Underwater Radiated Noise from Coastal Ferry Vessels: The Influence of Propellers and Operating Conditions

Cavitation-induced noise from propellers dominates the underwater radiated noise (URN) from ships – the largest source of underwater noise pollution worldwide. Vessel noise has often been related to travel speed, and slowdowns been found to be effective in reducing overall noise in high-traffic areas. However, radiated noise it not universally correlated with travel speed. We investigated field-measured radiated noise levels from eight coastal ferry vessels each operating at a range of speeds, and considered the relationships between noise at individual 1/3-octave band levels and vessel operating conditions in order to uncover the physical mechanisms and characteristic acoustic markers of the specific noise behaviors. We considered a gamut of propeller parameters including depth, pitch, and slip and determined that only speed was reliably related to URN. Among vessels for which speed and noise were anti-correlated at most frequencies, all of which used controllable pitch propellers, acoustic markers were found that indicated a change in the noise generation regime when the propellers were under-loaded. The acoustic signatures of these regimes appear to correspond to different types of propeller cavitation.