Inclusion of Whisker Spray Drag in Performance Prediction Method for High-Speed Planing Hulls

Abstract

The planing hull performance prediction method published by Savitsky in the October 1964 issue of SNAME's Marine Technology included only the viscous drag and pressure drag components in the bottom area aft of the stagnation line. While discussing the existence of an additional component of viscous drag in the whisker spray region forward of the stagnation line, a method to quantify this drag component was not developed in Savitsky (1964). Earlier studies by Savitsky and Ross (1952) and Savitsky and Neidinger (1954) discuss the potential whisker spray drag component but do not present a complete analytical derivation of its magnitude nor verification of the results by comparison with experimental data. The present study fills this void by developing a method for quantifying the whisker spray contribution to total hull resistance as a function of deadrise angle, trim angle, and speed, and incorporating the results into the SNAME published hull performance prediction method. The analytical results are compared with data from model tests conducted at three separate towing tank facilities and show fairly good agreement with these data. It is shown that for high-speed planing hulls, the whisker spray drag component can be as much as 15% of the total drag. In addition, (1) procedures are provided for the proper location, size, and geometry of spray strips to deflect the whisker spray away from the hull bottom; (2) the aerodynamic drag of the hull cross-sectional area above the waterline is also quantified and included in the final performance prediction method; and (3) the equilibrium trim angle identified in the prediction program (for prismatic hull forms) is, for nonprismatic hulls, related to the trim angle of the v4 buttock line (relative to the level water surface) when measured at the forward edge of the mean wetted length.