STUDY ON STERN FORM OF TWIN SCREW SHIP - TWIN-SKEG TYPE AND SHAFT BRACKET TYPE

Fluid dynamic and design problems of twin-skegs and shaft bracket sterns are discussed. The effect of the slope angle of the bottom tunnel of a twin-skeg stern form was investigated. When the slope angle of the tunnel in the stern bottom was enlarged, viscous pressure resistance increased. This was because the positive pressure gradient along the tunnel enlarged, and consequently the boundary layer was more developed, and the pressure resistance in the stern increased. In the model tests, even when this slope angle was about 26o, flow separation was not observed. The effect of the change of skeg form was then investigated. In the normal skeg form, propulsion performance was different according to the direction of rotation of the propellers, but in a skeg form twisted outwards, the difference was small. However, the propulsion performance of twisted skeg was worse than the normal skeg. Propulsion performance was best in a normal skeg form with the propellers rotating inwards. The effect of the velocity component within the propeller plane was large. The shaft brackets of a tested ship did not improve appendage resistance, the effective wake was improved by about 5%. It was found that there was a possibility of improving propulsion performance by making use of well designed shaft brackets.