For the container ship, as it was defined in ITTC Benchmark tests, the conditions were chosen for the ship to be marginally stable. Therefore, the ship experiences very large roll angles (Figures 4-6). As wave steepness becomes more linear or smaller, the difference between two numerical models decreases. However, 4DOF numerical model with static equilibrium in heave and pitch slightly overestimates the amplitude of the roll motion. 6 DOF numerical model also displays larger yaw deviations and slightly more stiff rudder turnings The difference between two numerical models in terms of maximum roll angle and with respect to wave steepness is shown in Fig. 7.