Influence of cavitation on propeller performance 1

In this paper the author summarizes some of the more important factors governing the onset of cavitation, the influence of the phenomenon on propeller performance and the technical difficulties involved in the process of correlating model propeller experiment results with those obtained on ship trials. Emphasis is laid upon the value to be derived from model experiments conducted under flow conditions simulating as closely as possible those at the ship, the need for which was fully recognised at the 9th International Towing Tank Conference in Paris, September 1960, and the importance of some of the recommendations by that Conference. It is only comparatively recently that attempts have been made to simulate the ship conditions in model experiments, and examples are given of work in this field by several authorities and in particular the technique of hull-propeller combination experiments is described.

The basic conditions which should stricly be met by such model experiments are outlined and some indication of the magnitude of errors which can arise if these conditions are not fulfilled is given. Apart from the simulation of the irregular flow at the ship propeller the important factors which prevent precise correlation between results of model tests of propeller designs subject to severe cavitation and the performance of the full scale propeller include the impracticability of simultaneous identity of Froude number, Reynolds’ number, advance coefficient and local cavitation number; tunnel wall effects; effects of the physical properties of the medium; and differences in experiment technique. Each of these is discussed in the paper which concludes with a suggested process by which the results for cavitation onset obtained from uniform axial flow model experiments might be more closely correlated with the cavitation onset observed at the ship. An approach on these lines appears to be a necessary first step in the determination of the effect of serious cavitation on ship propellers.

When the design requirements are such thai serious cavitation cannot be avoided the reduced performance may also be accompanied by serious damage to the material of the propeller blades and the paper attempts to summarise the mechanisms by which this may take place. In this connection the extent to which the fully cavitating propeller may be an advantage is briefly discussed.