Experimental Studies on the Hydrodynamic Forces Acting upon Full-Bodied Ships with Relation to Treir Course Stability

Abstract

Some of full-bodied ship models are unusually stable on course and others are unstable despite of almost same principal dimensions. This difference of course stability is caused by difference of hydrodynamic forces. The author measured the hydrodynamic forces of two typical models; one is unstable and another unusually stable. In the previous paper he assured that the difference of course stability was caused by the difference of hydrodynamic damping acting upon the aft-body. In this paper this difference of hydrodynamic damping upon the aft body is examined employing the surface pressure measurement, the flow survey in the wake field and flow observation on aft-body surface. From these results the following conclusions are obtained; The difference of hydrodynamic damping is due mostly to the difference of pressure acting aft of the s.s. 2 in bare hull condition. The difference of hydrodynamic damping is due to the difference of the stern flow in which the trailing vortices generated at the bilge part near the s.s. 2 play an important role. A remarkable flow named "reverse lateral flow" was observed between stern over-hang and propeller race, which flows from back side to face side against the general flow direction. The effect of such flow is not clear enough yet but certainly it has some concern to the empirical formula between the course stability and the stern profile of full-bodied ship models.