CHAPTER 2 Clean Bottom
Wave generation in non-displacement hulls The same criteria on fuel consumption and wave- creation for displacement craft apply to planing or semi- displacement vessels operating at sub-planing speeds. Such boats often leave a clean wake while planing but are inefficient in ‘displacement’ mode, pulling a huge wash as hull speed is approached. This places their operators in an invidious position in confined waters where traffic and speed limits abound. A semi-inflatable RIB, for example, will certainly make less wash travelling at 25 knots than at six, yet her skipper is constrained for reasons of safety from doing so. Very often, easing back from 6 knots to five can produce a dramatic result. Large, semi-displacement cruisers are especially prone to this phenomenon. The mathematics of what is going on is too complicated to allow general predictions. The only answer is to look astern and watch the wake.
Even a modest growth of nothing more than slime can knock a sailing boat’s windward performance back by 10% or more. Weed and barnacles will rapidly worsen the situation until it finally begins to affect the boat’s ability to handle properly. Such effects are less obvious in motor craft, as there is usually more reserve power to take up the slack. Fuel economy suffers, however, and if propellers themselves are allowed to foul up, loss of speed will soon follow. Whether under power or sail, all efforts to maximise a boat’s performance are a waste of time without a clean bottom.
Fuel factors in planing craft
In general, a well-designed planing hull covers more miles for a given amount of fuel at medium planing speeds than at the upper end of her displacement mode. Wave production attests to the truth of this, but the only sure ways to acquire empirical data are to consult a designer’s specification or set up a series of trials. One or other should be addressed before taking such a vessel seriously to sea.
Displacement craft
Planing craft
Although fuel consumption is about the same for both displacement and planing craft at low speeds, planing craft will go much faster for the same consumption as speed mounts.
MANUAL OF SEAMANSHIP | 17
Speed in theory and practice
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