Ship forward speed loss minimization using nonlinear course keeping and roll motion controllers

Ship forward speed can be significantly reduced by the added resistance generated by the effect of wave motion. Controlling the ship motions using the actuators available can reduce this speed loss. The use of ship rudder for both steering and roll reduction is considered here with the aim of reducing speed loss. The added resistance due to the wave effects is calculated by the extended radiated energy method and the added resistance in calm water, caused by steering, is also included. In order to judge the speed loss that occurs, a performance index is proposed to quantify the percentage of speed loss. A control system based on the sway-yaw and the roll dynamics is proposed to reduce the ship speed loss. The controller comprises two sliding mode controllers developed using ship dynamic models. Finally, an example of the control of a navy vessel with two rudders is presented to demonstrate the effectiveness of the proposed solution.