Effect of Ventilation on Cavity Formation on Stepped Planing Hulls

Abstract

A stepped planing hull is a type of high-speed boat with a transverse break in the bottom of the hull. The step allows the hull to run at a higher trim angle, and also reduces the wetted surface area at planing speeds because of a pocket of air that forms aft of the step, often reducing the frictional resistance. This paper summarizes model tests that were conducted on a prismatic planing hull, with two adjustable stern sections, to form either a single- or twin-step planing hull. Measurements were made of the resistance, heave, wetted lengths, and cavity pressures behind the steps for various speeds at fixed trim. The ventilation cases included natural ventilation, augmented ventilated by means of vent tubes located behind the step, and reduced ventilation, where the flow was blocked off from the step by closing the vent tubes and adding longitudinal fences to either chine to prevent air inflow. Blocking off the ventilation causes negative pressures to form and increases resistance. Tests conducted in waves showed that the size of the ventilation cavity behind a step is not affected significantly by small waves, indicating a shielding effect of the hull. Stepped hulls are becoming increasingly commonplace in high-speed craft for both military and recreational purposes. A stepped hull is a type of planing boat, in which there is a discontinuity in the bottom, resulting in flow separating from the step and reattaching father aft along the hull. This feature can reduce resistance in some cases. Although they’ve been in use for over a century, they remain a challenge to design and to physically model.