A Consideration on the Dynamic Action of Free Water on the Oscillation of Ships

Abstract

In this paper, the dynamic action of free water on the oscillation of ships is studied by making use of a dynamical model. The model consists of a wheel (corresponding to the ship's hull) which can rotate about a fixed horizontal axis and an open tank of rectangular cross section containing the free water and attached to the wheel under or above the axis of rotation. This model can be considered dynamically as a physical double pendulum, the wheel is the primary and the free water is the secondary pendulum. The dynamic action of the free water is treated to be possible to divide into two components, the bodily shift of the whole weight of the water and the hydrodynamic action due to the motion of the water. The natural periods and the resonance amplitudes of the wheel are calculated and are compared with the experimental results of the forced oscillations caused by the harmonic external force. This method of analysis is proved to be successful to some extent. But, considerablly exaggerated actions of oscillation absorption are gained by calculations particularly in the case of the upper tank. This is probably due to the presence of the effect of higher order terms in the velocity potential and the phase lag between the oscillation of the wheel and the hydrodynamic action of the water which are not taken into account in the present theory. Roughly speaking, the shallow free water has remarkable damping action to the oscillation of the wheel but the deep water has less or sometimes small negative damping action