Design of Propulsion Systems for High-Speed Craft

The demand for increased speed in medium and large craft challenges the designer to select propulsion systems which meet performance requirements economically throughout ever-widening operational profiles. The combined hydrodynamic characteristics of hull and propulsors result in a speed-thrust relationship for the environment in which the vessel operates. This speed-thrust relationship requires unique values of power and RPM input for each type and number of propulsors. Power and RPM are also sensitive to the mode of operation of the vessel whether at constant speed, accelerating to a greater speed or towing an object. Most vessels utilize fixed-pitch submerged propellers. Surface propellers are fitted to vessels designed to perform at very high speeds and waterjetpropulsors are being utilized with increasing frequency on larger vessels with high-speed operational profile. This paper discusses brake horsepower (BHP) and propulsor RPM relationships for vessel speed requirements based on the hydrodynamic characteristics of three types of propulsors: submerged propellers, surface propellers and waterjets. An example of predicted vessel performance regarding speed, power and propulsor RPM is presented which includes engine characteristics and BHP versus RPM. This latter format depicts the differences in power demand for three types of propulsors on a monohull vessel with regard to engine characteristics.