In order to satisfy the EEDI regulation introduced by IMO, ships tend to select a lower output engine, which might be dangerous in safety in adverse sea condition. Thus the assessment of load fluctuation in ship's main engine in waves is important, for which improvement in ship motion prediction, especially for surge motion, may be a key in enhancement of the prediction of load fluctuation in waves of ship's main engine. The strip method has been a robust and practical tool to estimate the seakeeping performance of a ship, but hydrodynamic radiation and diffraction forces for surge have been ignored in conventional strip methods, because the longitudinal-direction component of the normal vector on the ship hull surface (denoted as n_x ) is regarded as higher order, due to the assumption of slender ship hull. However, by retaining n_x -related terms in the formulation for hydrodynamic forces, we can expect improvement in the estimation accuracy not only in the surge motion but also in the other longitudinal motions.

In this study, a strip theory including the effects of n_x is proposed. First, the formulation for the radiation, diffraction, and restoring forces is presented, and then transformation of the forces and motion equations to those in another coordinate system is explained. Next, the effects of n_x -term in the diffraction problem are investigated by applying three conventional methods (NSM, STFM and the method by Mizoguchi and Watanabe). Calculated results by this proposed method are validated through comparison with model test results and numerical results by the conventional strip method. Furthermore, as a study on the load fluctuation of the ship main engine in waves, the results of surge motion computed with n_x -related terms are used in calculation of propeller effective inflow velocity in waves, and discussion is made for validation on the effects of the n_x -related terms.