ASYMMETRIC ROLL MOTIONS OF A SMALL FISHING VESSEL WITH A LIST

In the experiments reported in this paper, a model of a fishing vessel was restrained in all modes but roll and subjected to moderate amplitude regular waves. The effects of sway and heave restraint are well demonstrated in these tests. In particular, significantly different motions are observed depending on whether the model is biased into the waves or away from the waves. Moreover roll motions are much greater for a vessel with a bias. The implications of this result for unrestrained motions are not entirely clear. It is possible that the effect of sway restraint may be reproduced in a real situation by a steady wind or current. Generally a ship will move with the wave and the relative heave motion may not be large. If there is water on deck, the rate at which the water leaves the deck may cause a change of phase of heave motion relative to wave motion with a consequent magnification of relative heave. What is apparent from these results is that both heave and sway are significant in determining the roll response of a ship with static bias in severe seas and may not be ignored. This calls into question the validity of the use of the one degree of freedom roll equation to predict the extreme behaviour of ships with bias. For the restrained tests considered in this paper, it is possible to use the simple roll equation with suitable modifications to predict roll motions, since the 'relative heave' motion is of course known. In fact it is instructive to use this equation to single out possible factors that lead to the different responses for differing conditions. In addition to a 'simple' roll simulation using the classical GZ curve etc., a simulation of a much more sophisticated nature is employed to demonstrate the short comings of a 'simple' simulation and further to demonstrate the usefulness of simulation in understanding complex phenomena.