Boundary element methods in the prediction of the acoustic damping of ship whipping vibrations

Abstract

Damping of ship whipping vibrations following a slam due to wave impact is traditionally assumed to be primarily of material or struc- tural origin. However, several mechanisms of energy dissipation to the surrounding water exist, including gravity and acoustic waves. Nei- ther transports much energy for the lowest frequency modes, in which the acoustic wavelength may be an order or magnitude greater than the ship length whereas the gravity wavelength is at least an order of magnitude shorter than the ship beam. However, the acoustic damp- ing ratio increases as the fourth power of frequency, becoming signif- icant for higher frequency modes. This paper investigates at what frequencies acoustic damping is significant for a typical ship. The acoustic problem is solved using a three-dimensional exterior bound- ary element method, which is described. Acoustic damping is then compared with total damping from full scale ship measurements and material damping from laboratory tests, from which the relative con- tributions are deduced.