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TECHNICAL ARTICLES

Going fast on the water is an exhilarating experience but it also increases the risks

by Dag Pike


HULL DESIGN FOR HIGH SPEED

Speed Going fast on the water is an exhilarating experience but it also increases the risks. You need a sound boat and reliable machinery, and convention suggests that you play it safe with a conventional hull design. However, there are a number of exciting new hull concepts coming on to the high speed market and this is a good time to look at the alternatives. Let's first look at racing experience, because this represents the ultimate in performance. In offshore racing the catamaran rules and the racing boats in Class I are now reaching speeds of over 150 mph. It is a frightening speed where, when things go wrong, they go wrong in a big way, but the introduction of safety cockpits for the crews allows them to take the greater risks with a degree of impunity.

The hull design of these cats has been partly responsible for the very high speeds and these cats use asymmetric hulls with vertical sides in the tunnel and a vee on the outside. To get speed the hull shape is flatter with he shallow vee absorbing less power but still adequate to cushion the ride. Steps are incorporated into the planing surfaces to give better longitudinal stability and reduce the wetted surface. Weight is critical to speed which is why these cats are built from the latest lightweight composite materials.

These racing cats are for total performance and the concept doesn't translate easily into a pleasure boat because of the lack of space inside the hull. Prout Catamarans are virtually the only builders offering an asymmetric hulla as a yacht and they have shown it can work in larger sizes, but they have found it beneficial to fit a lift foil between the hulls to get better performance.

What si called the displacement catamaran is being used a great deal more for power catamarans. These cats rely on long, thin hull technology which is a very efficient hull form which can reach higher speeds with a minimum of power. The longer and thinner the hull, the more efficient it is, so again these yachts only tend to work in the larger sizes. The Awesome 770 is a 77 footer based on this technology and its efficiency can be judged by its 35 knot top speed, its 3000 mile range at 20 knots and its power of only 1200 hp.

A British builder has used similar technology to produce the VSV (Very Slender Vessel). As its name suggests this uses a long, thin hull and two large chines are built into the hull to generate lift and dynamic stability. With the same power as the Awesome the VSV has a 50 knot top speed and a 1000 mile range, but the limited space inside means that it is really only suitable for military work.

This VSV tends to go through waves rather than ride over them which is better from a comfort point of view, but it puts a heavy stress on the boat. This wave piercing type of craft takes some time to get used to and it does requires a very strong hull and superstructure to stand up to the stresses. Similar technology is used in the wave piercing catamaran, which, of course rides on two very slender hulls. In this type of craft, the side hulls pierce the waves to smooth the ride and a large centre hull normally rides clear of the water, but takes the strain if there is not enough buoyancy in the side hulls in heavy seas. The wave piercing cat is well proven in the fast ferry market and the first yacht of this type is now nearing completion in New Zealand.

The monohull is still the main type of hull used in the leisure market. When it comes to internal space for lusury accommodation combined with good seakeeping and performance, it has no equal. Designers can vary the vee in the hull to improve the seakeeping, the deeper the vee, the more comfortable the ride, but this tends to be at the expense of performance, so designers have to find a balance.

When you move into the high performance arena with the deep vee it is customary to have a vee angle of around 25 degrees. The hulls get longer and narrower and steps are introduced into the hull to reduce both the pitching and the wetted surface of the hull. The modern high performance deep vee hull is a highly refined design which balances performance and comfort and it can still be competitive in racing against the cats when the going gets rough.

The rigid inflatable is really a refinement of the deep vee hull with the air tube introducing new characteristics into the design. Because the tube will deform and change when it impacts with a wave, this has a shock absorbing effect in rough seas, but the tube also gives better stability at rest and makes the RIB one of the best sea boats of its size, and with RIBS now capable of running up to 90 knots they can combine this seaworthiness with speed. The secret of successful RIB operation is not to run with the tubes inflated too hard. Soft tubes help to cushion the ride.

So the deep vee and the catamaran are the two main types of hull, but a whole host of new concepts has been developed which can be used to modify these basic types. Hydrofoils have been around for a long time and they lift the hull clear of the water so it rides only on the foils. With computer control, these foils can produce an aircraft smooth ride at 50 knots.

Then there is the hovercraft where a pressurised air chamber is formed under the hull which lifts it up and reduces the resistance. This can be a full hovercraft which rides only on the air cushion or it can be the SES type where the air cushion is formed between the twin hulls of a catamaran which still stay in the water. The latest development is an SES catamaran with the air chambers under each hull rather than in the centre between the two hulls.

Another concept is the SWATH which is like a catamaran but where the hulls swell out under water so that most of the buoyancy is underwater below the waves so that the hull is less affected by waves movements. The emphasis with SWATHs tends to be on seaworthiness rather than speed which tends to range up to 30 knots.

With all these different hull concepts available, a designer can take something from each of them and produce a hybrid design which will have the right performance characteristics for a particular job. The catamaran with foils is one example but designers are constantly finding new combinations of hull technology in their endless quest to improve efficiency. Ride comfort and performance tend to conflict and the designer has to try and find a balance or a compromise between the two.

For real performance of 60 knots or above, the competition is between the deep vee and the cat, with the deep vee winning for the leisure sector and the cat when it comes to racing performance. It is hard to see this balance changing in the near future except with the advent of the ground effect craft. Already catamarans are virtually flying at speeds over 100 mph and aerodynamics are becoming increasingly important both to keep the boat stable and to generate lift. Monohulls have less facility to exploit aerodynamics but even some of the these boats are being fitted with wings.

Logic suggests that there is a lot of sense in leaving the water completely. After all, it's the waves which cause the problems at high speed. This is where the ground effect craft comes in, it skims the surface, exploiting the high lift effect generated between a wing and the ground (sea) when they are in close proximity. The result is a tremendous improvement in efficiency so that a 2 seater can manage 80mph with just 100 hp. The latest versions are hybrids with either tilting propellers or a hovercraft system which allows them to take off from land or with a very short run at sea.

It will be quite a few years before we see boating enthusiasts taking to the air rather than the water. In the meantime the refined deep vee hull will be the choice of most leisure boaters who want speed.