lnvestigation into Modeling and Measurement of Propeller Cavitation Source Strength at Blade Rate on Merchant Vessels

Abstract

The forces exerted on a ship's hull by propeller cavitation, modulated at blade rate, can be simply expressed as a function of the cavity volume velocity. Analytical models exist and are being refined that are used to predict this cavitation volume velocity for a given propeller in a given wake field. Very little data exist on how well these models work, primarily because of the difficulty in measuring cavity volumes on an operating propeller. This paper reviews recent work, sponsored by the Naval Research Laboratory, that has investigated the correlation between analytical models of propeller cavitation and ·measured cavity volumes on both full- and model-scale propellers (stereo photography data). These comparisons demonstrate that accurate prediction of cavitation volume is extremely dependent on accurate knowledge of input variables, especially the propeller wake field. These comparisons also point out limitations in the accuracies of the existing models. Results are presented of measurements of cavity volume velocities made by calibrating the ship vibration by using the principle of reciprocity. The accuracy of this reciprocal technique is demonstrated by a comparison of the results obtained with the acoustic pressures radiated by the cavity source. These measurements are analyzed for some of the time-varying characteristics of the cavitation. These results demonstrate that time-varying characteristics resulting from wake turbulence, rudder motion, ship motion, etc., introduce severe fluctuations in the source strength of propeller cavitation.