Unfavourable environmental conditions associated with an insufficient stability margin adopted in the design process may determine ship capsizing. One of the most critical situations is generated when the ship moves on longitudinal waves, at parametric resonance condition. In this paper, the theoretical and experimental investigations on transverse stability of a cargo ship on regular longitudinal waves are considered. During the seakeeping model tests on longitudinal waves, the occurrence of induced roll motions were observed and measured. The restoring moment on longitudinal waves was determined based on theoretical and experimental methods. A computer code has been implemented in order to calculate the main parts of the restoring moment i.e. Froude-Krylov, diffraction and radiation components. In order to reproduce the real physical phenomenon, captive and semi-captive model tests were correlated. The radiation and diffraction forces and moments and the restoring moment on regular longitudinal waves were measured. The agreement of the calculated solutions with the experimental results indicates the possibility of using the adopted hydrodynamic model in the design process.

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