Motions of mono/multi-hulled vessels in regular waves

Ballard, E.J., Du, S.X. and Hudson, D.A. (2001) Motions of mono/multi-hulled vessels in regular waves. In Proceedings of the Sixth International Conference on Fast Sea Transportation (FAST 2001). Royal Institution of Naval Architects. pp. 219-233 .

Abstract

The use of a time domain simulation technique for modelling ship motions allows for the evaluation of responses to arbitrary and/or transient excitation. Such a method also accounts for fluid memory effects, exemplified by the generation of motion-induced surface waves, introducing a dependence of the forces and moments on the past motion/excitation. The method used in this paper uses convolution integrals to describe both the radiation and diffraction contributions. The evaluation of the non-linear incident wave (Froude-Krylov) excitation is carried out at each step of the simulation by integrating the corresponding pressure over the instantaneous wetted surface. The corresponding, instantaneous hydrostatic restoring forces and moments are also accounted for by considering weight contributions at the instantaneous attitude of the hull. The effects of forward speed on a fast hull form (NPL5b) travelling at Froude number 0.5 in regular head waves are investigated. Both monohull and catamaran configurations are examined, the latter with two hull separations, allowing for interaction effects to be studied. Hydrodynamic data is calculated using frequency domain singularity distribution methods. Two frequency domain methods are used, one employing a pulsating source distribution with forward speed correction, the other using a translating, pulsating source distribution. The data from the frequency domain methods, referenced to a body fixed axis system, is transformed into impulse response functions using Fourier transformations. The impulse response functions are used as part of a convolution integral formulation to perform time domain simulations of motions in head-waves. Initially calculations are carried out in regular waves commensurate with the concept of linearity. Subsequently the effects of non-linearities are investigated by increasing the wave amplitude.

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Contributors

Author: D.A. Hudson

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