Parametric roll is an important stability failure mode in the second generation intact stability criteria, which are currently under development by International Maritime Organization (IMO). The roll restoring arm, which is one of the dominating factors for the prediction of parametric roll, should be calculated preciously. In this paper, firstly, the effects of the radiation and diffraction forces on the roll restoring arm (GZ R&D ) under different heeling angles and wave amplitudes are studied based on the body exact method, in which all the hydrodynamic forces are calculated by integrating wave pressure up to the wave surface. Secondly, the effects of radiation and diffraction forces on parametric roll are studied by the direct simulation using a coupled heave-roll-pitch model based on the three dimensional mixed source method. Finally, by taking one passenger ship as an example, the calculation accuracy of the body exact method is compared with the commonly used body linear method. The results show that the effect of radiation and diffraction forces on righting arm variation has nonlinear relationships with heeling angles and wave amplitudes, and the body exact method can provide more accurate prediction results for parametric roll in head waves than the body linear method. Therefore, the nonlinear effects of instantaneous roll angles and wave amplitudes on GZ R&D variation in waves should be considered for the prediction of parametric roll in head waves.
