**5. Discussion**

The thrust loading reduction in four listed sea states are favorable especially in the linear wave, but the reduction of torque on the rotor is not as satisfactory as the thrust; some findings are given in this section to discuss the noteworthy torque fluctuation.

#### *5.1. Blade Section Velocities*

In harsh winter sea states, the turbine moves heavily along the wave in the wave trough. The inertial velocity of the turbine, **U**T, is coupled with the wave–current velocity field and transferred to the local rotating blade coordinate system [24]. Two velocities, *U*1 and *U*1*y*, are obtained to modify the horizontal inflow and the apparent rotational inflow speed to calculate the relative velocity of the blade section. The new relative velocity results in a larger angle of attack, which generates a smaller lift force but a larger drag force on the blade section as shown in Figure 11. Therefore, the direction of the in-plane force is changed, which means the torque on the rotor has negative values and the blade is stalled. A blade profile that performs better than NRELs814 in larger angles of attack, or setting a pitch angle can be applied to avoid this phenomenon. However, this remains to be studied in the future.
