Abstract: Ships can experience serious difficulties in keeping a straight course when sailing in stern-quartering seas. Design modifications like the addition of stern passive fins, or the modification of active control surfaces, are common solutions to improve the ship course-keeping. However, the success of such design modifications depends on the delicate balance between the excitation forces induced by the waves on the appended hull, the stabilization forces provided by the lifting surfaces as appended fins, and the steering forces provided by the control surfaces. This research investigates which of these aspects of a ship design play a concrete role in improving the ship course-keeping in waves. The study is carried out with the intention of looking at the different behaviors of the ship originating from different stern appendages configurations. Three modifications of stern appendages on three different ship hulls were investigated in various mild-to-rough sea conditions. The behavior of the vessels were simulated using a time domain, boundary element potential method, with the addition of semi-empirical formulations for the modelling of the stern lifting surfaces. The simulations were carried out in long crested irregular waves at three different direction, using the JONSWAP spectrum. The results showed that although larger stern appendages improve the directional stability of relatively large and slow vessels, in most cases they worsen their course-keeping ability, increasing the yaw motions. For smaller and faster vessels instead, passive and active fins tend to improve the course-keeping, because at high speed the lift provided by the appendages stabilizes the vessel. This effect is compensated by the wave excitation force at lower speed. Similarly to yaw, the roll motions increases with larger stern appendages.
