An Experimental Study on the Motion Control of Semi-Submerged Ships

Abstract

It is well known that the semi-submerged ship possesses excellent seakeeping qualities in seaways, because of its long natural periods due to the small waterplane area. However, the small waterplane area also causes instability in the longitudinal attitude or large oscillations in following seas. Equipment of some control fins is desirable for making the ship stable at high speed and for reducing the oscillations at resonance.In this paper, the three components of hydrodynamic forces acting on the fin-body configuration were measured firstly by wind tunnel test and towing tank test. These data were analyzed putting emphasis on fin-body interactions and free surface effects.The free running test on the radio controlled SWATH type model were carried out secondly in the seakeeping basin. The trim, sinkage and oscillations in regular waves were measured in various conditions with fin control.The main conclusions obtained in the present study are summarized as follows ;1) The fin-body interaction and downwash effect of the forward fin to the aft fin can be evaluated by the method of Pitts et al.2) The free surface effect on the lift of fin can be estimated by taking into consideration of the flow field around the body which changes with the advance speed.3) The lift generated by the oscillating fin or the lift acting on the fixed fin in regular waves can be evaluated quasi-steadily by using the method mentioned in 1).4) At the free running test on our SWATH ship model, a large bow down phenomenon was observed near Froude number 0.4. This instability can be anticipated by the moment measuring test in towing tank.5) The usefulness of fin control for reducing the oscillations in waves was approved by the free running test.