THE WATER RISE IN FRONT OF A MODEL PLANING HULL

It is known that the water ''splashes-up'' or rises above the undistur bed surface immediately in front of a planing surface. This rise is gr eatest in front of a flat planing plate and a number of attempts have been made to reduce the experimental measurements of this phenomenon t o some kind of order. Since it was first independently proposed by bot h Schnitzer and Smiley in 1952, all attempts to correlate the flat pla te splash-up have started with the assumption that splash is only a fu nction of the immersed length of the plate and is independent of trim angle at least below about 20-degrees. In part, this was because the t hree early studies which compared this hypothesis with experimental da ta omitted those portions of the data which did not support the hypoth esis. The present paper concludes that this forty year old hypothesis is fallacious and that the water rise in front of any prismatic planin g surface is best approximated by d/square-root bl = k sin2tau where d is the vertical water rise at the water/keel intersection; b is the b eam; l is the submerged length of the keel; tau is the trim angle; k i s a constant determined from experiment, approximated br, k = 2e-2.5be ta, where beta is the deadrise angle in radians. It might be thought t hat this is a slight contribution, of little practical import, but for one thing. Starting in the 1950's most towing tank experimenters in t he United States abandoned the difficult measurement of model draft an d obtained only the ''actually wetted length'' from underwater photogr aphs. But theoretical planing force calculations require a knowledge o f the relationship between a hull and the undisturbed water plane. Thu s if modern experimental data is to be compared with theory, it is nec essary to estimate what the undefined splash-up or water rise was duri ng each experiment, in order to estimate the model's true position in space. The paper concludes by criticizing the format of some modern re ports of experiments with model planing hulls and suggests that, in ad dition to the usual graphical presentations, measured data should alwa ys be reported numerically. Also, that when relevant data is omitted f rom a plot, the facts of such omission should be clearly stated.