Hull vibration excitation due to monopole and dipole propeller sources

Abstract

An acoustic boundary element (BE) model of a modern twin-screw cruise liner is used to show how fluctuating pressures above the propeller, and hull forces that cause ship vibration, change relative to each other with the nature of propeller sources. The effects of fluctuating loads on the propeller, represented by dipole sources in different directions, are compared with the effects of monopole sources at one and two times maximum propeller blade passing frequency. Convergence of results for the cruise liner model is demonstrated using different element distributions. The results are specimen transfer functions that relate propeller sources, having general spectral characteristics, to hull excitation. They are presented in non-dimensional form to facilitate comparison with results for other hull shapes and to demonstrate the effect of hull scale. A simple analysis of dipole and monopole sources is used to interpret results and to demonstrate why solution of the Helmholtz equation, which includes the effects of a finite speed of sound underwater, is needed for accurate evaluation of hull disturbing forces. It is argued that such modelling techniques can facilitate more accurate future interpretation of model-scale experimental data from towing tanks or water tunnels, as well as allowing improved specification of ship acoustic requirements.