EXPERIMENTS WITH STEPPED PLANING HULLS FOR SPECIAL OPERATIONS CRAFT

Abstract

Many of today’s special operations craft and high-speed patrol boats can operate at volume Froude numbers high enough to justify the use of a stepped planing hull; however most of these boats are un-stepped. This paper describes experiments to determine the effect of adding a single step into the bottom of a planing hull with loading parameters consistent with modern special operations craft. In contrast to multi-step pleasure craft, which can operate at volumetric Froude numbers of 10, special operations craft often operate at volumetric Froude numbers of around 5, owing to the increased payload and decreased speed. Previous towing tests on a variety of stepped hull configurations indicated that a promising configuration for special operations craft may be a single step located near the transom (alternatively known as a hydrodynamic transom forward of the stern). The present experiments investigate this configuration further, by testing a single-step hull, with step located at 25% of the length forward of the transom. The step height is systematically varied to observe the effect on resistance, trim, wetted surface area and porpoising stability over a wide range of speeds. Of the configurations tested, the best reduced model resistance by 25% at the highest speeds tested, while increasing resistance by 10% in the hump speed regime. The stepped hulls tested had porpoising limits similar to conventional planing monohulls. A short method is provided to illustrate when a stepped hull may be advantageous for a given design.