The dynamic stability of a ship can become more and more of an issue as the ship speed increases. While many different forms of dynamic instability are known, often very little is known about their cause and only a limited amount of research exists. Therefore, an incident of dynamic instability observed during model tests of a semi-displacement vessel in calm water is investigated. The incident occurred at a very high Froude number of Fn = 0.9. Despite of the restrain by the gimbal yaw and sway motions develop followed by a roll motion in the end. First the approach used here is outlined. The numerical method and its extensions to consider sway, roll and yaw motions are introduced briefly. Then the computations carried out are described. Afterwards, the obtained results are presented and discussed.