Understanding boat design can be tricky. We’re all familiar with the questions that arise when looking for a new boat. Is it the right size and does it need work? How does it sail and is it within budget? Next you might look at the specifications – LOA, waterline, beam and draught – and maybe drill down a little deeper on the engine, for example.

The specifications give you a good framework, but more interesting is the information that’s hidden. Or, at least, appears hidden to the everyday boat owner. Aside from a test sail, much of a boat’s performance can be ascertained by looking at some calculations.

Designers use a variety of tools when creating a new boat. Using calculations they can predict things such as stability, seaworthiness and speed potential under a range of conditions. Keel percentage, form stability, VPP and STIX–- these terms may sound off-putting, but get to know them and you’ll better understand your boat.

Understanding boat design

Kasper Wedersøe is a partner and designer at CDE Danish Marine Design. He uses calculations frequently to predict how a boat will behave in the water.

“There are quite a few models that are useful, each in different ways,” he says. “But it’s important to remember that these dimensions are capable of predicting potential – not necessarily the actual performance.”

In other words, the numbers we get from dealers can be a bit optimistic. They’re based on ideal conditions, so not always representative, but can still be useful for comparing boats. Here, Kasper explains some of the more noteworthy terms.

Waterline length

Waterline length is a simple and useful parameter, because it says something about speed potential upwind. The longer the waterline, the faster the boat will be in displacement mode – that is, while sailing through the water. The calculation for the theoretical hull speed (in knots) of a non-planing (displacement) boat is the square root of the waterline length (in metres), multiplied by 2.43.

Keel percentage

Keel percentage is also interesting. It says something about stability. The keel percentage will quite simply tell you what percentage of the total weight of the boat is keel weight.

In my experience, sellers will quite often exaggerate when they inform customers about a boat’s keel percentage. Typically they count the structure holding the keel, or more precisely the framework that the keel is attached to. It does give a little bit of stability, but it sits very close to the boat’s centre of gravity, so it doesn’t supply stability even close to the keel itself. And furthermore, if you want to have the correct picture, you need to add draught. Having 100kg in the bottom of the boat will obviously not give the same stability as 100kg 2m below water. Traditionally you want the keel percentage to be close to 40%, but around 35% is more normal. Anything below 30% will affect performance. A boat with a high keel percentage stands well on its feet. It will heel to a certain point, and then stay there. A boat with a lower keel percentage will heel more in gusts. You will have to reef earlier, and your performance upwind will not be as good.

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Polar diagrams

Polar diagrams are based on VPP calculations. VPP is the abbreviation for Velocity Prediction Program. These calculations will provide a fairly precise picture of the boat’s speed potential on different wind angles and in different wind speeds. Most often they are provided by the producer of the boat, and they are likely to be a little optimistic.

Our experience is that ordinary sailors will struggle to achieve the speeds given. They predict speed when the boat is sailed perfectly, and in most cases on flat water. The reason is that calculations for waves are very complicated. A couple of very interesting things to look at is how much wind it takes to reach maximum hull speed upwind, and which wind angles and wind speed you need to make the boat exceed hull speed – in other words, to plane or surf.

Handicap/rating systems

The handicap or rating system is also interesting when it comes to understanding boat design. Just like the polars, the rating will tell you something about what boat speeds you can expect, but this time more overall. Ratings are nowadays based on the same data as polar diagrams. Rating systems such as IRC or ORC are directly derived from VPP calculations, while handicap systems such as the Portsmouth Yardstick are based on empirical data – actual results in races. If you study differences across different weather conditions, you will know more about a boat’s performance profile. Just remember that handicap or rating numbers work best when comparing performance with other boats. They are not an expression of divine truth.

Form stability

Form stability is the part of a boat’s stability caused by the shape of the hull. First and foremost it’s a function of the relationship between length and beam. A boat with a wide beam is quite simply harder to tip sideways then a narrow one, if all other factors are equal. Catamarans and trimarans, for example, are boats with an extremely wide beam, and therefore they have an extremely high form stability.

Form stability is most efficient at low heeling angles, and diminishes quickly after heeling reaches a certain point. Stability from the keel, on the other hand, is quite low at low-heeling angles, and will increase gradually until heeling reaches around 120°. This means that a high keel percentage adds more security than a high form stability.

Wide boats are becoming more and more popular. The main reason is probably that they offer more space below decks. But they do also provide a higher form stability. The downside is that they normally generate more resistance through the water, due to more wet surface. This picture will change with heeling, and vary with hull design.

Theoretical hull speed

Theoretical hull speed is quite simply the top speed a non-planing boat can sail. The maximum speed of a boat in displacement mode is limited by the wave it creates. A planing boat is capable of climbing over its own wave. As soon as the wave created by a displacement boat is as long as the waterline, it can’t go any faster. After this point, resistance rises exponentially. In short: Longer waterline means longer wave. Longer wave means higher speed. The formula, as mentioned earlier, is 2.43 multiplied by the square root of the waterline length (in metres).

Waterline length/ displacement

An interesting way to compare boats is to look at the relationship between waterline length and displacement.

Short waterline and high displacement indicates low speed potential. Stability can still be high, if keel percentage is high and CG (centre of gravity) is low.

Long waterline and low weight indicates high speed potential. But to make use of the potential, high stability is required. A boat like this will need to move less water, and the planing threshold will be lower. On the other hand, a boat with a long waterline and low displacement will carry less load (equipment, water, diesel etc). Traditionally, designers also tend to believe that this type of boat is less comfortable for the crew in a high sea state. The formula is calculated by taking waterline length and dividing it with the square root of displacement.

Sail area/displacement

The relationship between sail area and displacement is a bit like the relationship between horsepower and weight in a car. A heavy boat needs to move more water than a light boat. So, if sail area is the same, the lighter boat will theoretically be faster. To evaluate if this will happen in reality, though, you need to look at stability and hull shape too.

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When sail area/displacement is used to compare boats, make sure equal, comparable numbers are applied. For example: Is displacement noted with or without load (tanks empty/full, etc)? Is sail area noted as two triangles, or the actual surface? What about the roach or any overlap?

Stability index

STIX (Stability Index) is a compiled rating of boat stability, and one of the factors deciding which Recreational Craft Directive (RCD) category a boat is placed in – A, B, C or D.

The design categories help to quantify a boat’s degree of seaworthiness, based on the wave height and wind speed the boat is designed to encounter and handle. The further offshore the vessel is expected to venture, the higher are the expectations for construction strength, stability, freeboard, reserve buoyancy, resistance to downflooding, deck drainage and other seaworthiness criteria.

• Category A – Ocean: covers largely self-sufficient boats designed for extended voyages with winds of over Beaufort Force 8 (over 40 knots), and significant wave heights above 13ft, but excluding abnormal conditions such as hurricanes.

• Category B – Offshore: includes boats operating offshore with winds to 40 knots and significant seas to 13ft.

• Category C – Inshore: is for boats operating in coastal waters and large bays and lakes with winds to Force 6, up to 27 knots, and significant seas 7ft high.

• Category D – Inland or sheltered coastal waters: is for boats in small lakes and rivers with winds to Force 4 and significant wave heights to 18in.

Generally speaking, a higher STIX number will imply higher stability. Boats RCD-approved for category A (the safest, and approved for offshore sailing) must have a STIX number above 32, while boats approved for category D only need 5.

STIX numbers are rarely easily available, but all European boats built after 1998 are RCD approved, and the STIX number should be found in the boat’s manual.

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Speed table

This graph (below) compares a few different sailing boats based on a number of the calculations and formulas explained in this article (as well as some that are not included). The method might be a bit old fashioned, but it’s still a valid form of comparison.

 

The X axis (horizontal) shows the relationship between waterline and displacement.

The Y axis (vertical) shows the relationship between sail area (upwind) and waterline.

Diagonal curves show the relationship between sail area (upwind) and displacement.

The illustration demonstrates upwind performance only and does not predict speed potential directly, but indicates how the different models are likely to perform compared to each other.

For example, an X-37 has a relatively short waterline compared to its displacement, but has a large sail area. This makes it perform well in light air, compared to the other boats chosen here – but overall it’s not one of the fastest boats on the table.

A Bavaria 39C also has a relatively short waterline compared to displacement, but waterline is slightly longer than the X-37. The Bavaria also has far less sail area per kg, which deprives it of light air performance. Overall, it will be slower than most other boats chosen here.

A Faurby 460 E has a very large sail area and a relatively long waterline compared to displacement. This makes it potentially fast overall, with a slight advantage in light air.

It’s important to remember that had other boats been chosen, their relative positions would come out differently. Still, the tendencies described will help with understanding boat design and how this affects performance.

 

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