Boating NZ : FREE TO READ March 2014
of the hull to generate sufficient lift that the boat’s displacement is reduced and, with it, the vessel’s drag, allowing it to go faster than if still in displacement mode. Planing powerboats can generally be designed to operate well in a particular sea state and a particular speed with a particular payload. Yacht designers employ software to analyse and optimise the hull forms and indicate the power requirements, running trim, static stability and other characteristics through the speed range. What the software cannot do is tell you if the design is good or bad – experience and judgement are required at that stage. A good design is one in which the optimum design condition marries well with the most frequently encountered conditions – that varies from location to location; see Rule Three. A good, all-round design has a broader range of conditions in which it functions well, but at the expense of being less effective than the good design in its optimum conditions. See Rules Two and Five. So, when discussing whether a particular design is good or bad, it must be set in the conditions for which it was intended, whether or not it is operating in those conditions, and how it deals with them. It’s quite easy and often tempting to take a good design and re-power it with a bigger, more powerful engine. But, you might come up against the limit of the hull form’s ‘goodness’ and end up with a vice-ridden beast – through no fault of the original design because the longitudinal centre of gravity (LCG) has moved aft, and at the increased speed there is virtually no boat in the water. In 1964 Daniel Savitsky published a method for calculating the A range of performance prediction software dines on deadrise angle, beam at chine and longitudinal centre of gravity to help designers gain lift, control and speed. “” If the LCG is too far forward, the bow will be pressed down, reducing trim, increasing running length and wetted area, which – you guessed it – increases drag. subscribe online at www.mags4gifts.co.nz/boating-nz 49 Feature__future concepts_March14.indd 49 19/02/2014 3:10:43 p.m.
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