Time Domain Evaluation of Vertical Motions of High-speed Displacement Hulls

A nonlinear time domain seakeeping analysis has been performed for high-speed displacement hulls advancing in head seas with regular waves. An hybrid approach has been employed, solving the unsteady hydrodynamic problem in the frequency domain and the equations of motion in the time domain. A three dimensional Rankine panel method has been used in order to solve the hydrodynamic boundary value problem in the frequency domain. Radiation and diffraction forces, supposed linear, have been then transferred in the time domain, where Froude-Krylov and hydrostatic forces have been computed considering the actual wetted hull surface at each time step. Simulations have been performed on hulls derived from the NPL series form. Particularly, the Model 5b catamaran has been tested, considering two different hull separation to length ratios (S/L = 0.2 and S/L = 0.4) and the relative monohull. Comparisons between simulations, both linear and nonlinear, and experimental results are presented in term of heave and pitch motions, evaluated for two different wave steepness.