Propeller cavitation on small craft: Underwater noise measurements and visualisation from full-scale trials
T. A. Smith, Andrea Grech La Rosa, G. Piggott, J. A. N. Gaivota, S. S. McMorran
Abstract
Propeller cavitation is a significant contributor to vessel underwater radiated noise (URN). It is often assumed to be the major contributor for large vessels at higher speeds, but very little work is available in the literature on the role of cavitation on small boat propellers. In this work, data from two trials are presented to show how cavitation develops on small boats and how this contributes to the overall sound levels. Camera footage is combined with hydrophone measurements to determine the cavitation inception speed and this shows that tip vortex cavitation can appear at 5 knots. The emergence of cavitation is accompanied by a sharp rise in the URN levels. Cavitation due to gas bubbles being pulled close to the propeller blades is observed at speeds as low as 4 knots, leading to either bubble collapse close to the blades or the tip vortex cavitating downstream of the propeller. Wavelet analysis is used to investigate the makeup of the high frequency noise, providing insights into the types of cavitation that are present and how they scale with speed. This shows that high frequency noise from cloud cavitation increases far more substantially with speed than for tip vortex cavitation. • Camera footage shows the cavitation pattern on an outboard propeller. • The cavitation inception speed on all three boats is no more than 6 knots. • Tip vortex cavitation predominates on both outboard-powered vessels. • High frequency noise scales weakly with speed when tip vortex cavitation predominates. • High frequency noise scales strongly with speed when cloud cavitation predominates.