Do you want your boat fast, or do you want it stable? That is the question that would-be boaters always seem to have to face up to. Perhaps if your goal is speed, you might think stability is a reasonable price to have to pay. Until, that is, you come to take your first corner in your high speed boat.
The speed/stability compromise comes down the design of the hull. And when analysed for their suitability for sport and leisure activities, Barry Stimson of Design Development felt that conventional `sports boats’ left a lot to be desired. Hull design, he felt, had not changed much over the last 30 years. So he set about doing something about it with a radical design.
Design theory basically shows that to make a hull go faster the `wetted’ area has to reduce as speed is increased in order to decrease `drag’. So to achieve more speed, you have to lift the hull out of the water. But in effectively shortening the hull area remaining in the water as speed increases, you are at the same time making it less directionally stable and more `slammy’. And this is the source of the compromise.
Deep `V’ hulls retain length yet still reduce the `wetted’ area by lifting on `spray rails’ running fore and aft, and generally provide a softer ride into a `head sea’.
The disadvantage with such a hull design is the boat `chine riding’ – falling from one chine to the other – at speed. Boat hulls can also be reluctant to turn tightly due to the length of the wetted area, or can fall onto a chine in the turn, then fall into the hole created by the tight turn. As this type of hull tends to tip when at rest, it is not ideal as a work/leisure platform.
Barry’s answer is the St Tropez, a 5.2m long, 2.1m beam, boat designed to accept outboard motors from 40hp to 80hp. The key is the concave centre ski section – the ski runs the length of the boat. As speed is increased the boat lifts onto the centre ski, reducing the `wetted’ area of the hull, without reducing the `in water’ hull length. The chines have reverse dihedral `dipping’ into the water towards the rear of the hull, effectively balancing the boat, eliminating `chine riding’ at speed.
The concave ski enables high speed turns to be `carved’ rather like a `monoski’, and the concave area also reduces skidding. When running into a choppy head sea the centre ski area softens the slams, while with a following sea the hull remains buoyant at the transom due to lift from the `V’ hull, centre ski and reverse dihedral chines.
At slow speeds the centre ski section acts rather like a keel making even low speed forward and reverse manoeuvres positive and precise. At rest the ski area has no effect while the reverse dihedral chines create an extremely stable platform, suitable for diving, water skiing, or sunbathing.
Take two boats into the water? Not when you can have the best of both worlds with just one.
Design Developments Tel: 01705 379777