Swing Keels

The windward performance of sailboats is greatly improved by the use of a long keel or centerboard. The centerboard is the most simple type of stabilizing device used on sailboats. Usually, the centerboard is simply a long fin that protrudes from the bottom of the hull.

The centerboard keeps the boat on track when the wind is not moving in the boat's direction of travel. This is why sailboats can sail at different angles to the wind without being pushed to the side. A key characteristic of centerboards is that they can be raised and lowered, which is convenient on small boats that need to be trailered or beached.

Swing keels are similar to centerboards in that they can be raised and lowered, though they pivot on a hinge instead of sliding up and down in a truck. Swing keels are either recessed into the hull or held in a housing just below it, which usually also contains much of the boat's ballast. Swing keel designs free up cabin space that would normally be occupied by a bulky centerboard trunk.

Centerboards and most swing keels are an alternative to a permanently affixed keel. They're generally not considered to be as seaworthy as other hull designs, so their use is confined primarily to inland and coastal cruising.

Monohull Sailboat Hull Shapes

When in the water, it's difficult to distinguish between the different types of monohull shapes. In most cases, you have to pull the boat out of the water to figure out what hull shape you're dealing with. Next, we'll go over the most common monohull sailboat shapes and their uses.

Flat-Bottom Sailboats

Flat bottom sailboats are the easiest to build and often the fastest. These vessels have a very shallow draft and are often lightweight, so they slide easily and quickly across the water. Flat bottom sailboats make excellent racing boats and 'gunkholers,' which are primarily used for camping and hopping between shallow Islands.

Flat bottom sailboats usually have centerboards or swing keels, which makes them great for shallow water, beaching, and towing on a trailer. The use of flat bottom sailboats is confined primarily to inland and coastal waters, as a flat bottom does not handle well in swells and rough weather. Flat bottom sailboats pound hard on chop, and they lack the low center of gravity that's necessary for good stability.

Fin Keel Sailboat Hulls

The fin keel is a popular alternative to centerboards, and vessels utilizing this low-profile hull shape have proven to be quite seaworthy. Fin keels are popular on fast racing boats and lightweight cruisers. A fin keel resembles a centerboard, but it usually extends much further from the base of the hull.

The majority of a sailboat's draft comes from the fin keel, as the hulls of these sailboats tend to be rounded and shallow. They resemble flat-bottom designs, but slight rounding significantly increases comfort. Fin keel sailboats are ideal for racing and coastal cruising, and some models can be used for extended offshore passages.

Bulb Keel Sailboat Hulls

A bulb keel sailboat hull usually resembles most fin keel varieties. The hulls of these vessels tend to be shallow and rounded, with a long and thin fin extending from the base of the hull. A bulb keel is essentially just a thin blade with a bulb on the bottom.

Bulb keels are different from fin keels as they usually contain additional ballast weight for stability. The hydrodynamic properties of bulb keels are proven to be efficient. As a result, these boats can also be quite fast. In a direct comparison, a vessel with a bulb keel will likely be more seaworthy than the same sailboat with only a fin keel or a centerboard.

Bilge Keel Sailboat Hulls

The hull shape of a bilge keel sailboat usually resembles that of a bulb or fin keel sailboat, with one major distinction. Instead of one long and thin keel descending from the center of the hull, a bilge keel sailboat has two lengthier fins offset on the port and starboard side.

The idea behind the bilge keel design is that when the vessel heels to one side, one of the two keels will be straightened out. This, in theory, provides better tracking and improves stability. It also distributes ballast evenly on both sides. Bilge keels can also improve motion comfort, and they can reduce the vessel's draft by a small margin.

Bilge keel sailboats offer a balance between seaworthiness and speed. These vessels can be used as bluewater cruisers and coastal cruisers. They can also hold their own in any yacht club regatta.

While a bilge keel sailboat may not be ideal for cruising the North Atlantic during the winter, it can certainly make a safe and comfortable passage maker that can gain a knot or two of speed above its heavier counterparts.

Semi-Displacement Sailboat Hulls

Now, we'll look at some true bluewater cruising designs. The semi-displacement hull features a long and deep keel that runs from about the center of the hull all the way back to the rudder. Semi-displacement hulls get deeper the further back you go, reaching their longest point at the very aft end of the boat.

The offshore benefits of a long and deep keel are numerous, as this hull shape provides an enormous amount of stability and a very low center of gravity. The design itself it's quite old, and it's featured on many classic cruising sailboats and workboats.

Though less common in the modern era than more contemporary fin keel designs, a traditional semi-displacement sailboat offers easy handling and enhanced motion comfort. Semi-displacement hulls tend to have a deep draft and therefore are not ideal for shallow water. They handle confidently in all conditions, though they usually aren't as fast as newer designs.

Displacement Sailboat Hulls

Displacement hulls, also known as full keel hulls, are the bulldozers of the sailboat world. These traditional vessels are deep, heavy, relatively slow, and capable of plowing through the roughest weather conditions.

Displacement hulls have a long keel that begins at the bow and extends all the way after the rudder. Like semi-displacement hulls, full keel sailboats offer excellent motion comfort and confident handling.

Displacement hulls have the best directional stability and downwind maneuvering abilities. Their handling is more forgiving, and they're less jumpy at the helm. Many of these boats heel gently and give the crew more time to respond to changing conditions.

The primary downside to displacement hulls is their high cost and sheer mass. Displacement boats are large and take up a lot of space. They're usually too tall and heavy for trailering, so they tend to remain in the water most of the time.

Displacement hulls aren't made to just sit at the dock or jump around the lake; they're designed for real-deal offshore sailing. They also have the roomiest cabins, as the hull extends further down and longer than any other hull shape.

Now, let's examine multihull sailboat designs and why you may want to consider one. Some of the earliest seagoing vessels had multiple hulls, usually featuring one long hull (occupied by the crew) and a small stabilizing hull off to one side.

Multihulls have only recently become popular, and they make up a decent portion of the modern production boat market. This is because of their numerous design benefits and spacious cabins. Multihulls are almost guaranteed to be more expensive than monohulls (both new and used), and the used market is still saturated with expensive luxury cruising sailboats.

Modern multihull sailboats feature a large pilothouse in the center and plenty of cabin space in each full-size hull. They offer excellent motion comfort and achieve very high speeds. Due to their wide beam, they provide spacious living spaces and excellent stability. Here are the two main types of multihull sailboats.

From above, a catamaran looks like two thin monohull sailboats lashed together and spaced apart. Fundamentally, that's exactly what they are. Except catamarans have a very shallow draft and the capability to reach very high speeds.

Catamarans have two hulls instead of one, and each hull is typically a mirror of the other. They achieve their space using width rather than length, so a 30-foot catamaran has significantly more interior room than a 30-foot monohull.

Their primary drawback is that, due to their width, catamarans usually require two standard dock spaces instead of one. But at sea, they don't heel over dramatically like monohulls, which makes them much more comfortable to eat, sleep, and cook inside of.

Trimarans follow the same basic design principles as catamarans, except they have a third hull in the center. From above, a trimaran looks like a monohull with two smaller hulls lashed to the sides. Unlike a catamaran, the primary living space of a trimaran is in the large center hull. Trimarans are essentially just monohulls with stabilizers on the side, resembling ancient sailing canoes.

Trimarans have the same spatial and stability benefits as catamarans, though they can achieve higher speeds and better sea keeping. This is because of the additional stability provided by the center hall. Trimarans tend to be costlier than catamarans, though many sailors believe that the benefits outweigh the cost.

Best Sailboat Hull Shape for Speed

If we take wave height and weather conditions out of the equation, the fastest sailboats are usually the longest. Sailboats are limited by hull speed and sail plan size regardless of their hull shape. That said, the fastest sailboats tend to be flat bottom monohulls, fin keel monohulls, and trimarans.

Best Sailboat Hull Shape for Motion Comfort

The best sailboat for motion comfort is the catamaran. These wide and seaworthy vessels 'stance up' and minimize rolling. They also come close to completely eliminating heeling.

Wide and stable multihulls are popular because they alleviate some of the most common complaints of sailors. Trimarans are also an excellent choice for comfort, as their stabilizers minimize the effect of rolling in heavy seas.

Most Seaworthy Sailboat Hull Shape

Today, many people consider multihulls to be the most seaworthy design on the market. However, seaworthiness is more than just average stability in rough weather. Many Sailors argue that traditional displacement sailboat hull designs are the most seaworthy.

Displacement hulls have a low center of gravity which improves their knockdown survivability. In other words, in the (rare) event of a displacement boat knockdown, the weight of the keel is more likely to swing the boat back up and out of trouble. Multihulls cannot recover from a knockdown, as they like the pendulum-like recoil ability.

Most Spacious Sailboat Hull Type

The most spacious hull sailboat type is the catamaran. Catamarans have two nearly full-size hulls (one on each side) plus a large central pilothouse that resembles the main cabin of a large powerboat.

Many typical catamarans fit an entire kitchen into the Pilot House along with four private births and two full-sized heads in its hulls. Some mid-size catamarans even come with a bathtub, which is essentially unheard of on equivalent monohulls.

Spaciousness varies on small monohulls. Larger cabins are usually found on bulb and bilge keel designs, as swing keel and centerboard boats need somewhere to hide their skegs. Centerboard boats are the least spacious, as the centerboard trunk must occupy the middle of the cabin space.

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Daniel Wade

I've personally had thousands of questions about sailing and sailboats over the years. As I learn and experience sailing, and the community, I share the answers that work and make sense to me, here on Life of Sailing.

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Boats come in a wide variety of shapes and sizes—and so do their hulls. Despite the variety, all hulls are designed to do one of only two things: either displace water, or ride on top of it, which is called planing. Sailing boats, slow-moving boats, and large boats like cruise ships have displacement hulls. The combination of their weight and power means they move lower in the water, pushing or displacing water, rather than riding on top of it. Smaller, faster boats, like powerboats or personal watercraft, typically have planing hulls. Planing hulls are designed to rise up and ride on top of the water at higher speeds. Now let's look at some specific hull types.

There are four common types of boat hulls.

Flat-Bottom Hulls

Boats with "flat-bottomed" hulls are very stable, great for fishing and other uses on calm, small bodies of water.

Round-Bottom Hulls

"Round-bottomed" hulls are typically displacement hulls, and are designed to move smoothly through the water with little effort. An example of a round-bottomed hull is that found on a canoe. One drawback to the round-bottomed design is that it's less stable in the water and can capsize more easily. So, extra care needs to be taken when entering, exiting and loading these types of boats.

V-Shaped Hulls

"V-shaped" hulls are planing hulls, and are the most common type of hull for powerboats. Deep v-shaped boats are designed to plane on top of the water at higher speeds and provide a smoother ride through choppy water. These boats are usually equipped with a larger engine than flat or round-bottomed boats.

Multi-Hulled

Finally, let's look at "multi-hulled" boats. These boats can have either planing or displacement hulls depending on the shape of hull and size of engine. Multi-hulled boats are some of the most stable on the water. They also require more room to steer and turn. Examples of common multi-hulled boats are catamarans and pontoon boats .

Boat Terminology

Boat Engine Types

What is Sailboat Displacement? A Guide To The Basics

Are you considering buying a sailboat , or curious to learn more about sailboat displacement? If so, you’ve come to the right place! In this guide, we’ll be exploring the basics of sailboat displacement and how it’s measured.

We’ll discuss the factors that influence it, the importance of sailboat displacement, and how to optimize it.

Finally, we’ll go over common sailboat displacement measurements and provide tips for calculating sailboat displacement.

So, let’s dive in and get started!

Table of Contents

Short Answer

Sailboat displacement is the weight of the boat and all of its contents including fuel, water, and any passengers or cargo.

It is typically measured in long tons or short tons.

It is an important measure of stability and performance for a sailboat, as the displacement affects the sailboat’s ability to move through the water efficiently.

The displacement also affects the maximum speed that the sailboat can reach.

What is Sailboat Displacement?

Sailboat displacement is the amount of mass a sailboat displaces when it is in water.

This is determined by the total weight of the hull, any on-board equipment, and the crew.

It is an important factor in understanding the stability and performance of a sailboat, as it affects the drag and momentum when the boat is in the water.

Knowing the displacement of a sailboat is essential for optimizing the design and sailing performance.

The displacement of a sailboat is directly related to its weight.

A heavier boat will have more displacement, while a lighter boat will have less.

The displacement of a sailboat is measured in either kilograms or pounds, depending on the vessel’s scale.

The displacement of a sailboat is usually measured by submerging the boat in water and then measuring the volume of water displaced when the boat is afloat.

The displacement of a sailboat affects its performance in several ways.

A heavier boat will have more drag in the water, meaning it will move slower than a lighter boat.

Additionally, a boat with a higher displacement will be more stable in the water, as it will be less affected by waves or other environmental factors.

Sailboat displacement is also used to determine the optimal sail size for a given boat.

A boat with a larger displacement will need a larger sail, while a smaller boat will need a smaller sail.

This is because a larger sail will provide more power and speed, while a smaller sail will be less effective and provide less power and speed.

In conclusion, sailboat displacement is an important factor in determining the stability and performance of a sailboat.

Knowing the displacement of a sailboat is essential for optimizing the design and sailing performance, as it affects the drag and momentum of the boat in the water.

Additionally, the displacement of a sailboat is used to determine the optimal sail size for a given boat.

How is Sailboat Displacement Measured?

Sailboat displacement is measured by determining the amount of mass that a sailboat displaces when it is floating in water.

This is determined by taking into account the total weight of the hull, any on-board equipment, and the crew.

To measure the displacement of a sailboat, the boat must first be placed in the water and then weighed with a scale or dynamometer.

In addition, the weight of the crew and any equipment on the boat must be subtracted from the total weight to get the displacement.

This information can then be used to calculate the drag and momentum of the boat in the water.

Knowing the displacement of a sailboat is important for optimizing its design and sailing performance.

It is also important for ensuring that the boat is stable in the water and is able to move through the water with ease.

By understanding the displacement of a sailboat, sailors can make sure that the boat is balanced and that the drag and momentum of the boat are optimized.

With the proper knowledge and understanding of the displacement of a sailboat, sailors can ensure that their boat is operating at its best.

Factors That Influence Sailboat Displacement

When it comes to sailboat displacement, there are several factors that can influence it.

The first is the size and shape of the hull, which can affect the displacement of a sailboat depending on its length, width, and depth.

The hull’s shape also influences the drag and momentum of the boat as it moves through the water.

Additionally, the material used to construct the hull can affect the displacement of a sailboat, as more lightweight materials such as wood or fiberglass can reduce the overall displacement.

The quantity of equipment and supplies onboard can also influence the displacement of a sailboat.

Heavy items such as fuel tanks, batteries, anchors, and sails can increase the displacement of a sailboat, so it is important to carefully consider the weight of these items when calculating the displacement.

The number of crew members onboard will also have an effect on the displacement of a sailboat, as each person adds additional weight to the boat.

The type of water a sailboat is floating in can also have an effect on its displacement.

Seawater is denser than fresh water , so a sailboat will displace more water in seawater than in fresh water .

This can be an important factor to consider when determining the displacement of a sailboat in different environments.

Finally, the displacement of a sailboat can be affected by external forces such as wind and waves, which can cause the boat to sink deeper into the water and increase its displacement.

This is why it is important to measure the displacement of a sailboat in different sailing conditions to get an accurate measurement.

The Importance of Sailboat Displacement

When it comes to sailing, the displacement of a sailboat is an important factor that affects the performance and stability of the vessel.

Displacement is the amount of mass a sailboat displaces when floating in water, and is determined by the total weight of the hull, any on-board equipment, and the crew.

Knowing the displacement of a sailboat is essential for understanding the drag and momentum of a boat in the water, and therefore optimizing its design and sailing performance.

Sailboat displacement also affects the speed of the boat.

The higher the displacement, the greater the drag on the boat, so it will move slower.

Conversely, the lower the displacement, the less drag on the boat, making it faster.

Because of this, sailboat designers must take displacement into account when building a boat to ensure it is as efficient and fast as possible.

In addition, the displacement of a boat affects how it handles in rough waters.

A boat with a heavier displacement will be more stable, as it will be able to withstand the waves and wind more easily.

On the other hand, a boat with a lighter displacement may be more vulnerable to the elements, as it may not be as well-equipped to deal with strong winds and rough seas.

Finally, the displacement of a boat can also affect how much weight it can carry.

A boat with a heavier displacement will be able to carry more weight, whereas a boat with a lighter displacement will be limited in how much it can haul.

This is important to consider when planning a sailing trip, as a heavier boat may be able to carry more supplies for a longer journey.

In conclusion, sailboat displacement is a critical factor when it comes to optimizing the design and performance of a sailboat.

Understanding the displacement of a vessel is essential for ensuring that the boat is as efficient and fast as possible, as well as being able to handle the rougher waters.

It is also important to consider the displacement when planning a sailing trip, as a heavier boat may be able to carry more supplies for a longer journey.

Optimizing Sailboat Displacement

Optimizing sailboat displacement is an important factor when it comes to performance and stability.

Knowing the displacement of a sailboat can help you to make informed decisions when designing and sailing the boat.

The displacement of a sailboat is determined by the total weight of the hull, any on-board equipment, and the crew.

This is critical information when it comes to calculating drag and momentum in the water.

There are several ways to optimize sailboat displacement.

One of the most effective methods is to reduce the weight of the hull and any on-board equipment.

By lightening the load, the boat will have less mass and therefore less drag and momentum in the water.

Additionally, the use of lighter materials in the hull and equipment can also help to reduce the displacement.

Another way to optimize sailboat displacement is to minimize the crew size.

The fewer people on board, the less mass the boat has to displace.

This is especially important for smaller sailboats, as they may not be able to handle the weight of a larger crew.

Additionally, having fewer crew members can help to reduce the amount of gear and equipment that is brought on board, further reducing the displacement.

Finally, sailboat displacement can be optimized by optimizing the design of the boat.

This includes the shape of the hull, the placement of the sails, and the rigging of the boat.

By ensuring that the design of the boat is as aerodynamic as possible, the drag and momentum of the boat in the water can be reduced.

Additionally, the placement of the sails and rigging of the boat can help to maximize the amount of power that can be generated from the wind.

In conclusion, optimizing sailboat displacement is essential for maximizing the performance and stability of a sailboat.

By reducing the weight of the hull and equipment, reducing crew size, and optimizing the design of the boat, the displacement of a sailboat can be minimized.

This is important information to consider when designing and sailing a boat, as it can help to ensure that the boat performs at its best.

Common Sailboat Displacement Measurements

When it comes to sailboat displacement, there are two common measurements used to determine the amount of weight a vessel displaces when it is floating in water.

These measurements are displacement in kilograms (kg) and displacement in cubic feet (ft3).

Displacement in kilograms is calculated by taking the weight of the hull, on-board equipment, and crew and converting it into kg.

This measurement is the most commonly used and is typically the most accurate way to measure displacement.

Displacement in cubic feet is calculated by taking the volume of the hull and equipment and converting it to cubic feet.

This measurement is less accurate than displacement in kilograms, but is often used to compare the size of different vessels.

Knowing the displacement of a sailboat is important for optimizing the design and sailing performance.

It affects the drag and momentum of the boat in the water, so its important to have an accurate measurement when taking into account the stability and performance of a sailboat.

Additionally, displacement can be used to calculate the sailboats draft, which is the depth of the vessel below the waterline.

Ultimately, understanding sailboat displacement is essential for any sailor who wants to get the most out of their vessel.

Knowing the displacement of a sailboat can help you determine the design and sailing performance, as well as the draft of the vessel.

With the right knowledge and calculations, you can ensure that your sailboat is optimized for the best possible performance.

Tips for Calculating Sailboat Displacement

Calculating the displacement of a sailboat can be complex, as it depends on the total weight of the hull, any on-board equipment, and the crew.

However, there are some tips and tricks that can help make the process easier.

First, it is important to keep accurate records of any and all equipment that is placed on board the boat.

This includes any tools, spare parts, and supplies.

Keeping a running list of these items can help you accurately calculate the total weight of the boat and its components.

Second, make sure to account for the weight of the crew.

This includes any passengers, as well as the captain and any other personnel on board.

Third, it is important to consider the boats design when determining its displacement.

Some boats may have a keel or a centerboard, which can dramatically affect the boats weight and displacement.

If these features are present, it is important to factor them into the equation.

Finally, it is important to consider the effects of water density when calculating displacement.

Water density can vary depending on the temperature and salinity of the water, so it is important to consider these factors when calculating displacement.

By following these tips, you can be sure that you are accurately calculating the displacement of your sailboat.

Knowing the displacement of a sailboat is an important part of optimizing its design and sailing performance, so it is important to get it right.

Final Thoughts

Sailboat displacement is a key factor in determining the stability and performance of a sailboat.

Understanding how sailboat displacement is measured and the factors that influence it can help you calculate the displacement of a sailboat and make informed decisions about sailboat design and performance.

Now that you know the basics of sailboat displacement, get out on the water and start optimizing your sailboat for optimal performance!

James Frami

At the age of 15, he and four other friends from his neighborhood constructed their first boat. He has been sailing for almost 30 years and has a wealth of knowledge that he wants to share with others.

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Boat Hulls 101: Complete Guide to Boat Hull Types, Shapes, and Designs

Table of Contents

Last Updated on August 17, 2023 by Boatsetter Team

If you’re new to boating, then you may not have even considered a boat’s hull , its importance, and the way that it affects your time on the water. With the  hull  being the part of the boat in the water, it is perhaps the most important part as it gives your boat the ability to float. Not only that, but it affects every single characteristic of your boat and the smoothness of your ride. This article on boat hulls will equip you with the technical knowledge and expertise necessary to understand hulls and the way they work.

What is a Boat Hull?

First of all, we’ll go into a bit of detail on what a boat hull is. The hull is the body of the boat. It is sealed to prevent water from transmitting its way through and keeping your boat afloat. A hull can be open where you sit in it, such as a small dinghy, or a deck may cover it as you would find on a yacht.

When there is a deck placed on top of a hull, it opens up many more options for utilizing the space on your boat more appropriately as it is raised to the top of the hull, where more space is apparent. For example, on a deck, you can place a cabin -like you would find on a center console or even a mast and sail rigs to create a sailboat.

When the hull is open, options to use your space effectively are reduced as you sit at the bottom of the bowl shape. In addition to having less space, you also feel the rock of the water in a more pronounced manner as it is just the keel of the boat (the bottom) separating you from the water. Therefore, every wave and lurch in the water that rocks the boat is felt, which may cause you discomfort if you haven’t quite found your sea legs.

Why Are Hulls Important?

The knowledge of how a boat floats is fundamental if you are looking to get into boating. Without actually knowing, you put yourself at risk of compromising your boating activities and creating a danger that you cause your boat to sink. The key line to this knowledge is that the air encapsulating your boat must be denser than the water it sits upon. This not only includes the air but the items on your boat as they contribute towards the pressure that your boat’s hull puts upon the water.

The greater the amount of weight your boat holds, the further it pushes itself into the water, lowering or raising the level that your hull sits in the water. This force displaces the water to a level that is equal to the boat. If the average density of the boat is greater than the water, then the boat shall sink. You can see this in action if you have a small dinghy; the more people you place on it, you’ll notice that your boat edges itself ever so slightly more into the water as the boat’s weight is rising.

Different types of Boat Hulls

We’ll now walk you through the different types of boat hulls that you come across. The design of the boat’s hull changes the type of boat that you have. If you are browsing through our boat rentals, you’ll notice the various types of boats. Each of these boats has a different type of hull design. For example, a pontoon boat rental is designed for calm waters, whereas a giant yacht is designed for taking on the rough seas, meaning that their hulls vary greatly.

There are two main types of hull: displacement and planing. We’ll give you the rundown of both of these types and the other sub-varieties within them.

Displacement Hulls

The first variety of hulls that we shall examine are displacement hulls. These hulls are typically found on boats that need to carry a heavy load, such as a large fishing boat and big yachts. The hull sits deeper into the water, and the boat is supported by buoyancy, as opposed to its thrust.

Due to the boat sitting deeper in the water, it might be slower, but it will ride steadier. These larger boats are particularly good for the sea as they can handle stronger waves and currents as the boat can stabilize themselves better. This is why you’ll see container ships and other varieties that need to bear a heavy load using these types of hulls.

When it comes to boat rentals, you are most likely to find a sailing boat with a displacement boat hull. The hull is rounded at the bottom, allowing the sailboat to lurch strongly to one side while turning without any danger of capsizing. Thus, we can see the impact that the hull has on your boat rental as it gives your sailboat the extra capacity to lurch around sharp turns and enjoy some exhilarating fun.

Planing Hulls

The other main type of hull is the planing hull. This hull’s design allows the boat to accelerate to higher speeds due to less hull being placed in the water. When a boat with a planing hull is cruising at lower speeds, it operates similarly to a boat with a displacement. When it starts to hit around 15 knots, things start to change depending on the weight of the boat’s load. The flatter surface of the planing hull allows the boat to propel itself upwards to skiff itself across the water. This is what causes boats with planing hulls to obtain higher speeds. In addition, because the bulk of the hull is not placed below the water’s surface, there is less tension from the water holding the boat back, meaning that it can move through the water faster and using less power to obtain a speed that a boat with a displacement hull can.

There is not only one type of planing hull but instead many different varieties. We’ll look through these varieties to examine how it affects your boating experience so that you can make a more informed decision when choosing your next boat rental.

Flat Bottomed Hulls

As the name suggests, these hulls do not have the traditional curved hull that reaches a point at the bottom but has a flat surface instead. These tend to be small skiffs or fishing boats where you cast out from. Due to them having a flat hull, they are excellent for getting into shallow water where some of your favorite catches may lie. These boats don’t need much power for the planing power to come into action and reach quick speeds in no time. They also tend to handle well not just on the flats but also on the sea, with choppy water not being a big issue. So, if you’re looking for some gentle fishing on the flats or maybe out in some nearshore waters, check out our range of small flat bottomed hull boats to truly enjoy some great fishing experiences.

Pontoons  are one of the great boat rentals for cruising around and enjoying time with friends because the design of their hulls allows for more space to be created. Pontoons have two-cylinder hulls that sit parallel to each other on the surface of the water. The deck is placed atop these two cylinders, and because they are placed on cylinders, the deck can expand beyond the cylinders, creating more space. This allows for a comfortable seating/social area to be created on the boat, allowing you to use it for parties and some relaxed exploring with the wider family. These boats are best used on inland and flat waters. This is due to waves rocking them a lot more, and a storm at sea can even put them at risk of capsizing. For some fun on a lake, however, pontoons are hard to beat.

In recent years many tritoons have started to crop up on the boat rental market. These are similar to pontoons, but they have a third cylinder that gives them some extra stability. It also means that they can handle a more powerful engine that can bring them up to higher speeds than a pontoon boat. If a pontoon has an engine that is too powerful, then its planing hull can lift it too far above the water’s surface, causing great instability. However, when it comes to tritoons, the greater speeds that you can reach allow you to expand upon other activities and add in some wakeboarding or tubing action onto your party on the water!

V Bottom Hull

The shape of a v bottom hull has a sharper decline that accumulates in having a meeting point at the bottom, creating a v shape, as stated in the name. Because of the honed hull, one of these boats can cut through the water at decent speeds and are particularly good when out on seawater. However, they require a powerful engine for the boat to go into a planing mode. One of the most common types of v bottom hull boat rentals is center consoles. These are great vessels for going for some nearshore or offshore fishing or some general saltwater exploring. Their v bottom hull allows them to cut through the waves so that you can rush to the best fishing grounds in no time at all.

The tri-hull design is a variation of the v bottom hull. It has a v-shaped hull in the center and two parallel smaller hulls on either side of the main central one. This gives the tri-hull boat some extra stability when going forward . Additionally, this also allows the boat to have more deck space as the hull covers a wider range. One of the big drawbacks of the tri-hull – also known as a cathedral hull – is that the bat rocks more when it is in choppier water because the hull is wider. Nevertheless, tri-hulls make for a great option for fishing or exploring on lakes or calm coves.

Catamaran: A Multi-Hulled Boat

Perhaps the most popular multi-hulled boat is the catamaran. This type of boat has two separate hulls that run parallel to each other. These hulls sit on either side of the boat and the deck connects them. This type of design allows forecast amounts of space onboard . Many catamarans are luxury boats that can have the space to hold swimming pools and even helipads. Because they have dual hulls, catamarans can get themselves in shallow waters and lagoons where other luxury boats cannot. This makes them the perfect boat rental if you plan to visit a location where there are multiple small islands such as Hawaii or The Bahamas. The multi-hull system also provides a lot more stability and comfort, so they are perfect boat rentals if you are prone to suffering from seasickness . Catamarans are not only luxury liners as smaller versions with a trampoline-designed deck can also be found that make for great day adventures.

As we hope you have been able to discover in this blog post, the type of hull that your boat has affects everything about your boat. By having a little bit of knowledge on how the design of a boat’s hull has an impact on your boating experience, you can begin to make more informed decisions on which boat rental is best for you. To reinforce this information a little bit further, check out this  video !

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Hull speed is a phenomenon of displacement boats, and not of planing boats. Most sailing boats and all ships displace water—move it aside—as they plow through it. Planing craft, such as most motor boats, glide over the top like a surfboard. It takes more energy to push water aside than it does to slide over the top of it, and so displacement boats move at a more sedate pace than their lighter planing cousins. Some small sailing boats can be made to plane, but the general rule is that sailing boats are of the displacement type. Hull speed is usually an upper limit to the speed of displacement boats.* It is unsurprising that such a limit exists: we have seen how drag increases with speed, and so sooner or later drag will balance out the drive force and a sailboat will not be able to go faster. Yet there is a surprise in store for those of you who are not familiar with sailing: the hull speed of a given boat depends on its hull length at the waterline. It is not obvious from a simple consideration of drag why this should be so, but it is a well-attested fact, often quoted in the sailing literature, that the maximum natural speed of a displacement boat (in knots) is 4/3 the square root of waterline length in feet.

A key feature of the phenomenon, again well known to any sailor, is that hull speed has been reached when the bow wave of the boat lengthens to the waterline length. At lower speeds, there may be three or four complete waves seen to lap along the boat hull, but this number decreases as the boat picks up speed and reaches, pretty closely, one complete wave by the time the boat reaches her hull speed. It may be possible for her to go faster than hull speed, but this requires a disproportionate amount of effort. In other words, the hydrodynamic drag

* There is one trick by which a small displacement boat can exceed hull speed without expending enormous effort, and that is by surfing. Riding along the front of a wave is not the sole preserve of surfboards.

Figure 6.1. (a) Your hull-speed raft, viewed from above. Note the direction of motion. (b) When the bow wavelength is less than the distance between the long beams, drag is reduced compared to the case of (c). In (c) bow wavelength equals the distance between beams because the aft beam is more submerged. So hull speed is reached when hull length equals bow wavelength. Consequently, hull speed is limited by hull length.

force that is acting to hold back the boat increases rapidly once hull speed is reached. My goal in this section is to explain to you, in simple physics terms, why these phenomena occur.

Which is why I have press-ganged you into service onboard the undignified vessel illustrated in figure 6.1. She is a wooden raft with two long logs fore and aft that stretch way beyond her beam. These logs are not there to provide flotation, please note—we will suppose that the raft has enough buoyancy without them—but rather to illustrate hull speed. You set the primitive sail and drift off to the right. The forward log generates a bow wave which spreads out in the wake, as waves do. You notice something that you have seen many times before in other craft: the bow wave size (amplitude) increases as the vessel speed increases. This makes sense because the hull is pushing water aside, the displaced water has to go somewhere, and the faster you go, the more water is moved. So the wave size increases. Now you pick up speed, and so the wavelength of the wake, as observed alongside your hull, stretches out until exactly one wave lies between the two extended logs at bow and stern. The raft speed that gives rise to this condition is her top speed, you

Figure 6.2. Your hull-speed barge. Bow waves forward of the center of gravity, CG (open circle) exert a buoyancy force (vertical arrows) proportional to wave height that acts to rotate the barge hull counterclockwise. Similarly, waves aft of the CG act to rotate the hull clockwise. If we can assume that drag forces are proportional to counterclockwise torque (a dominant CCW torque means that the barge is climbing a hill created by its bow wave), we can show that hull speed occurs when bow wavelength equals hull length.

find. It is clear why: the aft log is now submerged, and so experiences more drag than it did earlier, when there was no wave crest at the hull stern (see fig. 6.1). So, drag force peaks when bow wavelength equals hull length, in this simple example.

Now we are able to see where the old formula for hull speed comes from. The speed of a bow wave, or of any other surface water wave,1 is c where c2 = gk/2p. Here l is the water wavelength, and g is the constant acceleration due to gravity. Now the raft speed, v, equals the water wave speed, c, so that v = VgL/2p (since hull length, L, equals water wavelength at hull speed, as we just saw). Substitute numbers and we arrive at the old formula.

The ungainly raft has served her purpose, and you can now abandon her. The lesson learned is intuitive, and yet it gives us a basis for understanding quantitatively what hull speed is about. Now I can do another calculation, this time a little more realistic. The math is more involved (you need not wade though it), but the basic idea is again quite intuitive. Figure 6.2 shows the profile of a steep-sided hull plowing through water and generating a bow wave, which oscillates along the line of the hull. This vessel is kept afloat by the buoyancy force, and we can see that the buoyancy force is going to be different at different points along the line of the hull because the wave height varies along the hull. Buoyancy that acts forward of the hull CG (shown in fig. 6.2) will create a counterclockwise torque that tends to twist the hull about the CG—trying to make it do a backflip. The buoyancy force aft of the CG produces a torque that acts in the clockwise sense. These two more or less cancel* but not quite. If the counterclockwise buoyancy torque is just a little bigger than the clockwise torque, the boat will tilt backwards, until her stern goes deep enough to generate a compensating torque. We would then be left with a boat that is going uphill, trying to reach the crest of her own bow wave.

Where am I going with all this? Roughly speaking, counterclockwise torque equates to uphill motion, and uphill motion leads to increased drag, for reasons that will soon be made clear. So, I am saying that increasing the unbalanced counterclockwise torque generated by a bow wave will increase drag. If this increase should suddenly take off at a certain speed, then we have found our hull speed. In fact, I can calculate the torque generated by the bow wave. You can see that as the bow wavelength changes, the torque will also change because the manner in which buoyancy force is distributed along the hull length changes with wavelength (fig. 6.2). The results of this calculation are plotted in figure 6.3. (For those interested, the math is provided in this endnote 2 in sufficient detail for you to reproduce the calculation.2) In figure 6.3 we see once again that drag force takes off for water wavelengths exceeding hull length, more or less.3

For simplicity, the hull of figure 6.2 was given vertical sides, but most boats don't have vertical sides, for a host of reasons. Recall that, in the Age of Sail, ships of the line were given a tumblehome cross section to deter boarders. Nowadays we are less likely to have to repel nefarious enemies swarming over our gunwales with cutlass in hand, casting a single bloodshot eye (the other being patched) in search of our gold doubloons. Hull sides are angled but the other way, with cross sections resembling a martini glass rather than a brandy glass. In plain language: more V-shaped. Here are some physics reasons for different hull cross sections.

——'Rounded hull bottoms are stronger than V-shaped hulls, but the latter will be deeper for the same displacement and so will better resist leeway.

*Just as well, because backflipping boats would be pretty uncomfortable.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 Water wavelength / L

Figure 6.3. Hull speed is limited by drag. In the simple model described in the text, the drag increases with water wavelength, l, as shown (L is hull waterline length). Here, drag force is set arbitrarily to 1 at zero speed. If the bow wave is assumed to have constant amplitude, independent of speed, then drag changes with speed as shown. For a more realistic model, with bow wave amplitude increasing with speed, the curve looks similar. In this simple model, hull speed occurs at l « 1.2L because for longer waves (higher boat speed) the drag force becomes too strong.

•—A large deck area is desirable, but large hydrodynamic drag is not. For a hull of a given displacement, the choice of hull shape is constrained by the trade-off between these two characteristics. •—'An angled hull—say one that is V-shaped—will have greater reserve buoyancy. That is, the righting moment will increase as the hull heels further and further. •—'During heeling, the waterline along an angled hull will not be symmetric about the longitudinal axis; the port side waterline length and shape will be different from that on the starboard side. This asymmetry can assist the boat to head up while heeling. Thus, even without aerodynamic assistance from her sails, a boat may automatically

point to windward when heeling solely because of hydrodynamic forces acting on the hull. •—'Different angled hull shapes beneath the waterline assist with planing. For certain boats, such as racers, this is important because planing requires less displacement, less wetted area, and so less drag—and hence increased speed.

The physics of angled hull shapes casts an interesting light on the capabilities of some ancient ships . Certain ancient ships were built with a lot of overhang at the bow and stern, but this practice is usually thought to have been of little value for the old square-riggers because these ships were supposed to be nippy only when running or on a broad reach. Today, such hull shapes are utilized to increase hull speed while heeling because the waterline length is increased when the hull is heeled over. This lengthened waterline increases boat speed on a beam reach, for example. It seems plausible to suppose that ancient vessels with overlapping bows and sterns may have been capable of traveling across the wind at speed. Indeed, such a hull design offers no other advantage for these square-rigged vessels. (An overhanging bow and stern increases deck area, but for merchantmen—and in ancient times most of the sailing ships were merchant vessels because warships were oar-powered—deck area was not such a big deal. Volume of the hold was what mattered.) For a downwind point of sail, extended hull length above the waterline will increase pitching motion when traveling downwind; this is bad, and yet the overhanging bow and stern must have conferred some advantage or these ancient ships would not have been built this way.

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Readers' Questions

Is it possible for a displacement boat to exceed hull speed?

No, it is not possible for a displacement boat to exceed its hull speed. Hull speed is the theoretical maximum speed that a displacement boat can reach, and it is determined by the length of the waterline. When a boat exceeds its hull speed, it starts to climb up on its own bow wave and create excessive drag, making it difficult to go any faster.

What can you say about the speed of a boat that makes a bow wave?

The speed of a boat that makes a bow wave is usually quite fast, as the bow wave is usually associated with a boat moving at high speeds.

How to calculate hull speed?

Hull speed, also known as displacement speed, is the speed at which a boat hull moves through the water. It is calculated by taking the square root of the waterline length of the boat in feet and dividing it by 1.34. The formula is: Hull Speed = √LWL / 1.34 where LWL = waterline length in feet.

What is maximum hull speed for a boat?

The maximum hull speed for a boat is typically 1.34 times the square root of the waterline length of the boat in feet. For example, the maximum hull speed for a boat with a waterline length of 20 feet would be about 24 knots (1.34 x √20).

Why catamarans sail faster than hull speed?

Catamarans sail faster than hull speed because of their unique hull design. Their twin hulls provide greater stability and lift than a single hull, which results in less drag on the boat. This reduced resistance allows the boat to move more quickly through the water, resulting in higher speeds than what is normally achieved with a traditional hull design. Additionally, the width of the catamarans hulls also distributes the weight of the boat more evenly, which further reduces drag and increases speed.

13 Types of Sailboat Hulls

Explore 13 sailboat hull types with photos! Ideal for travelers seeking the perfect sailboat for speed, stability, or rough waters.

A sailboat is only as good as the hull, and it ultimately determines how well you can navigate through the water. The hull of a boat plays a massive role in what type of water you can sail through and your overall speed. So, what are the types of sailboat hulls and how are they different?

The main types of sailboat hulls are planing hulls, displacement hulls, and semi-displacement hulls which offer the best of both worlds. Multi-hull boats such as pontoons and tritoons have even weight distribution and can handle rough waters. Flat-bottom sailboats are the most stable, but they don’t work well in deep waters.

Catamarans and trimarans feature space between each hull which adds stability and protects the deck from water. Choosing a sailboat with the ideal hull for you is essential in finding one that you will keep for years to come. Follow along as we explore the different types of sailboats and see what makes them unique.

Sailboat Hull Types

There are 13 types of sailboat hulls ranging from bilge keels and fin keels to displacement hulls. The ideal sailboat hull varies for you based on factors such as what type of water you’re in and weather conditions.  For example, some hulls, such as flat bottoms, are ideal for shallow and smooth water .

On the other hand, semi-displacement hulls are perfect for every application whether you’re in shallow or deep water. Let’s take a look at the different types of sailboat hulls and see how they differ.

1. Planing Hull

Planing hulls are the first of the three major categories of sailboat hulls. You can find planning hulls with 2 different shapes: v-shaped and flat-bottom hulls.  Planing hulls sit on top of the water and don’t sink deep like other types .

Many boaters and enthusiasts prefer this design because of how well a boat with planning hulls can move across the water.  Most fishing sailboats feature planning hulls because of how smoothly they can glide on the surface whether you’re on an ocean or lake . Boats with planing hulls can also move faster than other types of boats, and that is their main appeal.

2. Displacement Hull

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Boats with displacement hulls are slower than boats with planing hulls, but that doesn’t mean that they’re bad . While they don’t move as fast, many boaters consider displacement sailboat hulls to be much smoother.  This comes in handy if you live in an area with rough waters and strong winds .

A displacement hull is rounded instead of flat at the bottom like a planning hull. The main downside to sailboats with displacement hulls is that you will likely use more fuel than you normally would. That is because the shape isn’t as aerodynamic and you’ll need the extra engine power to move through the water.

3. Semi-Displacement Hull

Seattle Yachts

As the name suggests, semi-displacement hulls combine the best of both worlds between planing and displacement hulls. A semi-displacement hull is both flat and rounded at certain parts providing both speed and stability.  They aren’t as fast as a flatter planing hull, but they’re faster than a standard displacement hull .

The unique shape of semi-displacement hulls helps reduce resistance. This alone can help take a load off of your engine and let it work optimally under most water conditions. You also get the benefit of extra storage in most cases because boats with semi-displacement hulls have storage-friendly floor plans.

4. Multi-Hull

Multi-hull boats, such as pontoons and tritoons, are smooth and easy to sail . There are separate hulls on each side of the boat that provide stability and let you power through rough waters. On a pontoon, each hull is a large tube filled with air known as a toon.

Multi-hull boats generally sit higher above the water than most boats because of their unique design. They are popular for fishing, cruising, and entertainment.  A key downside to multi-hull boats is that they typically operate loudly because the propeller may not be fully submerged in the water .

5. Monohull

The vast majority of sailboats that you will come across have a monohull . They are easy to sail, transport, and even dock at a marina because of their simple design. As the name suggests, they only feature one hull and are suitable for calm and rough water.

You can save money with a monohull sailboat compared to a multi-hull sailboat like a catamaran. A key advantage to monohull sailboats is that they are incredibly safe. You don’t have to worry about capsizing as much as you would with a multi-hull sailboat.

6. Flat-Bottom

Flat-bottom hulls are essentially the simplest form of planning hulls. You can find flat-bottom hulls on the majority of sailing dinghies, and that’s what they are most suitable for.  They aren’t ideal for oceans or rough waters, but flat-bottom sailboats are perfect for rivers and lakes .

Rowboats also feature flat-bottom hulls, and they aren’t known for being particularly smooth. You get less precision with flat-bottom hulls, especially if you have to steer and turn unexpectedly. Otherwise, you won’t have trouble with a flat-bottom sailboat hull if you go out for a quick fishing trip in an area you’re familiar with that has smooth waters.

7. Catamarans

Catamarans feature a unique take on the traditional multi-hull design . They feature 2 hulls with space between them that usually features a deck. Sometimes, the space between each hull features a trampoline or even a small pool or tub.

They aren’t suitable liveaboard boats, but they are perfect for taking out for a day of cruising and fishing. Catamarans are as smooth as possible, but that sometimes comes at the cost of speed. However, they often feature multiple engines which can consume a lot of fuel but also put less strain on each engine.

8. Trimaran

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Trimarans are essentially a step up from catamarans because they feature a third hull. Many people prefer the stability that trimarans offer over catamarans.  The extra stability also helps increase the speed that you can cruise at with a trimaran .

They are also safer than catamarans because the multi-hull design allows for perfect weight distribution. That’s not to say that catamarans are unsafe, but the extra hull that trimarans feature is more durable.  Most of the weight lies on the center hull and the rest is distributed between the 2 outer hulls .

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Deep v hulls are another type of planing hull, but they are less common than some of the other varieties. Granted, high-end modern powerboats often feature a deep v hull, but they come at a high price.  The v design allows the hull to cut into the water easily which lets you easily control the boat in any type of water condition .

Generally, the deadrise goes between 21 and 26 degrees for a deep v hull which is ideal for many boaters. However, boats with deep v hulls are primarily geared toward anglers and aren’t ideal for cruising at high speeds. Deep v hulls are usually made out of aluminum which means that they will be loud as they glide across the water.

10. Bilge Keel

Bilge keel hulls are specifically designed to reduce the risk of a boat rolling . The strange shape of a bilge keel hull lets it stand upright whether you’re on the shore or in shallow waters. This makes them much easier to maintain than many other types of boats.

The bottom of a bilge keel hull features multiple fins in a row that helps ensure a smooth ride. They never feature more than 2 keels which means that they have a shallow draft and you can easily beach them. The one downside to sailboats with a bilge keel hull is that they are difficult to transport to a port because of the bottom.

11. Bulb Keel

Bulb keel sailboats feature a teardrop-shaped ballast that increases the boat’s stability.  They are even faster than bilge keel sailboats because of how hydrodynamic they are . Unlike some types of hulls, a bulb keel works just as well on the sea as it does on lakes and rivers.

They feature incredible weight distribution because of the inclusion of an extra ballast. However, you have to be careful with bulb keel sailboats in shallow waters near the shore. They are more susceptible to damage at the bottom so they can be difficult to bring to shore and require precision.

12. Fin Keel

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Unlike bulb keel and bilge keel hulls, fin keel sailboats are perfect for raising . Fin keel hulls improve the draft of a sailboat which comes in handy when you want to reach high speeds. Some sailors use fin keel sailboats to travel long distances across the water, especially if the weather is in their favor.

They are a perfect happy medium between flat and round-bottomed hulls offering the best of both worlds.  Fin keel sailboats are also quite comfortable because of the unique bottom shape that can easily handle choppy waters . With that said, they aren’t ideal for beginners because they can be difficult to steer compared to standard flat-bottom hulls if you are inexperienced.

13. Cathedral Hull

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Cathedral hulls get their name from their appearance which is similar to that of a classic cathedral. The unique appearance is one of the biggest benefits of cathedral hulls because the whole boat takes on that shape. They feature sharp bows and high sterns that are immediately recognizable.

With that said, cathedral-hull sailboats can be difficult to steer compared to flat-bottom or rounded hulls because of their bulky shape . You get plenty of storage with cathedral hulls which makes them perfect for long day trips with many people. They are incredibly stable because of the wide beams and wide berth, so there isn’t a serious risk of capsizing as long as you pack your cargo well.

What Type of Hull is Best For Rough Waters?

Any type of boat with a v-shaped hull is best for rough waters. Whether it’s a deep v or shallow v, this hull design makes it easy to cut through rough water without getting too much on your deck.  The last thing that you want is to go through rough waters and take on excess water weight onboard .

They are specifically designed to glide across the water without sinking low which is necessary for choppy waters. Boats with v-shaped hulls also often come with high-performance engines, so they offer the best of both worlds.  Generally, deep v hulls are the most precise and smoothest when it comes to rough waters .

They are perfect for keeping course which is essential if you’re in rough waters that can kick you off of your path. You can find many deep v hulls that are made out of fiberglass which is incredibly durable and withstand water exposure. Fiberglass v-shaped hulls are also easy to repair either by yourself or at a professional shop at a low cost.

What is The Most Stable Hull Design?

Flat-bottom hull sailboats have the most stable design for shallow water and multi-hull boats are the most stable in deep water. The inclusion of multiple hulls adds stability in deep water that prevents water from landing on the deck. This can save you expensive repairs and can also prevent your sailboat from capsizing.

Pontoons and tritoons are multi-hull boats and they are specifically popular because of their stability.  You can’t find a more stable design than a flat-bottom hull if you plan to cruise in shallow waters . Flat-bottomed hulls are also typically the fastest when you aren’t far from shore, especially if you are in smooth waters.

The box shape of flat-bottomed hulls is conducive to gliding across shallow water. However, they struggle to ride across waves and choppy waters because of the wide surface area. Multi-hull boats such as pontoons and tritoons are the best option if you plan to take your boat out to lakes, rivers, and oceans because they thrive in any scenario.

What is Better Flat Bottom or V-Hull?

Flat-bottom boats are better than v-hull boats for most uses, but v-hulls are better in choppy and deep waters. You can get by with a flat bottom in oceans and lakes alike, but they don’t always do well in deep water.  Conversely, v-hull boats can tear through rough and wavey water even at steep depths .

V-hull boats don’t do well in shallow waters and you are more likely to get stuck than you would be with a flat-bottom boat. Of course, you can always have someone push from behind when you depart, but that doesn’t help much when you return to shore.  V-hull boats are the better option for deep waters, however, even if you are in rough water .

Flat-bottom boats take on more water than v-hull boats unless you stay in calm waters. It’s always worth choosing a boat that won’t take on water that will weigh it down. However, if you’re looking for a reliable boat with a high capacity, then I would recommend looking into v-hull boats.

What Type of Hull Cuts Through Water?

Displacement hulls are the best at cutting through the water, especially when compared to planing hulls. They don’t rely on a powerful engine to cut through the water because of their design. Displacement hulls displace water once you lower them in from the shore.

This displacement isn’t ideal for speed, but it is perfect for rough waters and strong winds.  You can take sailboats with displacement hulls out on the ocean without having to worry about waves . They also work well in freshwater, but they are less necessary because of the lack of waves compared to the ocean.

Otherwise, you can get the best of both worlds with a semi-displacement hull . They aren’t quite as precise as displacement hulls, but they are better at navigating choppy waters than planing hulls. You sacrifice a little bit of speed, but the shape of a boat with a displacement hull lets you power through waves without veering off of your path.

How Long Do Sailboat Hulls Last?

Sailboat hulls last for an average of 15 years, but many of them can last for 20 years or longer. It ultimately depends on how well you maintain them and how often they are in the water.  For example, a sailboat that you always keep in the water and rarely store in a dry place may need hull repairs and replacement much sooner .

Sailboat hulls are susceptible to algae damage, and that is more likely if you always keep them in the water. Cleaning the hull of a boat is essential to protect them from algae and examine them for potential damage.  Fiberglass is the best material for a boat hull, even compared to aluminum which was the standard for years .

Fiberglass hulls can last for up to 50 years or more with regular cleaning and maintenance. A sailboat’s hull won’t last as long if it suffers damage neglecting maintenance or hits the shore too fast. The best way to increase the longevity of your sailboat is to take it out of the water every once in a while and scrub the hull to remove algae.

How Do You Inspect a Sailboat Hull?

The best way to inspect a sailboat hull is to take it out of the water and clean it . You can easily inspect a sailboat’s hull if it is clean and dry, or else you will mix cracks and dents. Cracks are the most important thing to look out for because it’s best to catch them early on.

You should be concerned if you come across cracks because they can eventually worse and threaten your boat’s structural integrity. This is especially true if you have a sailboat with an aluminum hull that you regularly take out onto saltwater.  Aluminum can eventually break down in saltwater so it’s important to inspect it regularly, especially after 10 years or more .

The hull is the first part of a boat that you should inspect because hull damage can cause a boat to sink. Always inspect your boat’s hull if you sail too fast in shallow water because that is when you risk the most trouble. Clean and dry your boat’s hull, then follow along it closely to look for spots that don’t glisten as much. This will indicate a weak point, scratch, tear, or dent.

Fastest Sailboat Hull Design

Multi-hull, trimarans, and flat-bottom boats feature the fastest sailboat hull designs . They are hydrodynamic which lets them glide through the water with minimal resistance. Trimarans move incredibly fast, especially in salt water, as long as they aren’t weighed down with too much cargo.

Careful packing reduces the necessity to haul less cargo because trimarans have incredible weight distribution. However, factors such as water conditions and what type of body of water you are on ultimately play a huge role.  Boats move up to 2% faster when in saltwater than in freshwater no matter which type of sailboat hull design you have .

Other factors such as your boat’s capacity and how much cargo you are carrying make a huge difference as well. Any type of boat with a planing hull is a safe bet if you want to move quickly through the water. Avoid sailboats with a displacement hull if you value speed because they often move the slowest.

So, What Are the Types of Sailboat Hulls?

The three main types of sailboat hulls are planing hulls, displacement hulls, and semi-displacement hulls.  Bilge keel, bulb keel, and fin keel hulls are similar but have different practical applications between freshwater and saltwater . Trimarans and catamarans feature sturdy hulls that are highly regarded for their even weight distribution and roomy storage.

Multi-hull boats such as pontoons and tritoons sit above the water and can withstand rough waters because of how high they sit. Displacement hulls are the best option if you need to cut through choppy waters and maintain your routing. Otherwise, consider a flat-bottomed hull if you primarily stay in shallow water because of how stable they are.

Kit Evans is a seasoned marine journalist and naval architect, bringing over 20 years of multifaceted experience in the boating industry to his writing and consultancy work. With expertise ranging from boat design and marine surveying to charter operations and vessel restoration, Kit offers unparalleled insights into all aspects of maritime life. When he's not penning articles for top boating publications or hosting his popular YouTube channel, Kit can be found sailing his lovingly restored 1960s Columbia 29 on the Chesapeake Bay, embodying his commitment to both preserving nautical heritage and embracing modern innovations in boating.

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Calculating power requirements for full displacement hull

Discussion in ' Boat Design ' started by Annode , Sep 2, 2019 .

Annode Previous Member

It seems to take very little power to move even large and heavy full displacement hulls up until about 6-8kts. Then the power curve vs speed seems to go up exponentially after that, and nearly vertical at about 13kts. Assuming that 8kts is sufficient, both in terms of efficiency and speed, what is the nominal (flat water) calculation for power vs weight. After that is known, what is the calculation for additional power for manoevering a heavy steel hull with a lot of inertia, and for wind, sea. Since its not trawling, dont need monster trawler engine, but something extra I imagine. How is this calculation typically made?  

DCockey Senior Member

Annode said: ↑ It seems to take very little power to move even large and heavy full displacement hulls up until about 6-8kts. Then the power curve vs speed seems to go up exponentially after that, and nearly vertical at about 13kts. Assuming that 8kts is sufficient, both in terms of efficiency and speed, what is the nominal (flat water) calculation for power vs weight. Click to expand...

to calculate engine power... which for some strange reason is measured in horsepower in boat engines - that only rev to 1500 or 2000 rpm thus disguising the the torque that the engine generates... a more relevant number since a turbo diesel 500hp can generate 2000 ft/lbs of torque  

Annode said: ↑ to calculate engine power... which for some strange reason is measure in horsepower in boat engines that only rev to 15 or 200rpm thus disguising the the torque that the engine generates... a more relevant number since turbo diesel 500hp can mean 2000 ft/lbs of torque Click to expand...

JSL Senior Member

how about we get all the facts* about this installation so contributors to the solution find it a bit easier. (* wl length, displacement, power, etc etc. Power, speed, & * etc. also governs propeller size.  

Mr Efficiency Senior Member

For practical purposes, rather than strictly theoretical ones, it is the wave making property of the typical displacement hull that causes the rapid rise in resistance, and is related to boat length, the wave system set up that is parallel to the direction of travel, propagates predictably according to speed, in that a given speed gives a given wave length, it is when the crest of the second wave falls behind the stern, that the resistance goes off the chart. Waves on the ocean behave according to similar rules, long wave length equates to speed, short wave lengths slow. Simply, physics.  

Ad Hoc Naval Architect

As already noted by JSL, there are far too many variables to consider before attempting to provide any kind of reply. Annode said: ↑ to calculate engine power... which for some strange reason is measured in horsepower in boat engines - that only rev to 1500 or 2000 rpm thus disguising the the torque that the engine generates... a more relevant number since a turbo diesel 500hp can generate 2000 ft/lbs of torque Click to expand...

> boats usually have "reduction gears" between the engine and propeller which reduce rotational speed while increase the torque. yeeeees ... just talking about engine right now. 800 - 2000 rpm for larger boats is normal I am told (with a 3:1 reduction in the gearbox) Specifics... no hab This is a THEORETICAL rough approximation discussion. I am going backwards from the power requirements for reasons that are not relevant right now. answers that are basically "why is that the question?" are not helpful. It is the question. so.. back to the topic...  

Chuck Losness Senior Member

Dave Gerr's "The Nature of Boats" discusses power requirements. It might be too basic and not give you what you are looking for. It would be a place to get started. I am sure that this topic has been discussed in the past. Try doing a search for calculating the power for displacement boats.  

fredrosse USACE Steam

A rough estimate of horsepower for displacement type hulls is fairly simple: 1. Maximum "hull speed", in knots, is equal to about 1.3 X the square root of the waterline length of the hull in feet. For example, say you have a displacement hull with a waterline length of 25 feet, then the hull speed is 5 x 1.3 = 7 knots. Trying to propel the boat faster than this will result in enormous increasing power requirements. 2. For displacement hulls of reasonable shape (not a square box, which would require more power, and not a rowing shell used for competition, using less power), about 1 horsepower per long ton of displacement will get the boat to hull speed in calm water. This value is based on a propeller of good efficiency, generally a large prop at relatively low RPM. Some additional margin is usually prudent to cope with wind and waves, or non-optimum propeller conditions, but anything over about 2 horsepower per ton of displacement is not required. 3. Much more detail is provided in the FAQ section of thesteamboatingforum.net, as virtually all of the steamboats use displacement type hulls, a general exception to the pleasure boat industry which has much faster boats.  

Fred. Thank you. thank you. That was a great answer. These numbers are for flat water and a reasonable shape hull obviously. OK. So the next part of the question is what facto do you use to give you some get out of trouble power. I have been reading on this forum for a while and one thread about a backup 9.9hp outboard and stories of string winds and currents got me wondering how you would calcualte a reasonable margin of "get out of trouble" power on top of the requirement to move at hull speed in clam seas. Obviously you can never have too much power, but these engines go up significantly in price with each step up in power. (for the sake of discussion lets assume a good size boat 25-30m with a steel hull and a weight of 100 - 150 tons. this is out of the category of small fibreglass hulls so things like wind, waves and inertia become significant.  

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KeithO Senior Member

Take a look at this paper. I think it is on topic for your thread. http://oa.upm.es/14340/2/Documentacion/3_Formas/Savitskyreport_conSemidesplazamiento.pdf  

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Understanding the True Definition of Full Displacement Hull Design Reveals the Kadey-Krogen Difference

Naval architect James S. Krogen once said, “To produce a successful design, you must make an honest determination of how the vessel really will be used and then prioritize every design decision to favor that outcome.” In the case of Kadey-Krogen full displacement trawler yachts, that outcome is a seakindly, live-aboard ocean crossing yacht. Achieving this objective might appear simple enough but it has long been our view that there is often confusion over what “full displacement” really means.

Unfortunately many people tend to think that all full displacement vessels are the same. Nothing could be further from the truth .

Describing a full displacement hull is very similar. We can differentiate between full displacement hulls based upon two form fundamentals: longitudinal symmetry, and displacement-to-length ratio (D/L)

“To produce a successful design, you must make an honest determination of how the vessel really will be used and then prioritize every design decision to favor that outcome.”

Longitudinal symmetry refers to the degree to which the stern shape matches the bow shape. A simple way to describe what is desired is that a hull with good symmetry will have V-shaped sections and sharp waterlines at both ends.

So why do you want a vessel with a symmetrical hull form? There are four compelling reasons.

First, the fine entry has superior wave-cutting ability making it more efficient compared to blunt, stout-looking forms. The fine entry also yields a softer ride, which means less pounding in head seas. Second, symmetrical forms track better in a following sea. The V-shaped sections aft slice following seas rather than surfing them, making for a safer and more comfortable ride. Third, the aft V-shaped sections also offer less resistance and drag than the broad waterlines of asymmetrical hulls. This results in better fuel economy, a key component of any long-distance trawler.

And fourth, symmetrical forms roll less. Yes, contrary to popular belief, an asymmetrical hull with an immersed transom and/or relatively flat aft sections will actually tend to roll more than a symmetrical hull in a following or beam sea. Why? Simple physics. The leverage (upward force) that wave action has on those flatter sections is greater than on a hull that is more rounded and/or tapered. The upward force on one side creates a downward force on the other side and voilà, you have roll. It is this roll (the downward pressure on the opposite side) that will then cause the vessel to veer off course (yaw). The appropriately symmetrical hull form is much more seakindly than one that is not. Vessels roll and heave as though they are on springs, with less symmetrical forms having stiffer springs (lots of force per immersion) whereas the Kadey-Krogen transom effectively rides on softer springs.

Contrary to popular belief, an asymmetrical hull with an immersed transom and/or relatively flat aft sections will actually tend to roll more than a symmetrical hull in a following or beam sea.

Displacement-to-length ratio

The displacement-to-length ratio (D/L) indicates whether a given displacement is carried over a long waterline length or a short one. It reflects the load the vessel has to carry on a per-foot of waterline basis. Lower ratios tend towards lean and slippery forms with fine ends, and higher ratios tend towards full-bodied and less efficient forms with blunter ends. The lighter the load per foot of waterline length the better the economy and hence the better the range will be. Low D/Ls result from either long waterlines or streamlined underbodies, or a mix of each. Both characteristics serve to improve hull efficiency and therefore fuel economy. Longer waterlines permit higher displacement speeds and streamlined sections result in a hull more easily driven through the speed range. For example, if a given displacement is stretched over a longer waterline, two things happen: The LWL increases (higher hull speed) and the ends get finer and sectional areas less full, i.e. streamlined (less hull drag). Of course, reducing a vessel’s fully loaded weight (displacement) will lower D/L and improve economy but the fully loaded displacement requirement is usually already more or less locked in by the voyaging requirements (living quarters, fuel supply, provisions, etc.).

Reducing D/L yields a more easily driven hull form thereby improving fuel economy and allowing for the use of smaller engines

This long waterline low D/L approach (those in the 260-310 range) can be compared to a bank account earning compounded interest. Reducing D/L yields a more easily driven hull form thereby improving fuel economy and allowing for the use of smaller engines. The improved economy reduces the weight of fuel that must be carried for the desired range and the use of lower horsepower engines will reduce the weight of propulsion machinery. All this weight reduction now results in even greater fuel economy resulting in more weight savings resulting in even better economy, and so the benefits compound.

Interestingly, D/L is often applied incorrectly in the evaluation of long-range trawlers.

Many inaccurate conclusions are made using this ratio. It is important to understand that low D/L does not mean lightweight, or a less substantial structure, or a long and narrow form. How did these misconceptions come to be? Back in the “early days” the market consisted predominantly of shallow flat-bottom coastal craft with insufficient internal volume for the fuel and supplies needed for long-range voyaging. These craft had very low D/Ls. To help bring some clarity to the topic, more than 50 years ago venerable designer and world-voyager Captain Robert Beebe published very useful minimum values of D/L, below which the vessel was said to have insufficient “heft” or carrying capacity for long-range voyaging. Therefore, D/L was used to verify that a vessel had sufficient heft instead of being used to rate hull efficiency. Heft was rightfully considered to be a good thing in that context. And back then, since most boats’ D/L ratios were way under the minimum for long range, the higher the D/L the better. Beebe and other experts agreed that the minimum D/L should be around 260. With many modern passagemakers in the 350+ range, that makes those in the 260-310 range seem “light” and some builders have tried to capitalize on this incorrect assessment.

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Hullspeed and the Speed/Length Ratio

So what gives one boat better hullspeed than another? This question was pondered long and hard by William Froude (1810 to 1869), a British engineer who had a special fascination with the sea and ships.

Funded by the Admiralty, who were clearly very keen to get some answers to this question, he built a tank testing facility at Torquay, where he experimented with various model hull forms.

As an early expert in model analysis he was well acquainted with the 'law of mechanical similitude' , which demonstrates among other things that there are few linear relationships in hull design.

So just what is the answer?

Let's take a look...

Hullspeed and the Matchbox Analogy

Consider your hull as a matchbox - not wonderfully efficient hydrodynamically, but stick with it for a moment.

Dissatisfied with the constraints of matchbox living, you decide to double its size. You add another matchbox ahead to double its length, two alongside to double its beam and four on top to double its draft.

Now wetted area has increased by four, volume and displacement by eight and stability - as the product of its mass and acceleration - has increased sixteenfold.

So by doubling a hull's dimensions, wetted area is squared, displacement is cubed and stability increases by the power of four.

With this knowledge and that gained by carefully measuring applied force and resultant movement, Froude was able to both calculate and demonstrate that a relationship existed between hull speed and waterline length - that relationship being known and described in the metric world as 'Froude Numbers'.

The Speed/Length Ratio

However, most of us more accustomed to units of feet and knots are probably more familiar with the Froude Number's close relation - the Speed/Length Ratio.

S/L Ratio = hullspeed (in knots) divided by the square root of the waterline length (in feet)

This discovery enabled Froude to compare the performance of boats of different length. For example a 25ft sailboat moving at 5 knots would have the same S/L Ratio at a 100ft patrol boat steaming along at 10knots, and consequently both would develop the same resistance per ton of displacement at those speeds.

For Froude's models, having no rig above the waterline to create windage, this resistance was caused by two principal factors; hull drag and wave making resistance.

Maximum Hull Speed

Maximum hull speed (in knots) = 1.34 x the square root of the waterline length (in feet)

These figures relate to a boat in displacement mode. If sufficient power can be applied to overcome hull drag and enable the boat to plane, then other criteria will affect ultimate hullspeed.

Any Questions?

What is the theoretical hull speed of a non-planing boat?

The theoretical hull speed is the maximum speed that a non-planing boat can achieve in displacement mode, when the wavelength of its bow wave is equal to its waterline length. Beyond this speed, the boat will encounter increasing wave resistance and will need more power to overcome it.

What factors affect the theoretical hull speed of a boat?

The main factor that affects the theoretical hull speed of a boat is its waterline length, which determines the wavelength of its bow wave. The longer the waterline length, the higher the theoretical hull speed. Other factors that may influence the actual speed of a boat include its hull shape, displacement, draft, trim, sail area, wind and sea conditions, and propeller efficiency.

What is the difference between planing and non-planing boats?

Planing boats are boats that can lift themselves partially or fully out of the water and ride on top of their own bow wave, reducing their wetted surface area and drag. Planing boats can exceed their theoretical hull speed and reach higher speeds with less power. Non-planing boats are boats that remain fully submerged in the water and cannot climb over their own bow wave. Non-planing boats are limited by their theoretical hull speed and require more power to increase their speed.

What is the 'half angle of entrance' and how does it affect wave resistance?

The half angle of entrance is the angle between the waterline and the centerline of a boat at its bow. The smaller the half angle of entrance, the finer the bow shape and the lower the wave resistance. A fine bow can slice through water with minimal disturbance, while a blunt bow can generate large waves and drag. The half angle of entrance is one of the key factors that determines the wave-making resistance of a boat.

How can I increase the speed of my non-planing boat?

There are several ways to increase the speed of your non-planing boat, such as:

• Increasing your sail area or using more efficient sails;
• Reducing your displacement or weight;
• Optimizing your trim or balance;
• Improving your propeller efficiency or reducing your propeller drag;
• Choosing a finer or longer hull shape;
• Sailing in favorable wind and sea conditions.

What are some common misconceptions about hull speed?

Some common misconceptions about hull speed are: - Hull speed is a fixed limit that cannot be exceeded by non-planing boats. In reality, hull speed is a theoretical estimate that can be surpassed by some boats with sufficient power or sail area, but at the cost of increased wave resistance and drag.

• Hull speed is the same for all boats with the same waterline length. In reality, hull speed can vary depending on the hull shape, displacement, draft, and trim of the boat, as well as the wind and sea conditions;
• Hull speed is the optimal speed for non-planing boats. In reality, hull speed is often too high for non-planing boats to maintain efficiently or comfortably, especially in adverse conditions. A lower speed that minimizes wave-making resistance and maximizes fuel or power efficiency may be more desirable.

The above answers were drafted by sailboat-cruising.com using GPT-4 (OpenAI’s large-scale language-generation model) as a research assistant to develop source material; to the best of our knowledge,  we believe them to be accurate.

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Ellis Boat Company

Ellis Downeast Semi-displacement Hull Design

True Downeast boats trace their legendary performance to a long line of ocean-proven work boats dating all the way back to Friendship sloops and Muscongus Bay schooners. For generations, Maine fisherman have relied on these rugged boats to take them to sea, function as stable, dependable work platforms, and then bring them home safely, burdened with the trip’s catch — in weather that most of us wouldn’t venture out in, 12 months a year.

Downeast boats, in general, are sea-kindly and safe in all kinds of weather. Stability is inherent in a design that includes a skeg to protect the underbody, propeller, steering gear, and drive train. It’s simply a very comfortable ride.

When Maine fishermen began switching from sail to power, the Downeast hull configuration evolved from a true displacement hull to a longer, wider, semi-displacement hull. In this long transition from sail to power, the builders shortened up the keel slightly, but kept the long and narrow hull. It was an easily driven shape with excellent sea-keeping characteristics. In fact, the early boats, with a ratio of three feet of length to one foot of beam, could be driven to hull speed with just 3 hp.

True Downeast hulls features a large keel, flat aft sections, and a fine entry, allowing them to slice cleanly and comfortably through the waves without significant loss of speed.

As engines became more readily available, and horsepower and displacement increased, the fisherman built their work boats with flatter aft sections to support the weight and bulk of the bigger engines, yet retained the boat’s traditional fine entry. The keel of the traditional Downeast hull is similar to a sailboat’s keel. Although it’s not ballasted, the keel performs two important functions: First, it helps steady the boat in a seaway, especially a beam sea, limiting side-to-side motion. More importantly, the keel and skeg serve to protect the vessel’s running gear in the case of accidental or deliberate groundings. The configuration allows lobstermen to come right up on a ledge at slow speed and ground–or “tunk” as it’s known on the waterfront–without damage.

Planing hulls tend to ride on top of the water at speeds where they pound in any kind of a sea. At slow speeds, forward visibility is restricted until the hull gets up on plane.

Today, with a single engine, Ellis boats can duplicate the performance of most deep-V boats with twin engines, but with single-engine efficiency and economy. The common hard chine or planing hull must utilize a great deal of power (with a corresponding leap in fuel consumption) to drive the boat up out of the water and onto a plane. Once on top of the water, the hull will pound and slap the waves, and has little lateral resistance to wave motion and, as a result, will pitch and roll, often lifting the props out of the water.

At slower speeds, these hulls point their bows high into the air reducing forward visibility to a dangerous level, and making for a very uncomfortable ride. In bad weather, then, the planing hull must either travel at a bone-jarring rate of speed to stay on plane (if the hull and crew can withstand the punishment) or reduce its speed to a crawl to maintain a relatively flat attitude.

Today’s true Downeast hulls use horsepower to get them up beyond displacement speeds, and yet these boats are still true to the central issue of providing predictable stability. They are simply about as stable a platform as you could ever want–and that means stable at 8 knots, stable at 20 knots.

Each Ellis hull is manufactured in our state of the art two-piece molds (with the exception of the Ellis 20, which is constructed in a single piece mold due to its more manageable size). These two pieces are hand-laid with stitched biaxial E-Glass and vinylester resin for superior strength and then fit together to form a one-piece hull while still in the mold. The center seam is filled with twelve layers of fiberglass which fuse the two halves together creating an extraordinarily strong keel. With the exception of the solid fiberglass keel and skeg, the hull is cored with vacuum bagged ATC Core-Cell® structural foam core to provide exceptional seaworthiness, impact resistance, sound deadening, strength to weight ratios and insulation properties.

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How to Calculate Outboard Motor Size for Sailboats

It seems so complex to pick the right engine size for your sailboat. I was done with complex calculations and tried to make it easier here.

How to pick the right outboard motor size for your sailboat? To get the right amount of horsepower needed to efficiently propel a sailboat, divide the displacement of the boat (in lb) by 550. You need approximately 1 HP per 550 lb of displacement or 4 HP per 2200 lb. Most sailboats don't need a motor with more than 30 HP.

In this article, I'm talking about small outboard engines for sailboats. We're talking about displacement hulls here, so in other words: keel boats. They need more power than flat bottoms.

But they're not powerboats - so it's not our mission to go fast. It's our mission to get decent speed, good control over the boat, and the best possible fuel efficiency. Without breaking the bank of course.

On this page:

How to pick the right motor size, other factors that are important for size, why is the right motor size important, is there a max hp for sailboats, in conclusion, related questions.

Sailboats need way smaller engines than powerboats. That's great news (unless your ultimate goal is speed), because it's cheaper to buy, cheaper to drive, and cheaper to maintain.

The amount of power you need is related to the hull displacement of your boat.

I like to use the simple formula:

HP = displacement (lb) / 550

So 1 HP for every 550 lb displacement, and 4 hp per 2200 lb.

Here, HP is the amount of horsepower you need to reach the maximum hull speed. This is in optimal conditions. So you have smooth water, no windage, a clean and polished hull, and so on.

If you want to get it absolutely right, you also need to correct for propellor size. And of course, a lot of other factors come into play (more on that later). But generally, these engine sizes will work with the following weights:

That sounds about right to me. But remember that these are all rough estimates: I just try to give you a ballpark figure. There is no one formula to get an exact number. The hull design, sailing conditions, and your personal preference are all very important.

If you're serious about getting a new engine, I definitely recommend to get advice from an expert . But you know, salespeople always recommend the Turbo version. Remember that you don't have to overpower a sailboat. Usually you don't need anything over 30 HP. So at least you now know what will work on average.

What is hull displacement?

• Hull displacement is the weight of the boat, or the amount of water the boat displaces.
• Maximum hull displacement is the weight of the boat when it's fully loaded, including crew.

The weight of the boat is the same as its displacement, because the weight of any object is exactly equal to the weight of the water it displaces (aka: pushes aside). This is called Archimedes Principle.

The weight slightly differs in saltwater from freshwater, because saltwater is heavier. In saltwater, the boat gets a bit lighter. So in theory you can use a smaller engine for a bluewater boat, but in practice this is offset by the stronger current and wind.

How to find the displacement of your boat?

Most manufacturers simply give you the displacement of your boat. If you can't find any data, because, for example, you own an old boat, you can weigh your boat on a truck scale. You can also haul it out and measure it (which is painstaking work).**

Tip: if you're gonna weigh your boat, simply drive it onto a truck scale, and retract the weight of the trailer from the total weight.

Of course, it's not so simple. This formula gives a rough estimate. But for me this was way clearer than all that black magic that I get when I ask people what size engine I should get.

Let's look at the things this formula doesn't take into account.

2-strokes are more powerful than 4-strokes. Two-stroke engines fire once every revolution and four-strokes fire once every other revolution. This makes the 2-stroke twice as powerful. They provide more torque at a higher RPM. But they also wear more quickly. The 4-stroke will last you a lot longer, and its also more fuel efficient.

The right propellor size is just as important as having enough horsepower. With a smaller prop diameter, it has to work harder to generate the same propulsion as a larger diameter. But you can't just go larger always. The prop affects the RPM of your engine, and you have to get in the right range (more on this later). You also have to check the maximum diameter that fits your boat.

Diesels have more torque, because the compression rate is higher than that of gasoline engines. So if you consider a diesel, you can do with less HPs.

High windage hulls (multihulls) need a bit more. A multihull (or larger hull in general) suffers from more friction because of the larger surface. So the engine needs to work a little harder.

If you sail longer distances under power , or against the wind it's a good idea to get a larger engine (but not too large). This helps you to save on fuel since you have lower RPM. Especially if you sail offshore or on open sea. The engine needs to work harder due to stronger wind and current.

If you're just sailing in and out of the marina under power, you may need less HP.

Smooth hull designs need less HPs than bulky hull designs, like the classic wooden clippers and crabbers for example.

It matters to get the right size outboard motor for a couple of reasons.

First of all: smaller engines are cheaper, so you save money on buying the engine.

Secondly: smaller engines use a lot less fuel, so you save money on using the engine.

Thirdly: smaller engines are cheaper to maintain: so you save money on maintenance.

So why not get the smallest engine and get the best fuel economy? There are a couple of advantages to getting a (slightly) bigger engine:

• More power means more control (easier to stop the boat, in case you need to)
• Finding the sweet spot might actually reduce fuel consumption

The sweet spot

To perform optimally, an engine should get up to speed. The problem with an overpowered boat is that the engine won't rev up to 80 - 90% of the RPM. This kills fuel efficiency and also the cooling system won't operate optimally.

• The optimal cruising RPM of the engine is about 85-95% of the maximal RPM
• You should reach cruising RPM at hull speed, so your engine should be at about 90% RPM

The propeller size is very important for the RPM. If your prop diameter is too wide, the engine can't get up to speed and struggles to build power. Bad for fuel economy, bad for the engine, and bad for performance.

On the other hand, if your prop is too small, you don't make use of the engine's full power.

If you struggle to get to high RPM, your prop is too large. If your engine is constantly in the red, you're underpropped.

So don't go too big on the prop, but also don't go too small. The easiest way to get it right is to check the engines manual and see what the manufacturer recommends.

You can definitely go too big on a sailboats engine. An overpowered yacht doesn't make any sense. True, it can look cool, but it can't feel cool. Every displacement hull has a maximum hull speed. That means that it cannot go any faster than the max speed. So if your engine can cruise at that speed, it's not getting any better.

The problem with displacement hulls is that they displace the water, or in other words: they push the water in front of them. They cannot move any faster than they can push away the water. And because the resistance increases as speed increases, there's an absolute, physical speed limit for each keelboat.

That's why powerboats have to get out of the water to reach top speed.

Fun fact: the longer your boat, the higher the hull speed. Want to know the maximum hull speed for your boat? You can find it in this article .

So, you can't go faster than your maximum hull speed, so a 50+HP engine is kind of ridiculous. Bear in mind that a large engine also has the following disadvantages:

First of all: larger engines are more expensive, so you spend more money when buying the engine.

Secondly: larger engines use a lot more fuel, so you spend more money when using the engine.

Thirdly: larger engines are more expensive to maintain: so you spend more money on maintenance.

Also, if your engine is too big, it doesn't reach the optimal cruising RPM, so your fuel economy also gets really bad FAST.

I suggest getting the smallest possible engine that gets you to maximum hull speed while it's at roughly 90% of the RPM. As long as it gives you enough control and good handling, it will get you there. If you give up on going fast, you can actually get really good fuel economy and your engine will last you probably 20 years.

If you want to go fast, a sailboat is not the right one for you. You should instead get a powerboat.

I'm just kidding. Read my 13 Reasons Why Sailing is Better Than Powerboating here .

Do sailboats have motors? Most sailboats are power assisted boats, which means they have a small auxiliary engine to cruise in light air. When a sailboat is sailing under engine power, it is considered a motorboat and it doesn't have right of way.

Thanks for answering my questions.

Taylor Bishop

Thanks for explaining how you can figure out what size you need for an outboard motor. You mentioned that you should find the displacement by weigh a boat on a truck scale. I’m interesting to learn if you need to regularly weigh it in case the hull displacement could change or if it will always be consistent.

Shawn Buckles

Hi MitI, you’re welcome, my pleasure.

Hi Taylor, my pleasure.

You don’t need to weigh your boat regularly, as the hull displacement will stay consistent. You could literally see the hull displacement as the amount of space your hull takes up in the water. So as long as you don’t make any major changes to the hull shape or ballast of your boat, you should see no differences in displacement.

Roger S Johnson

How do you measure for shaft size, most outboard motors are for flat bottom and say measure to the bottom of the boat, most sailboats tapper to the aft. Where do you measure for a tapered bottom sail boat?

Will a 5 horse Honda 4 stroke be ok for a 25 foot Pearson Commander sail boat. Thanks for your time Luke

I think it would be Luke.

Great post, thanks for the info. A naive question from a soon-to-be sailor: I’m considering buying a 28 ft sailboat, with 2500 kg (ca. 5500 lbs) displacement. The engine is in pretty good condition, but is old and the original one (from 1977!), so I am also thinking of an alternative scenario in which it fails. I know that in my area replacing an inboard engine will cost double the price I’m putting down for the boat, and since I’m on a budget, that simply won’t be an option and outboards seem to be cheaper. So the question is: is it possible to put an outboard engine on all boats? Is there some factor that would make it impossible to mount an outboard engine on the boat? Thanks!

Garth Powelson

What is minimum length that a sailboat can go without an outboard. Does a 29’ “require by law” to have engine?

Hello Mr. Buckles, Thanks for the informative article. I’m looking to get the smallest possible outboard for my 1.5 ton displacement fiberglass monohull Hood 23’ sloop. Can I get away with a 4HP?!? What size prop would I need?!? (I’m only going to use it when there is NO wind, and, if I can stay 4HP or below, I am not required to register my vessel—which is pretty cool, so here’s hoping!)

Thanks again, Ship

Hi, I’ve got a older Pearson 39’ . I’m looking to remove the old 40 ho westerbeke and go electric. Unsure of what hp is going to be needed?

emilio h javier

i am purchasing a catalina 22 ft. i have in mind a 4 HP motor. what would be the length of the shaft.

I am considering buying a 25 ft sailboat with a 7200 lb displacement. The boats top speed is listed at 7knots per hour but the diesel motor does not work. The owner has a 9.9hp outboard that can be purchased with the boat. Is 9.9hp enough to power the boat to at least 5 to 6 knots per hour? Thanks. Rick

What weight outboard would be too much for a 20’ Santana, displacement 1,350 lbs? I don’t want too much weight at the back. I want the boat to be seaworthy.

I have not seen this amount of BS in years :) I’m not a marine engineer, yet physicist & avation engineer. You even can’t tell the difference between mass of the vessel and diplacement :D Fcking genius.

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