Theoretical Prediction of Cavitation on Propellers

One of the consequences of cavitation on marine propellers is the risk of damage to the propeller in the form of erosion and bent trailing edges. Other detrimental effects of cavitation are the large amplifications of vibration-exciting hull forces and the emitted noise. These problems associated with propeller cavitation have become matters of great concern. In this paper a method is described for the assessment of cavitation inception and for the calculation of the type and extent of cavitation on marine propellers. The adopted theory is suitable for application to nonuniform flows such as exist behind ships. Some primary effects associated with viscosity are also included, in particular the problem of Reynolds number scaling such as occurs when testing models in cavitation test facilities at speeds lower than at full scale. It is shown that the described theory leads to reasonable correlations with actual cavitation patterns for lightly and moderately loaded propellers. For heavily loaded propellers such as those of tankers and tugs, the calculated results are less satisfactory. The argument is made that this is due to the lack of knowledge regarding the change in the wake flow into the propeller due to the propeller load. For minimizing the occurrence of cavitation in subcavitating propeller design, long-standing experience in both the design and testing of propellers in cavitation test facilities is normally required. With the use of the described theory, however, it is possible, by systematically varying design parameters, to arrive at a successful design in a straightforward manner.