The present article introduces a new and innovative shape for the stern flap which modifies the flow of water underneath and around the transom stern. This is done in such a manner that the transom wave known as 'rooster tail' is reduced to a minimum, at different speeds. This hydrodynamic effect can be considered a radar signature reduction. Fuel consumption and resistance reduction are other advantages of the rooster tail reduction. To explore the effects of transom flap, a complete set of parametric studies is conducted and a three-dimensional numerical simulation of the transom stern flow, based on the FVM-VOF schemes in Ansys-CFX software, is presented. Three Froude numbers at four different trim angles are chosen and effects of transom flap on the rooster tail are probed in each case. Comparison of the numerical solutions with results of the existing empirical formula gives favourable agreement.