Yu Bi
Ansys Employee

Hi, you can have a check of the stiffness of the cylinder in the oscillating direction to see if it is too small. Besides, if the gravity center is too high to make the hydrostatic righting moment negative, the cylinder also cannot return to its original position. You can add in additional damping manually to cover the viscous effect if you think the current radiation damping is not enough. Hope these give you some hits to fix the problem.