Dynamic Behaviour of Rigid Mono- And Multi-Hulled Vessels in Waves, Incorporating Non-Linear Excitation

Abstract

This chapter presents a time domain model for the prediction of ship motions in waves. Fluid forces and moments acting on the ship are represented by convolution integral expressions, thus accounting for fluid memory effects. The required impulse response functions are obtained from transforms of frequency domain data evaluated using a three-dimensional potential flow analysis based on a source distribution over the mean wetted surface of the vessel. Convolution integrals are used to describe both the radiation and diffraction contributions to the ship motion problem. Nonlinear restoring and Froude–Krylov excitation forces are determined at each time step using the instantaneous underwater portion of the hull. The applicability of a time domain method, formulated using body fixed axes, impulse response functions for radiation and diffraction influences, and nonlinear incident wave and restoring actions, in predicting motions for mono- and multihulled vessels traveling in regular waves is illustrated for various speeds and headings. For the same fast hull form in catamaran configuration and moderately high speeds, the requisite frequency domain data were obtained using a translating, pulsating source distribution over the mean wetted surface to better idealize the interactions between the demihulls. The applications of the partly nonlinear time domain method in oblique regular waves have, so far, shown that the nonlinearities in the incident wave excitation and restoring actions do not appear to have a significant influence on predicted heave and pitch RAOs.