The nonlinear vertical responses of a containership advancing in regular waves are investigated experimentally. The paper presents the results of an experimental program carried out in a seakeeping tank with a model of the ITTC S-175 containership in head regular waves. All the wave frequency ranges of interest were tested, and for each wave frequency several wave amplitudes were used ranging from small to large amplitude. In this way the influence of the wave amplitude on the nonlinear characteristics of the responses was assessed. The measured responses include the absolute and relative motions, vertical accelerations, and cross-sectional loads at midship and ¼ Lpp from the forward perpendicular. Strong nonlinear effects were detected, especially on the loads, which can be identified by the variation of the transfer function with the wave amplitude, the asymmetry of the time histories, and the presence of higher harmonics.

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