In this study, numerical simulations for the prediction of hydrodynamic coefficients and longitudinal motions of a trimaran vehicle in regular head waves with varying wave slopes are presented for a wide range of wave conditions. Wave excitation and radiation terms in 2DOF equations of longitudinal motions of the ship in regular head seas are obtained, using an unsteady Reynolds averaged Navier-Stokes (URANS) solver based on finite volume method. A trimaran hull form is used for the

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... at a relative high Froude number of 0.628. Numerical simulations are performed for eight different regular wave conditions with varying wave slopes. Firstly, the verification study is performed to determine an optimal mesh system and time step for CFD simulations. Secondly, numerical simulations are carried out to predict hydrodynamic coefficients and longitudinal motion responses and the results are compared with those obtained from the fully URANS approach and experiments. Finally, the relationship between the hydrodynamic coefficients and longitudinal motions are investigated with varying wave slopes. Systematic studies of the numerical simulations demonstrate that the numerical results have a reasonable agreement with the experimental data. The nonlinearities effects of wave slopes on the hydrodynamic coefficients and vertical motion of the trimaran are studied and analyzed taking the viscous effect and free surface nonlinearities into consideration.