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

Damping of ship whipping vibrations following a slam due to wave impact is traditionally assumed to be primarily of material or structural origin. However, several mechanisms of energy dissipation to the surrounding water exist, including gravity and acoustic waves. Neither 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 damping ratio increases as the fourth power of frequency, becoming significant for higher frequency modes. This paper investigates at what ∗School of Engineering, University of Tasmania, Hobart, Australia. mailto:[email protected] See http://anziamj.austms.org.au/V45/CTAC2003/Hol2/home.html for this article, c © Austral. Mathematical Soc. 2004. Published August 10, 2004, amended August 11, 2004. ISSN 1446-8735 ANZIAM J. 45 (E) ppC845–C856, 2004 C846 frequencies acoustic damping is significant for a typical ship. The acoustic problem is solved using a three-dimensional exterior boundary 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 contributions are deduced.