Influence on the rudder gap cavitation by the scaling of its clearance

The main objective of the present study is to investigate the characteristics of gap flow around a semi-spade rudder as a lifting body using visualization techniques and pressure measurements. To reduce the scale effects as much as possible in the given experimental conditions, 1/28.5 scale-down models of a propeller and a rudder are employed. Since the propeller slipstream comes with some angle of attack to the upper part of the rudder in the port side, the highly accelerated leakage outflows are separated significantly at the discontinuities of the gap to generate strong cavitation at the lower pintle on the suction side. In the case of 1/28.5 scale-down gap clearance, the high pressure region in the suction side disappeared because the horn can hide the sharp edge of the movable part. However, it may play a role of the stagnation point as the gap clearance increases to 1/16.7 scale, reducing the horn's shading effect. The protrusion inside the gap does not control its flow in the 1/16.7 scale-down gap clearance. The augmentation of the gap clearance can blow the flow developed along the horn surface and delay the flow separation, causing the separation type of cavitation on the movable part, at the rudder deflection angle larger than 9° although the movable part has a severely deformed shape.