Hydrodynamics of Surface-Piercing Outboard and Sterndrive Propulsion Systems

Work has been done recently at Mercury Marine in the area of characterizing the performance of surface piercing drive systems used for outboard and sterndrive propulsion. These drive systems operate behind the boat at the air-water interface and are required to produce steering and vertical forces as well as thrust. The ability to produce these forces efficiently is a primary advantage of this type of drive system. The complexity of the physics presents a significant analytical challenge. Theoretical, experimental and computational methods are presented. The content should be of particular use to boat designers who would like to integrate these types of drive systems into vessels. Gearcase lift, drag, and side-force are reported as functions of speed, drive trim angle, drive steering angle, and drive height, including the influence of cavitation and ventilation. Some considerations for surfacepiercing propellers are reported as well.