FEM/BEM analysis of diesel piston-slap induced ship hull vibration and underwater noise

Numerical prediction of vibration transmission from a ship diesel via a resilient mounting system to a stiffened cylindrical hull is performed aiming to provide a clearer insight into the significance of piston-slap in the diesel excitations to the hull vibration, and consequently, the underwater radiated noise. Finite element method (FEM) is employed to simulate the vibration response of the hull due to the excitations of diesel piston-slap and vertical inertia force of reciprocating masses. Effects of the rotational stiffness of resilient mounts on vibration transmission are also numerically investigated through coupled multi-DOF isolation analyses. Finite element solutions of the hull vibratory velocity are further used as boundary condition of the hull boundary element model for consequent underwater radiated noise calculation. The numerical results show that (1) piston-slap imposed rolling moment on the diesel frame may cause a higher level of ship hull vibration and underwater radiated noise than that due to the excitation of the vertical inertia force of reciprocating masses; (2) rotational stiffness of elastic mounts for resilient mounting system plays an important role in the diesel vibration transmission to the hull, especially as exciting frequency increases; and (3) neglect of the excitation component of piston-slap moment can lead to overestimates of hull vibration in some cases.