Experimental Analysis of Stern Flap Effects on the Pressure Distribution at Transom Stern

Abstract

In this study, the flow characteristics past transom stern vessel fitted with and without a stern flap are studied experimentally. A 1:35 scaled model of a high-speed displacement vessel in even keel condition was used. These experiments were conducted to measure the pressure distribution along the hull due to the fitment of the stern flap and its effect on resistance for Froude numbers (0.16-0.47), representing the vessel’s operating profile. The scaled model was fitted with a stern flap at the transom end with a constant chord length of 1%L <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OA</inf> and a constant span of 58% Breadth with downward angular deflections of 5°,10° and 15° w.r.t centreline buttock of the model. Hull pressure was measured using pressure taps. At low Froude numbers, the pressure at the transom was observed to decrease due to the decrease in hydrostatic head as Froude no increased and presence of flap had no appreciable effect on pressure. With the onset of ventilation (Fn=0.31), the pressure was observed to return to atmospheric pressure when the transom is fully dry. At post ventilation Froude numbers, the pressure at the transom end increases with Froude number. This pressure recovery is attributable to the concave curvature induced by the deflection angle at the underside of the stern flap. It was experimentally shown that this recovery of pressure leads to a decrease in the total resistance.