A Universal Parametric Model for Waterjet Performance

The successful selection of a waterjet for a high-speed marine vehicle requires a system-wide analysis of that jet's performance across the vessel's entire range of speeds. Selecting a waterjet based on a single top-speed design point often results in poor vessel behavior, notably in acceleration, heavy seas or at the planing "hump-speed" regime. Current waterjet selection practices typically involve graphically plotting the "speed-thrustpower" relationships and matching them to vessel resistance. The analysis of equilibrium engine RPM is notably absent from this process, yet it is vital. Only by extending the "speed-thrustpower" analysis to include the proper selection of impeller, engine RPM and transmission can one define a truly successful system. The author proposes a new parametric model of waterjet performance based on a set of "universal waterjet coefficients". These coefficients describe both the "speed-thrust-power" relationships and the equally important "power-RPM" relationships. The performance of any commercial waterjet can be defined and predicted with these coefficients, providing the critical part of an off-design system analysis, including equilibrium RPM, thrust, torque, and power.