Analytical and experimental studies were made of the origin and characteristics of the two distinctly different appearing spray patterns associated with prismatic planing hulls. It was found that these two spray patterns, identified as "whisker spray" and "main spray," are a consequence of the same basic hydrodynamic flow phenomenon and transform seamlessly from one into the other. Similar to the analytical treatment developed for swept-wing aircraft, where the oncoming free stream velocity is divided into components normal to and along the leading edge of the swept wing, the free stream velocity for the planing surface is divided into components normal to and along the stagnation line. Combining this orientation of velocity components with the results of analytical studies of Wagner (Uber stoss-und gleitoorgauge an der oberflache von flussiglaseiten. Z.f.a.M.M., 2, 4, 1932) for two-dimensional planing of infinitely long flat planing surfaces with those of Green (The gliding of a flat plate on a stream of finite depth, Proceedings of the Cambridge Philosophical Society, 31, 1935) for planing of a two-dimensional, finite length surface, explains the origin and characteristics of both the whisker and main spray patterns. It is shown that the main spray originates in a small local area at the intersection of the stagnation line with the chine. Relatively simplistic equations that define the three-dimensional location of the spray apex are developed, and the results presented in three graphs. An illustrative example is presented that demonstrates the ease of application of the method to define the main spray geometry of a typical planing craft. Model tests were conducted to define the maximum height of the main spray and its lateral and longitudinal positions relative to the hull as a function of deadrise angle, trim angle, and speed coefficient. These data substantiate the analytical results.

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