Designing of planing hulls with longitudinal steps: CFD in support of traditional semi-empirical methods

The paper presents an integrated semi-theoretical/numerical (CFD) method for the design of V-stepped planing hulls. Considerable resistance reductions with respect to conventional hull forms (in the order of 20%) have been obtained by the authors following this design procedure on a number of cases. The proposed hull form design is suitable to be applied for retrofit of existing high speed crafts and it is suitable for fast yachts having a top speed in excess of 50 knots. The focus of the paper is on the combined theoretical/numerical method to effectively design optimum step configurations for high speed planing hulls, including dynamic stability. The method combines a systematic series of CFD calculations with the semi-theoretical formulations proposed by Clement based on the analogy with lifting surface theory with CFD to design the geometry of the step and wetted portion of the forward body. An application example of the combined method to the redesign of an existing high speed Deep-V planing hull in a transverse V-stepped hull is presented: a reduction of the resistance of 19% is confirmed by CFD in this case. Details of flow solution are detailed in terms of pressure distribution, shape and position of the spray root line, free surface wave pattern and global parameters such as friction and pressure forces, dynamic trim angle and sinkage for the original and modified transverse-step version. The analysis of the CFD results on the equivalent hull forms permits to draw interesting conclusions on the effectiveness of transverse steps on the reduction of drag in planing hulls. © 2013: The Royal Institution of Naval Architects.