Boating NZ : FREE TO READ March 2014
incurring a drag penalty from the excessive wetted area, and it would exhibit a high degree of instability in roll. In other words, it would tend to flip sideways. Savitsky’s method seeks equilibrium between two separate but inter-dependent force systems: drag/thrust and weight/lift. The prop’s thrust is trying to overcome the hull’s drag; in propelling the boat forward, the planing bottom generates lift at the longitudinal centre of pressure (LCP) of the hull bottom. If the LCG of the boat is aft of the centre of pressure, the stern will be pushed down, increasing the trim angle and, with it, the drag. 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. See LCG fwd, aft & equilibrium diagrams. What deadrise does, amongst other things, is to offer a vee’d form to the waves which softens the impact and, importantly, allows the stern to sink so the bottom can take up an angle of attack. It does this because it is an inclined plane; ie, the vee shape, which means the lift it generates is not straight up and so the vertical component is reduced. There is an optimum angle of attack – generally between 2.5 and 5 degrees – at which the drag from the trim angle and the drag from the wetted area are at a combined minimum. Varying the deadrise at the design stage allows the amount of lift – and its location – to be varied and optimised for a given speed range. The flatter the vee, the further aft 0314-59 Coming on to the plane, as buoyant lift is replaced by dynamic lift. subscribe online at www.mags4gifts.co.nz/boating-nz 51 Feature__future concepts_March14.indd 51 19/02/2014 3:16:54 p.m.
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