Waterjet Modeling to Calculate High Speed Vessels’ Performance

Abstract

Similar to the matching process between the main engine and propeller in a conventional vessel, there is a basic necessity to match main engine and waterjet in a high speed craft. Unlike the ability of the naval architect to review the overall impact of the engine and propeller selection in terms of thrust, delivered power, fuel consumption and vessel range, when coming to match a waterjet to a particular vessel, the performance data presented by the waterjet manufacturers is in some ways simplistic and does not contribute to the naval architect's effort to assess the complete performance picture of the propulsion system. In this article, a simple parametric modeling of the waterjet is presented which is common to all waterjets and enables the naval architect to match and assess the interaction between main engine and waterjet much like the process when matching main engine to a propeller. It will be shown that the constant power lines for thrust and rotational speed of a waterjet can in fact be consolidated into a single nondimensional set of curves similar to the nondimensional KT/KQ/ηo of a conventional propeller. This process will allow in fact the overall assessment of the matching problem between the main engine and the waterjet. Some waterjet manufacturers provide data for different impeller designs in which case this process can provide the naval architect a tool to choose the optimal impeller. Unlike the graphical nature of the performance assessment of the conventional data presented by the waterjet manufacturers, this method of modeling allows the digital/analytical solution in any intermediate speed and power setting. For the sake of simplicity, this article does not deal with the cavitation phenomena and the loss of thrust when the waterjet is cavitating although these are important issues that should be considered when assessing waterjet performance.