Opposite: the spectacular Orma 60 Grand Prix were usually filmed using a variety of high-speed custom chase boats… but few of them would keep up reliably. The prototype of the Wilhelmina motorboat hull (above) at rest in Dartmouth after a 50nm delivery trip from Fowey in Cornwall – at an average speed of 9.4kt with a total fuel burn of 13 litres running at 10hp. Maximum power is 14hp
motorboat for sailors – that kind of thing. Better say right away that such a boat won’t be cheap, simply because what we are talking about is unlikely to become a high-volume product. On the other hand, it will probably only be of interest to that second home owner – or at least someone who has retired to live by the sea, so the price tag may not be a problem provided the boat delivers as promised. Here are some basic observations on
how we ourselves approached that design challenge… Rule 1 is that the design of any motor
boat can only be truly optimised for a single speed. A displacement hull is limited to an absolute speed that is about 1.4√Waterline Length (ft). So a 12m (39.4ft) displace- ment hull won’t/can’t be pushed at much more than 8.8kt. If it is well designed it will, however, deliver the quietest and most satisfying ride in the range of zero to around 7.5kt (speed/length ratio of 1.2). To travel faster than that (which most
buyers will want to do) it is normally assumed that your boat needs to plane on the surface of the water to break free of the above constraint. To do so it needs to have a hull that is flat enough to provide the required amount of lift. Unfortunately that flatness is the last thing you need once you start to encounter waves. The boat soon becomes uncomfortable as it makes an impact on each wave. You can opt for a deeper, more V-shaped hull, which will cope better in a seaway but will then need a bigger, more noisy/thirsty engine. When on a planing boat you can feel
when the hull is struggling as it reaches the displacement hull speed limit (called the ‘hump speed’). Once it has made that tran- sition the speed will increase quite rapidly. In fact, you should find that once the boat has broken through the hump speed into-
planing mode you can actually pull back the throttle somewhat with no loss of speed – which is how a planing hull is intended to work. The problem is that these boats are
going to need a lot of power to plane properly. That means they’ll have to be able to carry a lot of fuel to go the distance. Although a planing hull can, of course, be very efficient it’s true to say that going properly fast – say upward of 30kt in a 30-footer – does burn an awful lot of fuel, however you look at it. A pair of 250hp engines (for example) will burn some 110 litres (91kg) of fuel per hour. A 100-mile (ie cross-Channel) trip will need over 300kg of fuel (plus a reserve and the extra burn due to the extra fuel weight carried at the start). In the wake of the Pêche-Promenade
experience our plan has always been to buck the trend by producing hulls that are not unusably extreme but do lean towards Slippery Thoroughbred ideals. Thinking on a clean sheet of paper led us to a first result that was called the Rangeboat. The chosen design was a 12m LOA
(39.4ft) launch destined to take a different route as it explores a sort of middle ground that lies between the true displacement hull and the planing hull (sometimes called semi-planing… whatever that means.) Seventeen Rangeboats were produced
over a number of years. They displaced around 5 tonnes and only had what you’d have to call ‘weekend accommodation’ for a couple in the forward cabin. There was a small deck-house amidships and the rest of the boat was dedicated to a very roomy cock- pit. The boats were fitted with a single 100hp or 200hp engine, the former providing a top speed of 16kt and the latter some 20kt – at which speed the hull starts to lose its advan- tage in terms of efficiency. To be capable of this kind of operating
performance the design needs to have the required low Displacement/Length ratio (our acronym LDL) that defines these hulls. It allows the hull to iron out that ‘hump’ speed so that it is barely noticeable, so that speed can be adjustable through the range as desired and according to sea state. Often a planing hull boat will encounter
a sea state that makes planing too uncom- fortable for the crew – or possibly too hard on the boat. To make matters worse a planing hull that is operating at sub- planing speeds is a handful to control – especially downwind. The low displacement/length require-
ment means that the finished boat is relatively light for its length, so we’re talk- ing long, quite skinny hulls that are nonetheless quite wide aft to make sure the transom is not allowed to squat. The downside is that wetted surface is
not reduced as much as it is on a true planing hull as speed increases – which is why these hulls are not suitable for truly high-speed operation (above a speed/ length ratio of about 2.5). An LDL hull should be kept light
enough simply by having less accommoda- tion in that skinny hull. It’s also very important the weight of the engine doesn’t stray too far aft. The 25hp engine in our current 9.5m (31ft) test MINA class boat weighs a mere 113kg, but engine beds, box and a huge amount of sound insulation add up to almost as much again. A prop shaft angle of less than 8° is also
pretty desirable for efficiency so again it’s useful to keep the engine somewhere near amidships. The operational window for which these
boats are optimised is a Speed/Length ratio of between 1.4 and 2.5 so the higher end of this range feels more like ‘Swift’ than truly ‘Fast’. This is consistent with the notion
SEAHORSE 39
NIGEL IRENS
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