The Effect of Controlling Fins on the Motion of Resonance-Free SWATH (RFS) Platform

Abstract

The speed reduction, additional resistance or slamming, which are caused by the large amplitude of ship motions, should be restricted completely for oceangoing large fast ship, because of the strict time-punctuality and high value of the cargo. In the present work, comparison of seaworthiness, such as the motion responses in head seas, is made among four types of ship hulls, i.e. Mono-hull, Ordinary SWATH, Trimaran and Resonance-Free SWATH (RFS). The last one can be regarded as a special type of catamaran. As a result of the extremely small water plane, the restoring moments are negative for RFS hull, which leads to free of resonance in the motion responses. Experiments in towing tank and theoretical calculations are carried out to examine the motion responses in waves. Calculations are generally based on potential theory modified by adding the viscous effects and including fin lifts. The results of the motion responses of RFS are compared with those of mono-hull ship, ordinary SWATH ship as well as trimaran ship. Where the motion of RFS is controlled using movable small underwater fins. The motion of ordinary SWATH is examined with and without fin control. It can be seen that the heave motion responses of RFS are very small in comparison with those of other ship hulls. Also, the pitch motion responses of RFS are much smaller than those of mono-hull or trimaran or ordinary SWATH without fin control, and are smaller than those of ordinary SWATH with fin control. Fin control actions, i.e. proportional and derivative control, are applied in the experiments and calculations. It is considered that the derivative control action is more effective on the motion responses of RFS than the proportional one as predicted. The larger the derivative gain value is, the smaller the motion response of RFS is.