Fuel-efficient rudder and propeller control allocation for marine craft: experiments with a model ship

We derived a control allocation algorithm for low-speed marine craft using propellers and rudders. Active use of rudders has advantages in a low-speed operation by decreasing the need for propeller power and fuel. However, at low speed, a rudder is effective only for positive thrust. This complicates the thrust allocation problem which can no longer be solved by convex quadratic programming. In fact, the existence of local minima introduces discontinuities in the commanded thruster signals even if the desired control force is continuous. Discontinuous signals cause excessive wear on the thruster system and must be avoided. This paper suggests an analytic, 2-norm optimal method that can ensure continuity of the solutions. Being analytic, however, its limitation is the capability of handling only configurations where one single thrust device is subject to sector constraints at a time. Experiments with a model ship illustrate the potential for fuel saving. For this particular vessel, the energy consumption was halved. An output feedback tracking control law with integral action was simultaneously derived and analyzed. Semiglobal ship controllers like this one rely on the yaw rate being bounded, and an admittedly conservative method for determining this upper bound was proposed.