## Yachting Monthly

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## 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.

Continues below…

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## Boat handling: How to use your yacht’s hull shape to your advantage

Whether you have a long keel or twin keel rudders, there will be pros and cons when it comes to…

## Sailing in waves: top tips to keep you safe at speed

Sailing in waves can make for a jarring, juddering experience and long, uncomfortable passages and at worst, a dangerous, boat-rolling…

## How to cope with gusts and squalls

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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.

## Enjoyed reading Busting the hull speed myth?

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## Hull Speed Calculator

Table of contents

Welcome to the hull speed calculator . If you've ever seen a boat go so fast that its nose started rising, then you've seen the concept of hull speed in action. In this article, we'll explain what hull speed is and what it means for a ship's design. Later, we'll show you how to calculate hull speed with the hull speed formula, so that you can work out how to calculate hull speed for your own boat.

## What is hull speed?

Hull speed is the speed at which a vessel with a displacement hull must travel for its waterline to be equal to its bow wave's wavelength. A displacement hull travels through water, instead of on top of it as a planing hull (like a kiteboard ) would, thereby displacing water with its buoyancy as it sails. The pressure that this displacement exerts on the water creates a wave; this wave is known as the vessel's bow wave . A slow-moving boat's bow wave might make small waves, but, as the boat sails faster, the bow wave's wavelength λ \lambda λ grows. When the wavelength meets the waterline length (that's also when the bow wave's first and second crests are at opposite tips of the waterline), the boat is said to be traveling at hull speed. Take a look at the picture below to see what we mean:

## Why does hull speed matter?

Although it's not perfect, hull speed remains a useful concept that can help us answer questions about how fast a sailboat can go, and the optimal amount of thrust you need to keep a boat moving forward.

A boat's hull speed limits how fast it can travel efficiently. When traveling at hull speed, the boat's bow wave and stern wave have synchronized and constructive interference occurs, which allows the boat to move very efficiently. However, at speeds greater than hull speed, a vessel's nose automatically starts rising as the vessel tries to climb its bow wave. This process is called planing , and it wastes lots of energy. Trying to move faster than the hull speed will therefore require more and more thrust (whether that comes from sails, rowing, or engines) in exchange for smaller and smaller gains in speed as more energy is wasted angling the boat upwards. Hull speed can therefore be said to impose a flat limit on how fast a sailboat can go.

## Shortcomings of hull speed

Although the physics behind hull speed is sound, it is heavily dependent on the hull's shape. Long and thin hulls with piercing designs can easily break their hull speed without planing. Such hulls are found on:

- Catamarans; and
- Competitive kayaks.

A hull's design can enable it to circumvent the workings of hull speed. It is for this reason that hull speed is not used in present-day ship design; naval institutions nowadays favor more modern measurements of speed-to-length ratio, such as the Froude number .

## How to calculate hull speed

The formula for hull speed only needs the length of the vessel's waterline in feet, denoted as L waterline L_\text{waterline} L waterline . With this length, the vessel's hull speed in knots can be calculated with

If you want to instead work out exactly how long your new boat's waterline must be for it to have a certain hull speed, you can invert the formula to obtain

## How to use the hull speed calculator

The hull speed calculator is just as easy to use as the formula.

Enter your vessel's waterline length into the first field. This is the length of your boat's hull at the height of the waterline. Your vessel's hull speed will then be calculated and presented in the second field.

You can also use the hull speed calculator backward to work out how long a vessel's waterline must be if you know its hull speed.

You can freely change the units of your measurements without interfering with the hull speed formula.

## How can I increase my boat's hull speed without changing its hull?

Load your boat heavier! If you think about a normal displacement hull, it's usually narrower near the bottom than at the deck. So pushing it down with some weight will lengthen the boat's waterline, and so its hull speed is increased. Of course, heavier boats are harder to move, so while your loaded boat now has a higher hull speed, you would need more power to move it.

Waterline length

The length of the ship at its waterline.

The speed at which the ship's waterline length equals its bow wave's wavelength.

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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)

20 feet 25 feet 30 feet 35 feet 40 feet 45 feet 50 feet |
6.0 knots 6.7 knots 7.3 knots 7.9 knots 8.5 knots 9.0 knots 9.5 knots |

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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## Hull Speed Calculator – Optimize Your Boat’s Performance

This tool calculates your boat’s hull speed based on its waterline length.

Hull speed is the speed at which the wavelength of the bow wave of a boat is equal to the boat’s waterline length. It is often used as a measure of the speed potential of a displacement hull. This calculator uses the formula:

Hull Speed (knots) = 1.34 * sqrt(Length of Waterline)

To use this calculator, enter the length of the waterline of your boat in the text field and select the desired unit for the hull speed result. Then click the “Calculate” button to compute the hull speed.

How it works:

- Length of Waterline (ft): Enter the length of the boat’s waterline in feet.
- Choose Units: Select the unit for the output speed (Knots, Miles per hour, or Kilometers per hour).

Limitations:

- This calculator assumes the hull is a simple displacement hull and does not account for other hull designs.
- Water conditions, hull shape, and weight distribution can affect actual hull speed.

## Use Cases for This Calculator

Calculate hull speed based on waterline length.

Enter the waterline length of your boat to quickly determine its hull speed. The calculator will instantly provide you with the maximum speed your boat can achieve before it starts creating excessive waves, helping you plan your journeys efficiently.

## Optimize Boating Efficiency with Hull Speed Calculation

Use the hull speed calculator to find the ideal speed for your vessel based on its length. By staying within the recommended hull speed range, you can enhance fuel efficiency and reduce the drag on your boat, ensuring a smoother and more enjoyable boating experience.

## Determine Maximum Safe Speed for Your Boat

By inputting your boat’s waterline length into the calculator, you can ascertain the maximum safe speed for your vessel. Avoid overloading your boat or pushing it beyond its hull speed limit to maintain stability and safety on the water.

## Plan Your Sailing Adventures Wisely

Before embarking on a sailing trip, use the hull speed calculator to calculate the optimal speed for your boat. This helps you estimate travel time accurately and adjust your speed to ensure a balanced and comfortable journey.

## Understand the Relationship Between Boat Length and Speed

Explore how the length of your boat influences its maximum hull speed with the help of this calculator. Gain insights into how different boat lengths require varying speeds to operate efficiently and make informed decisions while navigating water bodies.

## Enhance Boating Safety by Knowing Hull Speed

By understanding your boat’s hull speed, you can navigate unpredictable waters more safely. This calculator gives you a clear speed limit that ensures your boat moves smoothly without excessive drag, reducing the risk of accidents.

## Calculate Hull Speed Quickly and Accurately

The hull speed calculator offers a fast and accurate way to determine the maximum speed your boat can achieve. Simply input the waterline length, and the calculator will provide you with the optimal speed limit in seconds, facilitating efficient planning.

## Improve Performance by Following Hull Speed Recommendations

Follow the hull speed recommendations provided by the calculator to optimize your boat’s performance. By staying within the suggested speed range based on your boat’s specifications, you can enhance maneuverability and overall efficiency on the water.

## Prevent Wave Interference with Hull Speed Information

Learn how hull speed impacts wave interference and boat control through this calculator. By staying within the designated speed range, you can prevent excessive wave creation, maintain better control of your vessel, and enjoy a smoother sailing experience.

## Make Informed Speed Decisions for Different Boats

Whether you own multiple boats or rent vessels for recreational activities, this calculator helps you quickly adapt to various watercraft. Input the waterline length of each boat to determine their individual hull speeds, assisting you in making informed speed decisions across different vessels.

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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.

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## Hull Speed Calculator

Are you looking to understand the maximum hull speed of your boat? In this blog, we’ll dive into the concepts, and we also have a handy hull speed calculator to help you figure out your own boat’s speed.

In short, hull speed is the theoretical maximum speed a boat or ship with a displacement hull can travel, regardless of the boat’s size or engine power. Exceeding a boat’s hull speed is highly inefficient and dangerous.

So why does it matter? What causes these speed limits? How is it calculated? Read on to find answers to all of these key questions.

## Key Takeaways

- Hull speed is the boat's theoretical top speed based on waterline length.
- Exceeding hull speed is inefficient and can be dangerous.
- Checking your hull speed helps pick the proper boat, engine, and speed.
- The hull speed of a boat is calculated using a formula involving the square root of the waterline length.
- Doubling a boat's waterline length will only increase its hull speed by 40%.
- Wave interference at speeds exceeding the hull speed causes increased drag, greater fuel consumption, and reduced stability.
- Unlike traditional displacement boats, the speed of planing hull boats isn't limited by wave interference and displacement.

## How to Calculate Hull Speed? The Displacement Hull Speed Formula

The calculation uses a simple formula based on the boat’s waterline length. Here is the hull speed formula:

Hull Speed (knots) = 1.34 x √Waterline Length (feet)

So, for example, a boat with a waterline length of 20 feet would have a theoretical hull speed of:

1.34 x √20 = 8.2 kn

This formula shows that hull speed increases with the square root of the waterline length. Therefore, doubling the waterline length will increase hull speed by about 40%.

## Using the Hull Speed Calculator

Here is a calculator to help you determine the theoretical hull speed for a displacement hulled boat.

Simply enter the waterline length of your vessel, choose between feet and meters, and click the button to get the hull speed in knots instantly. Want to try again? Click the “Clear” button to reset all fields.

## What Causes the Displacement Speed Limit?

But why does this formula hold true? What limits a vessel’s maximum speed?

The answer lies in wave interference. As a boat moves through the water , it creates a bow wave and one at the stern. When the boat begins to exceed its hull speed, these begin to interfere with each other. This interference causes the waves to become steeper, requiring tremendous amounts of energy to climb.

Essentially, the boat hits a speed “wall” where its own wave creation prevents it from accelerating. This is why huge amounts of engine power cannot push a boat past its hull speed. The physics of wave creation forms an upper limit.

## Why Does Hull Speed Matter for Your Boat?

Knowing this can help you in several ways:

- Operate your boat more efficiently. Exceeding hull speed wastes fuel and stresses the engine.
- Avoid dangerous situations . Pushing past hull speed can reduce control and stability.
- Set realistic speed expectations. Understanding hull speed helps you pick the right boat and engine size.
- Improve navigation and safety. Accounting for max speed allows better trip planning.

## Vessel Hull Speed Chart & Table

Waterline Length (feet) | Waterline Length (meters) | Hull Speed (knots) |
---|---|---|

10 | 3.05 | 3.37 |

20 | 6.10 | 4.77 |

30 | 9.14 | 5.84 |

40 | 12.19 | 6.74 |

50 | 15.24 | 7.54 |

60 | 18.29 | 8.25 |

70 | 21.34 | 8.89 |

80 | 24.38 | 9.49 |

90 | 27.43 | 10.07 |

100 | 30.48 | 10.61 |

## Explanation of the Graph

The curve on the chart shows how the speed changes as the length increases, following the formula.

## Final Thoughts

Understanding your boat’s hull speed is paramount for maintaining safety, efficacy, and optimal control at sea. Our guide has provided invaluable insights on its relevance, implications, and calculation method using the waterline length. Use our calculator to determine your boat’s hull speed.

It’s not entirely about owning the longest boat or having the most powerful engine; it’s about understanding the physics of your vessel and how it interacts with water at different speeds . Remember, being a skilled mariner isn’t just about managing the boat; it’s also about knowing your boat inside and out.

The hull speed is the speed at which a boat’s waterline length equals the bow wave’s wavelength. To calculate it, you follow this ratio: Hull speed in knots = 1.34 * sqrt(LWL in feet), where LWL is the length of the boat’s waterline.

Hull speed calculation is vital because it defines how fast a boat can go under its own displacement hull without needing significant thrust or horsepower. This calculation provides an optimal speed limit for the boat.

As the boat speeds up, the wavelength increases. Once the boat travels at a speed where the bow and stern wave synchronize, further increases in speed result in higher energy consumption but not necessarily in an increase in speed. Therefore, preventing the boat from outrunning its bow wave is beneficial to maintain optimal energy use.

This refers to any ship or boat designed to displace water equal to its own weight. These vessels have a hull speed, the theoretical maximum speed they can achieve without riding on the plane.

The hull speed is the speed at which the length of the bow wave equals the waterline length of the boat. As the boat starts to move at this speed, it will take a significant increase of energy to make the boat move even slightly faster. That’s why a traditional sailboat can’t naturally move faster than the hullspeed.

The hull speed is generally considered the maximum efficient speed for a displacement boat; exceeding it requires exponentially more energy. However, powerboats, planing boats, and other specific designs can go considerably faster than their hull speeds.

When a boat reaches 1-to-2 knots over its hull speed, the wave interference effect quickly increases drag and instability. Fuel usage and stresses also rise exponentially.

Planing hulls are designed to lift up onto the water at speed, “planing” across the surface. So they do not face the same hull speed limits as heavier displacement boats. But each hullhas performance limits.

A longer waterline length only increases hull speed by the square root. So doubling the size only increases hull speed by 40%. Often it is better to pick an efficient hull design instead of just a longer boat.

## Anchor in Deep Water: Tips and Techniques

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\n") msg.document.write("\n") makeChartBar(msg, boatName[i], boatName[j], "LOA", LOAMax, LOA[i], LOA[j]) makeChartBar(msg, boatName[i], boatName[j], "LWL", LWLMax, LWL[i], LWL[j]) makeChartBar(msg, boatName[i], boatName[j], "Beam", beamMax, beam[i], beam[j]) makeChartBar(msg, boatName[i], boatName[j], "Displacement", displacementMax, displacement[i], displacement[j]) makeChartBar(msg, boatName[i], boatName[j], "Sail Area", sailAreaMax, sailArea[i], sailArea[j]) makeChartBar(msg, boatName[i], boatName[j], "Capsize Ratio", capsizeRatioMax, Math.round(capsizeRatio[i]*100.)/100., Math.round(capsizeRatio[j]*100.)/100.) makeChartBar(msg, boatName[i], boatName[j], "Hull Speed", speedMax, Math.round(speed[i]*100.)/100., Math.round(speed[j]*100.)/100.) makeChartBar(msg, boatName[i], boatName[j], "Sail Area to Displacement", sailAreaToDisplacementMax, Math.round(sailAreaToDisplacement[i]*100.)/100., Math.round(sailAreaToDisplacement[j]*100.)/100.) makeChartBar(msg, boatName[i], boatName[j], "Displacement to LWL", displacementToLWLMax, Math.round(displacementToLWL[i]), Math.round(displacementToLWL[j])) makeChartBar(msg, boatName[i], boatName[j], "LWL to Beam", LWLToBeamMax, Math.round(LWLToBeam[i]*100.)/100., Math.round(LWLToBeam[j]*100.)/100.) makeChartBar(msg, boatName[i], boatName[j], "Motion Comfort", motionComfortMax, Math.round(motionComfort[i]*100.)/100., Math.round(motionComfort[j]*100.)/100.) makeChartBar(msg, boatName[i], boatName[j], "Pounds/Inch", PPIMax, Math.round(PPI[i]), Math.round(PPI[j])) msg.document.write(" Report any problems to . will continue to host Carl's Sail Calculator on his Web site; please direct correspondence to him. |

Carl's Sail Calculator v3.55 . For multihulls, try this siteThis page works with all standard browsers on Mac OSX, Windows 7 or later, and Linux. It does not render properly on Apple iPads and iPhones running iOS 10. This is an OS problem beyond my control.

Some data were moved and recalculated from earlier versions. If you find any basic measurements that you know to be incorrect for any of the boats please send the corrections to Tom .

: When you select a boat, its parameters appear below in . |

") for(i=0;i ")} // --> | ") for(i=0;i ")} // --> |

Select one boat in each column above, and press |

: Note that length overall, length of waterline, and beam are in feet, displacement in pounds, and sail area in square feet. Do not use or in your numbers, which should be in the form, for example, 1000.50. Note that this site uses the American standard, with a period instead of a comma as a decimal delineator. you follow number entered with the letter " " and then click on the page anywhere outside the entry box. Doing this will convert each of your entries to the native units (feet, square feet, and pounds0) used by the calculator. Thus if you enter 1000m for the displacement in kilograms, it will be converted to 2204.6 pounds. |

* |

Press . |

to e-mail the data on your boat to Tom: |

: This area displays the parameters of the boat selected. Do not enter values here. Click on any of the Derived Quantities boxes for an explanation of the box. |

: You can search for boats in the database you selected in Part 1 by their parameters. Select any number of conditions. |

: You can find your 'ideal' boat by doing a weighted search. For example, you can search for the boat that has the highest combined normalized scores in 'Motion Comfort' and 'Sail Area to Displacement' giving one a 60% weight and the other 40%, or whatever! You can also do low searches, for example, you can search for the boat that has the highest normalized score in 'Motion Comfort' and the lowest normalized score in 'Capsize Ratio' giving one a 30% weight and the other 70%, or whatever. A 'high' search is done as a percentage of the highest boat in the parameter. So, if the boat with the highest Sail Area to Displacement has a value of 48, a boat with a Sail Area to Displacement of 24 would receive a value of .5. For a 'low' search it is the inverse. That is, if the boat with the lowest capsize ratio has 1.3, a boat with a capsize ratio of 3.9 would receive a value of 0.33. Only boats within the specified length range and in the database chosen in Part 1 will be searched. You can also eliminate any type or types of boat from those searched by entering their names separated by commas in the first field below. For example, entering 'Herreshoff,Bolger' would eliminate any boat with either name in its name. The results (the top three boats, their scores and the average score for boats searched) are reported in the text area below. |

Output Field: | |||||||

Minimum Length: | |||||||

Capsize Ratio | Hull Speed | SA/Disp | Disp/LWL | LWL/Beam | Motion Comfort | Pounds/Inch | |
---|---|---|---|---|---|---|---|

Weights: | |||||||

Search Direction: |

: The material here is taken from an article by in (February 2001. pp. 81-84) entitled . To really understand the numbers calculated below you should consult this article or his book . A note on the Maximum Sailing Speed calculated below: This is also from Gerr's work. He has determined that the classic formula for Hull Speed ( 1.34 Sqrt(LWL) ) does not always apply, the 1.34 is not a constant, leading to, in some cases, much higher speeds. However, Gerr observes: " |

To use this form, select a boat, enter a Horsepower and Prop Type. |

\n") for(i=0;i ")} // --> |

Press |

## A Complete Guide to Displacement Hulls (Illustrated)

The displacement hull is the classic go-to hull design for sailboats and one of the most recognizable ones out there. In this guide, I explain all there is to know about them.

What's a displacement hull? A displacement hull is a boat hull design that uses buoyancy to support its weight. It lies partially submerged and displaces water when moving, hence its name. The amount of water it displaces is equal to its weight. It's very stable in rough waters. That's why this design is widely used on cruisers and sailboats.

Displacement hulls are great and reliable. Below we'll talk all about that. But they all have one major setback. Read on to find out what.

## On this page:

Displacement hull features, how a displacement hull actually works, why it's so fuel-efficient, setback: maximum hull speed, advantages & disadvantages of displacement hulls, who might like this type of hull, in conclusion.

Nearly all sailboats have displacement hulls. Displacement hulls are great for operating in rough waters. They are less affected by waves than planing hulls. Because they're so steady, they are to go-to design for many ocean-going boats. Examples of boats with displacement hulls are: sailboats, canoes, and fishing boats and trawlers.

The displacement hull is:

- the most reliable & efficient hull in rough water
- the most fuel-efficient hull
- the most buoyant hull
- the hull with the largest cargo capacity

I'll explain all these points later on, but first, I want to just describe the hull design for you.

## Design Features

Displacement hulls are pretty bulky. They have round bilges. The bilge is where the boat's bottom curve meets its vertical sides. The hull itself is round. It's round because that creates less resistance when moving through the water. That roundness is what makes it such a comfortable ride, even in waves.

But that roundness also makes it easy to roll (think of canoes, for example). That's not a good feature in heavy weather. To offset it, sailboats have a heavy keel that runs deep into the water. This counterbalances any roll, making the boat very stable. Sailboats with a long keel are very difficult to capsize.

The hull is rounded throughout, running from bow (front) to aft (back).

The displacement hull is generally pretty heavy. That's okay, since it is supported by its buoyancy, so it doesn't need a lot of power to propel (more on this later). The weight actually helps it be more stable and unbothered by nature's pull. I think it's fair to call the displacement hull with the whale among boats . It uses the water's upforce to carry it, and gently peddles along.

## How Fast Is It?

Since this hull needs to move a lot of water before going anywhere, displacement hulls are pretty slow. Actually, it very well may be the slowest hull type out there. On average, their cruising speed lies anywhere between 6 - 8 knots. They can go faster, but most boats with displacement hulls don't have the power to do so.

They are great at low speeds. Thanks to their shape, they are easy to move and don't require a lot of power. They're actually one of the most fuel-efficient designs out there.

## Compared to other hull types:

- Displacement Hull - Partially submerged, buoyant, moves water
- Planing Hull - Glides over water surfaces, generates lift |
- Semi-Displacement Hull - Displaces at low speed, lifts partially at cruising speed

I've written an Illustrated Guide to Boat Hull Types , where I go over 11 different examples of the most common boat hulls . That article will give a great and quick overview to get you up to speed, so if you don't know anything about boat hulls yet, that article is a great place to start.

The shape of the hull creates a sort of air bubble that floats on top of the water. At the same time, the weight of the boat pushes down (or actually, gravity pulls it down). This submerges the boat a bit, anchoring it, in a way. This push-pull gives it its characteristic reliability, making it more stable and better at keeping course.

As with anything that is really good at floating, it doesn't require a lot of energy to propel it. Since it can use the water to carry it, it's great for carrying cargo. You can really load her up without drastically increasing fuel consumption.

A planing hull needs to get up to speed before it generates lift, and until it does, it's absolutely rubbish in terms of a smooth ride. That's why planing hulls can get so uncomfortable in waves. They can't get up to speed, and their hull isn't made for displacing - rather flying - so it becomes a terrible ride.

The one major setback for displacement hulls is the upper-speed limit. As I've noted before, they are pretty slow. But the thing is: they can't go beyond their upper-speed limit, even if you gear her up with massive outboard engines and so on. The reason for this is called the maximum hull speed .

To understand the maximum hull speed and how it works, I want you to think of yourself lying in the Mediterranean Sea. That's just arbitrary, but since I can pick any sea I like in these kinds of visualizations, I prefer the Mediterranean. So you're lying in the Meds and along comes a sailboat. The sailboat hauls a rope behind it (I know, a line). You grab on to the rope and hold tight. The sailboat gently drags you along. It accelerates. The pull increases, you have to grab on even tighter. It accelerates even more. You have to really clench now.

The reason you have to increase your grip when the sailboat accelerates is simple. Your body displaces water when you move. When the speed increases, it has to displace the same amount of water, but faster. The water resistance (drag) increases.

The power needed to displace water increases exponentially with speed.

So now you can probably imagine that there will be a point where you can no longer hold on and have to let go. You have to slow down. That's your maximum hull speed working.

In the same way, there's a point where the boat's drag becomes so large, that it becomes almost impossible to propel it, no matter the amount of power. That speed is called the maximum hull speed. Every displacement hull has one, and it is a direct correlation with the boat's length. If you want to check out the maximum hull speeds for different boat lengths and learn how to easily calculate it yourself, you can check out a previous article. In it, I go over average sailboat speeds and the formula for calculating maximum hull speed .

By the way, the reason planing hulls can go faster, is that they generate lift at a certain speed. In terms of our story just now, that's the same as if you got yourself a wakeboard. Then, when the boat accelerates, at one point you pull yourself out of the water, and glide over the surface, instead of lying in it.

As with anything, this design has both pros and cons. I'll go over each one briefly down below.

- handle well in rough waters
- very hard to sink
- smooth ride
- large cargo capacity
- requires little power: very efficient
- very dependable
- can be very heavy
- large range

## Disadvantages

- has a maximum hull speed
- tends to roll
- can capsize if it has no keel
- if it does have a keel, it has a deep draft

If you don't care about speed and are all about range, safety or comfort, the displacement hull is the way to go. It's by far the most comfortable ride of all hull types and will get you anywhere. You can cross oceans, cruise inland - it doesn't really matter. It has the largest range of all the hull types, and the fuel-economy is really impressive. With cruising speeds averaging between 6 - 8 knots, this hull type is the slowest, but also the steadiest. The perfect boat for long-range cruisers and liveaboards.

Displacement hulls have been around for centuries, and they are the most well-known hull for a reason. They're reliable and efficient. Those are perhaps the two most important trades when you're at sea. Nearly all sailboats have displacement hulls, and for cruising, the benefits outweigh the drawbacks big time. If you like speed, however, you should consider getting something with a planing hull or semi-displacement hull. You can learn everything about semi-displacement hulls here .

## Jacques Burgalat

Great tutorial ! Do you know of anyone (or company) who could help me with using a fully electric power train on a 19m/40 ton Tjalk? I (and the tjalk) are currently on the Saint Johns River (Florida) which is more akin to a lake or canal than an actual “river”, so fighting waves and currents is not an issue.

Thank you for your help.

## Leave a comment

You may also like, the illustrated guide to boat hull types (11 examples).

I didn't understand anything about boat hull types. So I've researched what hulls I need for different conditions. Here's a complete list of the most common hulls.

## Semi-Displacement Hulls Explained (Illustrated Guide)

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## CRUNCHING NUMBERS: A Better Way to Estimate Hull Speed

It’s been a while since we discussed all those mysterious numbers that both boat designers and journalists are always throwing around to confuse us when they talk about boats. You’ll recall last time I bloviated about displacement/length ratios . One big reason it’s a good idea to go to the trouble to calculate a boat’s D/L ratio is that you can use this number to refine your estimate of the boat’s maximum speed potential beyond the relatively rough estimate afforded by the classic hull-speed formula we discussed earlier.

This more accurate method of finding a boat’s potential top speed, devised by designer Dave Gerr and published in his tome, The Propeller Handbook , can also be used to estimate the hull speeds of multihulls (both catamarans and trimarans). Dave warns, however, that catamarans with very narrow hulls (for mysterious reasons that no one really understands) will often exceed the speeds predicted by his method.

To comprehend Dave’s formula, we first need to comprehend that the famous multiplier we used in the classic hull-speed formula (1.34) is in fact what is known as a speed/length ratio (S/L ratio). This ratio quantifies the relationship between a boat’s speed (BS), whatever it happens to be at any given point in time, and its waterline length, according to a formula that holds that S/L ratio equals a boat’s speed in knots divided by the square root of the boat’s load waterline length (S/L ratio = BS ÷ √LWL).

The brightest kids in class will instantly note that this is simply the classic hull-speed formula (HS = 1.34 x √LWL) run backwards to solve for the speed/length ratio instead of speed. What the classic hull-speed formula assumes is that 1.34 is the maximum S/L ratio that can ever be achieved (due to the characteristics of waves we discussed before) and thus always serves to limit a boat’s top speed potential.

What Dave’s formula does is estimate a boat’s maximum S/L ratio based on its D/L ratio so as to more accurately reflect the fact that lightweight boats are more capable of exceeding their nominal hull speed. Once we’ve arrived at a new and more accurate S/L ratio for a given boat, we can then plug it into the classic hull-speed formula to derive a new, more accurate estimate of that boat’s nominal hull speed.

Dave’s formula holds that a boat’s maximum S/L ratio equals 8.26 divided by its D/L ratio to the .311 power (Max S/L ratio = 8.26 ÷ D/L ratio↑.311). For a 12,000-pound boat with a 28-foot waterline and a D/L ratio of 244, we thus get the following results: 244 to the .311 power equals 5.53 (you’ll obviously need a scientific calculator to figure that out!), therefore 8.26 ÷ 5.53 equals a maximum S/L ratio of 1.49. Plug 1.49 into the hull-speed formula (1.49 x √LWL) and you get a new nominal hull speed of 7.9 knots (1.49 x 5.29 = 7.88), as compared to the boat’s old nominal hull speed of 7 knots (1.34 x 5.29 = 7.08).

This in itself is an appreciable difference, but it grows even larger as the boat grows lighter. Assume, for example, that our 12,000-pound boat has shed 3,000 pounds to become a 9,000-pound boat with the same load waterline length, and its D/L ratio drops to 183. Its old nominal hull speed, based solely on its LWL, remains exactly the same at 7 knots, but its new nominal hull speed, figured according to Dave’s method, now becomes 8.6 knots!

Things get even more exciting if you bear in mind that this revised hull-speed estimate still does not account for a boat’s potential to plane. That is, we’re still only talking about the top potential speed that may be achieved by a hull in displacement mode. Dave further warns that getting the extra hull speed his method predicts will require a lot of extra power, but he does maintain that his method is ultimately more accurate than the old one.

I, for one, happen to believe him. Running Dave’s formula on your own boat and finding out that its top speed is higher than you thought is an easy way to put a smile on your face. Unless you own a classic CCA-style cruiser with long overhangs. These boats have exaggerated D/L ratios, due to their short static waterlines (which get longer when the boat starts sailing and heels over) and when you run Dave’s formula on them you usually get a lower top speed. In cases like this, I always assume the classic hull-speed formula is more accurate.

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I love your blog

Thank you for this information it’s kind of you to share it.

This may be a reasonable estimate for BOATS, but it is not scalable to larger SHIPS. Let me give you an examples. — The Iowa Class Battleship has a displacement of 50,000 long tons and a length of 860 ft. This gives a D/L ratio of 78.6 and a S/L ratio of 2.12. This then indicates that Hull Speed = 2.12 x Sqrt (860) = 62.17 knots. We know that is bullshit because the Iowa has 212,000 shp for it’s 50,000 tons or greater than 4 shp per ton. We also know that it is a 33~35 knots ship depending on load (and hence draft). There is no way her hull speed is 62 knots. — The Queen Mary 2 cruise ship is about 87,000 tons and 1132 ft long. This gives a D/L ratio of 136.8 and a S/L ratio of 1.79. THis then indicates that the Hull Speed = 1.78 x Sqrt (1132) = 59.9 knots. The QM2 has 115,300 shp delivered via her four mermaid pods or about 1.33 shp per ton. In service she is a 28 knots vessel which reached 30 knots during her trials. Again, there is no way her Hull Speed is 60 knots.

Great article but I have a question. How can this be applied to displacement hulls? Specifically to kayaks or surfskis. Does the constants and formulas hold true for this kind of vessels?

I have an Epic v7 surfski which measures 17′ long and weighs 53 lbs. According to the standard hull speed calculations, it has a theoretical hull speed of 5.52 knots. However, if I try to use these formulas, the Boat Speed comes to 20.89 knots. Its creator, an olympic medallist, is able to push it to 7.18 Knots for long distances which could very probably translate to a slightly higher speed for shorter distances; but no one on muscle power alone could drive it to 20 knots or even close to that.

Something clearly needs to be adjusted for it to apply to small displacement hulls.

Did you factor in the rower’s weight? I get 12.9 kts with a 150 pound rower.

Interesting. I am currently boatless and looking at purchasing something in the 30′ range (sail) and comparing potential hull speeds. One boat has a 24 ft waterline and the other 25. I assumed the longer wl would give me a bit more speed as the old formula shows. However the longer WL boat also displaces 1,400 lbs more so by this new formula the shorter boat has the same theoretical hull speed. Good to know. Thanks!

The math shown in the example doesn’t work. 12000/28=428.57 not 244. Raise that to the power of .311 and you get 6.585. 8.26/6.585=1.254

Mark: you need to scroll up and click on the previous article on D/L ratios; (in red) The exact number works out to 243.6, rounded to 244.

Did I miss something, or does David’s sailboat speed-prediction formula ignore sail-area. ,,,& is based only on D/L & boat-length?

So then, is Dave saying that sail-area is irrelevant to a sailboat’s speed?

Hi Michael! Thanks for the comment. The answer is yes, Dave’s formula ignores sail area and treats only the maximum speed potential of any given hull. But no, sail area is obviously not irrelevant. Dave’s formula, like the traditional speed-prediction formula it replaces, merely assumes a sailplan powerful enough to drive the boat at its maximum speed.

I ran into this formula in your book The Modern Cruising Sailboat. Running the numbers for our 31ft double ender gave pretty interesting results:

Traditional hull speed: 6.8kt Gerr’s hull speed (light ship): 7.1kt Gerr’s hull speed (2t load): 6.4kt

Now, there are no polars (or much other performance material) available on our boat, so I’ve constructed polars based on the actual recorded wind speeds and angles vs. actual sailing speeds during last two summers of cruising. And you know what is the top speeds I get there? 6.4kt! Coincidence? Probably not.

I always find these exercises interesting. And probably very worth while to designers, as an end user the only thing that matters is real world sailing characteristics. To reflect on theoretical speed potential of my boat, which has over 700 sisters and an established racing record would just be frustrating. When shopping for a new boat I would look at established history and PERF ratings. They are a long term average of a particular hull form and should predict what the sailor can expect. Theory is would only come into my calculations if I were comissioning a new design, but once she is built the proof is in the boat.

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- Sailboat Calculators

## The Sailboats Calculators below will enable you to calculate the main Sailboat Ratios, using data that you can retrieve from the Boat table or your own data.

We will be adding more calculators along the way and more in-depth explanations of how they work and what they can help you with., hopefully you will enjoy them and find them useful to search or understand the characteristics of your or any given sailboat ..

## SA/D range of values

16 to 18 Heavy offshore cruisers 18 to 22 Medium cruisers 22 to 26 Inshore cruisers, racing boats 26 to 30+ Extreme racing boats

## Ballast/Displacement:

A Ballast/Displacement ratio of 40 or more translates into a stiffer, more powerful boat that will be better able to stand up to the wind.

## Displacement/Length:

The lower a boat’s Displacement/Length (LWL) ratio, the less power it takes to drive the boat to its nominal hull speed.

less than 100 = Ultralight;

100-200 = Light;

200-275 = Moderate;

275-350 = Heavy;

350+ = Ultraheavy;

## Comfort Ratio:

This is a ratio created by Ted Brewer as a measure of motion comfort. It provides a reasonable comparison between yachts of similar size and type. It is based on the fact that the faster the motion the more upsetting it is to the average person. Consider, though, that the typical summertime coastal cruiser will rarely encounter the wind and seas that an ocean going yacht will meet.

Numbers below 20 indicate a lightweight racing boat;

20 to 30 indicates a coastal cruiser;

30 to 40 indicates a moderate bluewater cruising boat;

40 to 50 indicates a heavy bluewater boat ;

over 50 indicates an extremely heavy bluewater boat.

Comfort ratio = D ÷ (.65 x (.7 LWL + .3 LOA) x Beam^1.33), where displacement is expressed in pounds, and length is expressed in feet.

## Capsize Screening Formula (CSF):

Designed to determine if a boat has blue water capability. The CSF compares beam with displacement since excess beam contributes to capsize and heavy displacement reduces capsize vulnerability. The boat is better suited for ocean passages (vs coastal cruising) if the result of the calculation is 2.0 or less. The lower the better.

## Hull Speed Calculator

Hull speed calculator is a simple calculator that determines a vessel’s hull speed based on the length of the vessel’s waterline.

## Boat Speed Calculator

The boat speed calculator calculates the top speed of a boat based on the boat’s power and her displacement. If you try to understand how fast a boat can go, this calculator will help you answer that. The boat speed calculator utilizes a constant known as Crouch constant which differs based on the type of the boat.

## FOR MULTIHULLS ONLY:

Bn – bruce number:.

The Bruce Number is a power-to-weight ratio for relative speed potential for comparing two or more boats. It takes into consideration the displacement and sail area of main and jib. 100% fore-triangle only, no overlapping sails.

Chris White, “The Cruising Multihull”, (International Marine, Camden, Maine, 1997), states that a boat with a BN of less than 1.3 will be slow in light winds. A boat with a BN of 1.6 or greater is a boat that will be reefed often in offshore cruising.

Derek Harvey, “Multihulls for Cruising and Racing”, International Marine, Camden, Maine, 1991, states that a BN of 1 is generally accepted as the dividing line between so-called slow and fast multihulls.

BN = SA^0.5/(Disp. in pounds)^.333

## Kelsall Sailing Performance (KSP):

Another measure of relative speed potential of a boat. It takes into consideration “reported” sail area, displacement and length at waterline. The higher the number the faster speed prediction for the boat. A cat with a number 0.6 is likely to sail 6kts in 10kts wind, a cat with a number of 0.7 is likely to sail at 7kts in 10kts wind.

KSP = (Lwl*SA÷D)^0.5*.05

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- Tools and Calculators

## Boat Speed Calculator

Most of us want to know our boat’s top speed but don’t know how to calculate it. And, just as likely, we don’t think we need to. GPS is a good way to figure out your boat speed with no effort. It does all the work for you with just a glance. But it’s not always going to be available, especially if there’s a service disruption or an issue with your power. Plus, what if you want to know before you get on the water? Fortunately, calculating boat speed doesn’t have to be that hard. Let’s take a look at what you need to know.

## The Basics of Calculating Boat Speed

You’ll need to know a handful of factors when calculating boat speed. This is why most of us don’t like to calculate it ourselves. And why a calculator tool is so much more helpful. However, we thinking knowing the basics behind how and why the calculator works is important, too. Even if you never need to write it out by hand.

You’ll need to know your boat’s shaft horsepower as well as its displacement. You’ll also need to include Crouch’s constant which varies based on the kind of boat we’re talking about. This number is something boat designers use when designing hull types. For instance, average runabouts have a Crouch constant of 150. A racing catamaran can be up to 230. High speed runabouts will be 190.

Horsepower works out to 550 foot-pounds per second. That equals 746 watts of energy. A number of factors affect how much horsepower is ideal for your boat. It relates to size and hull shape as much as everything else does. The general rule of thumb for horsepower is that for each horsepower you need about 5 to 40 pounds of weight. Too little horsepower and you can burn your engine out far too quickly. Too much horsepower can be dangerous. In fact, every boat is required to have a maximum horsepower rating. If you swap out your outboard motor for a more powerful one you risk damaging the boat, losing control, and worse. When it comes to HP, bigger is not always better.

## Displacement

Displacement refers to the volume of water your boat displaces. This is then converted to weight to, in practical terms, you can consider displacement the boat’s weight. A racing hydroplane might displace 6700 lbs, for instance.

## A Speed Calculation Example

Speed = square root of (horsepower/displacement) X Crouch Constant

Let’s take a look at an example to get a better idea with a smaller boat.

Speed = square root of ( 50 hp/800 lb) X 150

Speed = 37.5 mph

In a pinch you can work this out on your own with a pen and paper. Or, more likely, the calculator on your phone. A calculator tool makes it super easy, of course. But if you’re ever in a pinch with no power handy, it’s good to know the math behind it.

## Horsepower Calculations

Another big concern for many boaters deals with horsepower. Like we said earlier, you need to have the right amount of horsepower for your boat. Too little is a struggle that can burn your engine. Too much can damage the boat and lead to accidents.

Remember, when your boat was designed, it was designed with these calculations in mind. The hull and transom are meant to support only a certain amount of pressure and weight. Even a small increase in horsepower can dramatically increase the pressure on your hull. It will also increase the torque on your transom. If it goes too far beyond manufacturer recommendations you could collapse the hull entirely.

Does this mean you can never exceed the horsepower rating of your hull? Not exactly. Accommodations need to be made. You would have to reinforce the hull and transom to handle the higher horsepower. Obviously we’re into some heavy work at this point. If you’re not sure right now how to reinforce a boat hull, you may want to stay within the established limits. Another thing to consider is whether or not you have a self-draining cockpit. A new engine could throw off the balance of your boat. That could make water enter the scuppers and soak the boat.

If you need to buy a new engine you can calculate the horsepower using the same formula. That’s the beauty of any math formula, you can solve for any single number in the equation if you know the others. So, if you want to know horsepower to achieve your desired top speed, do a reverse calculation. Let’s say in this case you want your boat to hit 50 miles per hour.

Horsepower = (speed/ crouch) squared x displacement

HP = (50/150) squared x 800

In this case, with your small boat displacing 800 lbs, if you want to reach 50 miles per hour, then you need a 90 hp engine.

One thing to remember about upgrading an engine is weight. Usually, a higher horsepower engine is also going to be heavier. The change in displacement obviously changes the figures. That said, it’s not always the case. Many modern engines do a very good job of keeping weight down.

## What About Hull Speed?

Another formula for calculating hull speed for a displacement hull you might see is fairly simple. This one does not require as many numbers but also doesn’t give you the most accurate answer. The formula is

1.34 x the square root of the waterline length in feet, or 2.43 x the square root of the waterline length in meters.

For example, if you had a 16 foot boat, the square root is 4. So the formula would be 1.34 X 4 = 5.46 knots.

1 knot equals about 1.15 miles per hour so you can calculate this to mph if that’s easier for you. That takes you to 6.3 miles per hour, give or take.

This is the theoretical top speed of the vessel. That said, many factors can get in your way. How much horsepower you have, propeller slip, the condition of your boat and more alter this. Even water conditions and hull cleanliness can change your top speed.

## Doesn’t Waterline Length Change?

Why does this formula exist and the other formula as well? This calculation is older and not as accurate. For instance, your waterline length can actually change as your speed increases. Thus, the accuracy is very suspect. Plus, when you add power sufficient enough to overcome hull drag, this number no longer applies. That means when you’re using your motor for propulsion, our original equation is far more useful. This one here is really more something you should be aware of. You may find it when you Google boat speeds and wonder why the different formulas exist. Even boat manufacturers ignore this calculation these days. It just doesn’t apply to modern boat making in any reasonable way.

## Insurance Issues

A final note you might want to consider if you’re looking to soup up your boat. Let’s say you can get a higher horsepower engine and really boost your overall speed. That can be fun if you do it safely and, of course, safety is the number one concern. But there is another issue that may make you think twice. At the very least you’ll want to research it further to make sure it’s not a problem. Insurance.

Check with your insurance company before installing a new engine, especially if it boosts the horsepower. If you go past what the manufacturer recommended, you could be in trouble. If an accident occurs your insurance company may deny a claim.

Worse, if you are in an accident with an overpowered engine, the fault could automatically become yours. You may be considered responsible or negligent for damage caused as a result. Your insurance will not cover you and the result could be lawsuits coming your way. As such, check with any state boating regulations before you commit to anything. No sense spending time and money on something you can’t or shouldn’t do.

## The Bottom Line

Knowing how to calculate horsepower and boat speed is a very useful skill. Having a handy calculator is also worthwhile. This allows you to get a better idea of how long any trip will take, how much weight your vessel can carry, and more. Plus, let’s be honest, it’s kind of cool.

My grandfather first took me fishing when I was too young to actually hold up a rod on my own. As an avid camper, hiker, and nature enthusiast I'm always looking for a new adventure.

Categories : Tools and Calculators

## Craig Dahlke on March 19, 2022

Great article. In the late 1970’s I drove a recitative’s 8′ hydroplane that had a 40hp Mercury. I kneeled on the floor, and it had a dead man’s throttle. It was the fastest boat I have been in. Roughly how fast could this hydroplane go? Craig

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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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## COMMENTS

Hull speed can be calculated by the following formula: where is the length of the waterline in feet, and is the hull speed of the vessel in knots. If the length of waterline is given in metres and desired hull speed in knots, the coefficient is 2.43 kn·m −½.The constant may be given as 1.34 to 1.51 knot·ft −½ in imperial units (depending on the source), or 4.50 to 5.07 km·h −1 ·m ...

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.

The hull speed calculator is just as easy to use as the formula. Enter your vessel's waterline length into the first field. This is the length of your boat's hull at the height of the waterline. Your vessel's hull speed will then be calculated and presented in the second field. You can also use the hull speed calculator backward to work out how ...

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 ...

Calculate Hull Speed Based on Waterline Length. Enter the waterline length of your boat to quickly determine its hull speed. The calculator will instantly provide you with the maximum speed your boat can achieve before it starts creating excessive waves, helping you plan your journeys efficiently. Optimize Boating Efficiency with Hull Speed ...

Hull Speed Formula. Theoretical displacement hull speed is calculated by the formula: velocity in knots = 1.35 x the square root of the waterline length in feet. Example: The Odyssey 18 rowboat has an overall length of 18' and a waterline length of 17'-7". On the chart 17'-7" is about half way between 17 and 18 feet, so hull speed is 6.5 mph.

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).

As a very general rule the maximum speed of any displacement hull-commonly called its hull speed-is governed by a simple formula: hull speed in knots equals 1.34 times the square root of the waterline length in feet (HS = 1.34 x √LWL). Thus, for example, if you have a 35-foot boat with a waterline length of 28 feet, its hull speed works ...

Here is the hull speed formula: Hull Speed (knots) = 1.34 x √Waterline Length (feet) So, for example, a boat with a waterline length of 20 feet would have a theoretical hull speed of: 1.34 x √20 = 8.2 kn. This formula shows that hull speed increases with the square root of the waterline length. Therefore, doubling the waterline length will ...

The Hull Speed (V h) of a boat can be calculated using the following formula: V h = 1.34 × √L wl. Where: V h: Hull Speed in knots; L wl: Length of the waterline in feet; Impact on Society. The Hull Speed concept has played a significant role in the design and construction of ships and boats. It has helped marine engineers and naval ...

The hull speed can be further mathematically expressed as: 1.34 times the square root of the vessel's length, simplifying the above terms and replacing the wave speed term with the given vessel's speed as both are equal. So, hull speed can be numerically defined as: 1.34 X (L)^ (0.5), where L is the overall length of the vessel.

As a very general rule the maximum speed of any displacement hull--commonly called its hull speed--is governed by a simple formula: hull speed in knots equals 1.34 times the square root of the waterline length in feet (HS = 1.34 x √LWL). Thus, for example, if you have a 35-foot boat with a waterline length of 28 feet, its hull speed works out ...

Evaluate and compare your sailboat to 3000+ others or find the ideal boat for you. This is the latest version of the Sail Calculator (3.55). ... He has determined that the classic formula for Hull Speed ( 1.34 Sqrt(LWL) ) does not always apply, the 1.34 is not a constant, leading to, in some cases, much higher speeds. However, Gerr observes:

That speed is called the maximum hull speed. Every displacement hull has one, and it is a direct correlation with the boat's length. If you want to check out the maximum hull speeds for different boat lengths and learn how to easily calculate it yourself, you can check out a previous article. In it, I go over average sailboat speeds and the ...

Plug 1.49 into the hull-speed formula (1.49 x √LWL) and you get a new nominal hull speed of 7.9 knots (1.49 x 5.29 = 7.88), as compared to the boat's old nominal hull speed of 7 knots (1.34 x 5.29 = 7.08). This in itself is an appreciable difference, but it grows even larger as the boat grows lighter. Assume, for example, that our 12,000 ...

It takes into consideration "reported" sail area, displacement and length at waterline. The higher the number the faster speed prediction for the boat. A cat with a number 0.6 is likely to sail 6kts in 10kts wind, a cat with a number of 0.7 is likely to sail at 7kts in 10kts wind. KSP = (Lwl*SA÷D)^0.5*.05.

2.43 x the square root of the waterline length in meters. For example, if you had a 16 foot boat, the square root is 4. So the formula would be 1.34 X 4 = 5.46 knots. 1 knot equals about 1.15 miles per hour so you can calculate this to mph if that's easier for you. That takes you to 6.3 miles per hour, give or take.

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 ...

Specs are unconfirmed but she is expected to have a maximum speed of 16 knots and a cruising speed of 14.5 knots. This Italian shipyard recently shared its new Admiral Quaranta model with BOAT International.The first unit was sold in May 2024, with highlights that include an aluminium hull for minimised fuel consumption and a flexible internal layout typical of yachts 20 metres longer.