Dynamic stall and cavitation of stabiliser fins and their influence on the ship behaviour

The lifting characteristics of stabiliser fins and their efficiency are evaluated most of the time with a static and steady approach. Usually the same is applied in numerical simulations, both in the frequency or time domain. Although this approach can be assumed to be correct when the fins are working at low angle of attack in mild sea conditions, strong non-linearity appears when reaching their limits in rougher sea conditions. Non-linearity originates from dynamic effects on lift and drag, stall or cavitation. When stall occurs while sailing, the degradation in roll motion of the vessel is such that the fins can hardly recover their lift until lower waves are encountered. Hysteresis loops are then observed in the lift slope curve. At large fin angle of attack and high ship speed cavitation will also occur, resulting in lift degradation. In the present paper experimental investigation on the dynamic behaviour and characteristics of lift and drag is presented. Results of cavitation tunnel tests are presented and compared with experimental data obtained with a freerunning model in waves, both for a high aspect ratio and a low aspect ration fin. The results of these experiments will be used in the future to obtain a more accurate description of lift characteristics under dynamic conditions and thus a more accurate prediction of roll motions.