Waterjet System Performance and Cavitation Test Procedures

Different from marine propeller designs, which undergo standard stock and design propeller test programmes with detailed assessments of the final design (for the overall propulsive efficiency, the cavitation performance and the pressure fluctuations, etc.), waterjet systems of a final design are seldom tested for their system characteristics, the intake loss and the cavitation performance. It could be both due to economical reasons that waterjet system tests are relatively expensive. It could also be due to technical reasons, such as, that the operating point of a waterjet system does not vary too much for different operational conditions. This means that it performs also very well for all other operational (off-design) conditions once a waterjet system is well-designed for its design condition. However in practice, mismatching of power absorption and shaft rotational rate, and cavitation erosion, are now and then found after the sea trials of the waterjet propelled ships. Remedial action is needed then. In some cases, removing cavitation erosion can be rather difficult and simple modifications may not solve the problem. In order to prevent those kinds of problems from the early design stage, waterjet system performance and cavitation tests of the final design are strongly recommended. Taking example of a Fast River Passenger Ferry, test procedures are discussed in detail in the present paper. The scale effects and the extrapolation method are also addressed. The results provide a good data set for CFD validation too.