The steering and course keeping qualities of high-speed craft and the inception of dynamic instabilities in the following sea

Small high-speed craft are the most vulnerable to the severity of the sea: achieving a design which pairs good performance and acceptable levels of safety is not a trivial task. The seakeeping and manoeuvrability of these vessels play a crucial role in following sea conditions: dynamic instabilities, namely broaching-to and surf-riding, are more than a rare eventuality and threaten the survivability of the vessel and the life of the mariners. This study investigates the effects of the steering qualities on the broaching-to behaviour of a high-speed craft when it is sailing in following and stern-quartering waves. The motions and loads of the vessel are simulated by means of a 3D time domain blended potential flow boundary element method (BEM), validated using captive model tests in regular waves carried out at the Seakeeping and Manoeuvring Basin (SMB) of MARIN. The hull directional stability and turning ability of the high-speed craft were artificially modified, showing that an increase in the directional stability as well as in the effectiveness of the steering can be beneficial to avoid the inception of broaching-to, but they have different consequences on the dynamics of the vessel's loss of control. •A potential flow model (panel method) was validated and corrected for the prediction of the manoeuvring loads of a high-speed craft in the following sea. This task was accomplished carrying out numerical and experimental captive model tests in following and stern-quartering regular waves.•A vessel with enhanced steering effectiveness is less vulnerable to the broaching-to inception in the following sea. However, a greater steering force might increase the risk of capsize in stern-quartering sea.•An increase of directional stability has a limited effect on the inception of broaching-to in the following sea when compared with an increase of steering force.•A combination of enhanced course keeping and turning ability allows an optimal level of dynamic stability in the following sea. This is preferable because the modifications of the stern appendages configurations, aimed to improve the ship manoeuvrability, are often limited by other design constraints.