Copyright 2017 - Woods Designs, Foss Quay, Millbrook, Torpoint, Cornwall, PL10 1EN, UK
  • home built Flica 37

  • plywood Romany 34

  • lightweight 14ft Zeta mainhull

  • Strike 15 trimaran at speed

  • 28ft Skoota in British Columbia

  • 10ft 2 sheet ply Duo dinghy

  • 24ft Strider sailing fast

  • 36ft Mirage open deck catamaran

Introduction

As I've said elsewhere, I only like to design boats that are fun to sail. I also know from personal experience just how much effort is involved in building even the smallest boat. I've found that it is the psychological effort that's particularly hard, especially if you are a home builder building alone in your spare time. I also know that there are other people, like myself, who's keeness to build is not matched by manual dexterity.

So I try to design boats that are straightforward to build. In simple terms, if I can build it then anyone can! To do this I try to keep to simple shapes and use flat panels where ever possible. Flat decks in particular have many advantages. For a start they are easier to walk on, while coming alongside and boarding from a marina pontoon or dinghy is a lot safer. Flat panel hulls may not offer ultimate speed, but to be honest, few cruising sailors need the fastest boat while I've found that most people don't have either the skill or daring to sail a racing boat to its full potential.

You have to remember that a cruising boat, especially, isn't just for sailing. It has to be a practical floating cottage as well. And the design of that often over rules otherwise desirable sailing features. And also remember that boats have to be usable in harbours and marinas. It's not like the "good old days" when Slocum and even the Pardey's first went to sea - with no engines and few marinas or even cruisers. So all cruising boats MUST maneuver reliably under power and be easy to board from both the dock and from a dinghy.

That is one reason why I don't now like canoe sterns. They make boarding so much harder than a boat with transom steps (the acid test I always use - "could my mother get on board?"). Safe maneuvering in a small harbour is another reason I like small boats. I also find a trimaran much harder to handle than a catamaran when coming alongside, as it is so difficult to reach the outrigger bows to fend off, especially when compared to the big wide decks of a catamaran. Successful designs are ones that work in every situation, not just those that sail or motor fast in a straight line.

I always try to visulise what a particular design would be like when sailing to windward at 2am in the rain. Or when reefing. Or of course when drifting downwind on a very hot humid day.

I tend to own a fleet of multihulls. Sometimes I just go for a day sail, sometimes I race for the weekend, and most years I spend a long time living on board (I spent every Christmas living on board a boat from 2001 - 2009). All this experience means that I have personally faced nearly every situation you can meet when sailing and I use that experience in my designs.

Hull Shapes and Performance

In this article I will talk solely about hull shapes in relation to performance. Comfort, seakindliness and load carrying are also major factors affecting hull shapes and will be discussed in more detail in future articles.

People try to simplify hull design and performance predictions, formulae like the Bruce Number and KSP spring to mind. These coefficients rely only on basic sail area, displacement and length dimensions yet purport to give an accurate indicator of performance. It's easy to show that these formulae cannot be relied on if you consider that a Tornado would have the same rating whether it was sailing forwards or backwards! I suspect the latter is slower! Its probably as accurate as predicting car speed from the kerbside weight and engine horsepower. In fact hull design is a hugely complex subject while different sailing conditions require different solutions. For example, inshore boats can have a flatter rocker while offshore cruisers should be more veed forward to prevent pounding when sailing to windward in waves.

Some factors affecting yacht design are based on scientific principles and are unalterable, so always apply, whatever ones basic design philosophy and regardless of cross section shape (ie whether one uses a Deep V or round bilge hull for example). Everything else is just styling or dressing up the same proven concepts in a slightly different way. As with all moving objects, speed is the result of the combination of resistance to movement (drag) and available power. In sailing boats the power is related to the sail area while in simple terms drag comes in two forms - friction drag and wave making drag.

Frictional drag is primarily dependent on the Wetted Surface Area (WSA). Less is always better than more and WSA is the biggest factor affecting lightwind speed. The minimum WSA for a given displacement (or boat weight) is the hemisphere (eg half an orange). A longer, thinner hull has proportionately more WSA and so in light winds suffers from more drag and thus is slower but conversely is significantly faster as the wind gets up. In fact this is one reason why monohulls - which are much more orange like, do well in light winds. Spray also adds to wetted surface, one reason why powerboats have spray rails. Lots of spray makes a boat look as though it is sailing fast, but it is actually very inefficient. As an example, because of their heel and deeply immersed lee outrigger, trimarans make a lot more spray than catamarans. But we usually find that they are actually slower, particularly reaching, than an upright, low spray producing catamaran. Round bilge hulls have the lowest WSA and deep V hulls the most.

Many people think that, because multihulls have relatively thin hulls, wave making drag is non-existent, but in fact, nothing could be further from the truth. The size of waves that a hull makes depends on several factors. The Slenderness Ratio (SLR) or Displacement Length Ratio (DLR) is a measure of the fineness of a hull and is the technically correct coefficient that naval architects use. However, it is easier to visualise the hull waterline length/hull WL Beam ratio (LWL/BWL), so that is more commonly used. That's acceptable, as for a given cross-section shape, the SLR is directly related to LWL/BWL. Higher ratios result in smaller waves. Typically the LWL/BWL ratio will vary from 10:1 for a good cruising boat to 16:1 for a racing boat. (Team Philips has a LWL/BWL ratio of 35:1!) Finer hulls are more efficient at high speeds, but as we've just seen suffer from more WSA and so for normal cruising catamarans in average conditions a ratio of 11:1 - 13:1 seems optimum.

The Prismatic Coefficient (Cp) is a measure of the fullness of the ends of a boat, the higher the number the fatter the ends and - surprisingly - the more efficient at high speeds. Intuitively you'd think that a diamond shape would cut through the water best, but that's not actually the case. A high Cp also has the advantage of reducing pitching. But to complicate matters, the lower the SLR the lower the Cp can be. Typically a monohull has a figure around .56, while a properly designed multihull will be about .63. Although such a shape is slower than a lower Cp in light winds that is not a problem as one then has the sail carrying power to add extra sail to compensate. It is when sailing fast in strong winds that you need an efficient hull because you then don't have the stability to carry more sail. As an example a 30' boat with a Cp of .63 will be 1/2 knot faster than one with a Cp of .55 when sailing in 25 knot winds for EXACTLY the same sail area (and crew effort etc). Boats with a low Cp try to race faster by keeping too much sail up and it was these types that often used to capsize 25 years ago. Add in the fact that the high Cp boat won't pitch as much and its clear which is going to be the better boat.

In simple terms the Half Angle of Entry is the angle that the waterlines make to the hull CL at the bow. If it is too low then the boat is wet to sail, and, in extremis, if it is hollow there is a pressure build up further aft which slows the boat. If too fat it is also wet to sail as the bow wave goes vertically up the sides of the boat. All sailors, no matter how skillful, sail slower if they can't see where they are going because they are being blinded by spray! In both cases the correct Cp has to be maintained. So a 10 degree angle seems a good compromise. Vertical bows look fast but its actually very difficult to draw a vertical bow on a hull with both the correct Cp and one that has good reserve buoyancy. Read my articles about the Cape to Rio race to discover what I learnt first hand about the perils of vertical bows!

When I was a design student I took the opportunity to do some tank testing on catamaran models and I investigated the drag from the wave interference between the hulls. I found that this interference caused significant drag at certain speeds - in fact up to 20% when compared to hulls at an infinite spacing. So it's vital to reduce this interference as much as possible. The simplest way is to have a hull spacing wide enough so that the bow waves meet at the stern rather than under the boat. This has the added bonus that there is significantly less bridge deck slamming. In the past designers said that the optimum L/B ratio was 2:1. (In fact they were talking about overall length and beam when obviously it is waterline length and beam that are the crucial measurements.) The reasons given for this ratio were the theories that wider boats would break up and be hard to tack. In practice I've heard that limiting beam had more to do with the width of the boatyard doors! Our Strider Turbo has a LWL of 6.6m and a hull centreline spacing of 4.2m yet I've always thought it was a better sailing boat than the standard Strider. So the general trend is to go as wide as one can. But structural strength is still a problem, even with modern techniques and materials. Wide boats are heavier than narrow ones and that ultimately limits the LWL/BWL ratio to about 2:1 on cruising boats with full bridgedeck cabins.

Turning now to the hull above the waterline, vertical topsides reduce space inside dramatically and in addition are not good news when sailing offshore. As a boat moves in waves so it heaves up and down, causing discomfort and slowing the boat. Flared topsides help counteract this heaving because as the boat sinks the buoyancy picks up more quickly than with vertical topsides. The result is a smoother ride and as a bonus, better load carrying ability for, by the same token, the hull sinks slower so there is less increase in WSA and wavemaking as the boat is loaded. Clearly, freeboard adds to windage and slows the boat. Traditionally yachts had low freeboard because they were large (J Class etc) so people could fit in the accommodation easily regardless of freeboard and it's easy to make a low freeboard yacht look elegant. More importantly, it was hard to make a conventionally caulked and planked boat strong and watertight if it had too many planks (ie too much freeboard). As boats got smaller and as grp took over freeboards had to, and could, increase. Adding a few centimetres (inches) of freeboard adds enormously to interior space and at the same time results in a boat that is drier and more comfortable to sail. Fortunately, in practice I have never found that high freeboard slows the boat down appreciably and certainly not by enough to worry any except the most ardent racer.

Load carrying considerations are an important factor for cruising boats. In general it's natural for people to add weight aft because it is easier to load stores near the companionway than forward. Engines and their associated tanks, generators, a/c units etc are also always aft. So I always try to add extra buoyancy near the stern. That means that when empty the boat will probably float stern up. Too many poorly designed multihulls float stern down and drag their transoms.

Pros and Cons of popular hull shapes

The deep V is a simple to build hull shape that matches the human body as it is narrow low down and wider high up so it is a good choice if the accommodation is only in the hulls. It can make to windward without keels or boards - just - but it's more maneuverable and makes less leeway with them fitted. Deep V hulls pitch more than any other hull shape, particularly if they have canoe sterns. Hull asymmetry is needed to reduce pitching, a canoe stern is obviously as pointed as the bow so it's bound to pitch more. They have more WSA than any other hull shape so are slow in light winds.

The flat bottom hull is also easy to build and has the added advantage that it is self supporting during building and transport. It needs Veeing forward for offshore sailing or it will pound. Then it becomes a hard chine/single chine hull. Carefully drawn such a shape is a close approximation to a round bilge hull, but without any complicated building.

The round bilge hull is the only hull shape that can be varied over it's length so one get exactly the shape one wants. It has minimum WSA, and so is also the optimum hull shape but it is the slowest to build. A topsides knuckle helps deflect spray, adds interior volume and makes it easy to join flat topsides to the curved bottom. It also makes a nice styling line.

From the start of my design career I have always tried to design balanced, undistorted hulls that sail easily on all points but are not too extreme. However, I have made a few changes to my hull shapes over the years. First I have increased freeboard (in common with most designers, monohull and multihull). I have also increased the centreline spacing and where appropriate, drawn a bigger knuckle. I haven't designed any deep V boats for a long time because of the pitching and light wind speed problems. I have found the flat bottomed or single chine hulls are as simple to build and are more efficient hull shapes.

Finally, I am one of the few designers who use all feasible hull shapes and so can choose the most appropriate one for the intended use. I'm not committed by dogma to any one hull design. The performance differences between different hulls are easy to see, however I have not noticed any practical difference in seaworthiness between them.

The following sketches are typical hull cross sections. Please note, these are not to scale and are not real boats, instead they are just examples of the different hullshapes we use in our designs. (For those not familiar with lines plans: Only half sections are shown. The forward half of the boat is shown to the right, the aft half is to the left of the vertical centreline.)

 

 

 

 

 

 

 

 

This is the "Dory" hull used on the Janus and Gypsy as well as the Strike trimaran mainhulls. Note that the Janus does not have a V'eed area forward (as shown) as the bottom is narrow enough to prevent slamming on such a small boat.

This chined hull is used on Flica, Mirage and Romany and is a close approximation to a round bilge hull, but built in flat panels

This is a deep V hull used on Surfsong, Windsong and Mira (deep V version)

This is the chined V hull used on Meander, Rhea and Ondina. If these larger boats had a conventional V hull then either the gunwale or keel panel would have to be very wide so that the hull had the correct displacement. By adding a soft chine the lower hull section can be well flared, while the topsides remain nearer vertical. This hull shape has the added advantage that the hull panel is stiffer and, as each section is smaller, it can be easier to make.

 

 

 

 

 

 

 

 

 

This continuously curving hull shape is used on Wizard, Sango and Wizzer. It has a similar below-waterline shape to a Strider hull (for example) but the bulge in the topsides allows a vertical bow to be drawn while keeping a good flare forward to prevent nosediving. It also adds to the interior room, especially at shoulder level. These hulls can be built in strip planking or foam sandwich but it is harder to build than the small knuckle hull shape.

 

 

 

 

 

 

 

 

This "Small Knuckle" round bilge hull is used on Strider, Shadow, Merlin, Gwahir, Skua, Gypsy (round bilge version), Mira (round bilge version), Scylla, Nimbus, Rhea (round bilge version) and Cirrus. This shape is easier to make than the one below. The knuckle is small and is usually made from solid timber (eg 2" x 1"). Even so it has proven effective at reducing spray and slamming. The hull bottom can be double diagonal plywood or strip plank. The topsides of both this hull shape and the one below can be strip plank or sheet plywood. Alternatively both knuckle designs can be built in foam sandwich with a flat panel topside panel.

 

 

 

 

 

 

 

 

 

This is the "Large Knuckle" hull used on the Scorpio, Javelin, Sagitta, Eclipse and Transit. It is the most sophisticated shape I draw, and takes the longest to build. The large flare increases space inside and cuts down on spray. The angle of the hull at the WL is actually higher than on other hull shapes. That means that it sinks relatively slowly as you add weight. A big advantage for cruisers. But it also means the boat doesn't pitch and heave so much, ie vertical movements are reduced. That is because it is (slightly) harder to make the boat sink as it goes through waves. All minor differences but they add up if you are looking for the best all round shape.

Having said that, if you are planning on using LAR keels rather than daggerboards then you will probably be better off with a flat panel hull. There is no point in taking just one part of the overall design to the limit, you have to balance the trade off for the whole boat. So don't fit daggerboards and cheap sails! Makes no sense to me.