Blade rate radiation from cavitation volume modulation on merchant vessels

Abstract

The radiation of acoustic energy from merchant vessels at blade rate frequency is dominated by the mechanism of propeller cavitation modulation. These modulations are generated by the response of the propeller blade to the spatially nonuniform vessel wake. Since these blade rate lines and their harmonics are a major contributor to low-frequency ocean ambient noise, the prediction and measurement of these merchant ship spectra have recently been the subject of considerable interest. This paper briefly reviews the results of an ongoing effort to predict the cavity volume time history for a given merchant ship propeller. These results show interesting limits for the unsteady propeller cavity volume which imply a limit for the maximum blade rate radiation from a given propeller. These limiting volumes are compared to volumes measured on merchant ship propellers using stereo photography, yielding relationships for typical cavitation on merchant vessels. The acoustic radiation from this volume source is reviewed and the developed relationships for unsteady propeller cavity volumes are compared to volumes inferred from recently measured levels of blade rate lines from three merchant ships.