A power system for a marine vessel comprises an engine and a controllable pitch thruster (14) drivably coupled to the engine. A speed of the controllable pitch thruster various with a speed of the engine. A control unit (16) receives a thrust command value indicating a thrust required from controllable pitch thruster (14). The control unit (16) determines a current speed of the controllable pitch thruster (14) and calculates a pitch of the controllable pitch thruster (14) that produces thrust in accordance with the thrust command value at the current speed of the controllable pitch thruster. The power system allows for operating the engine at variable speeds to allow for a reduction of the engine speed under partial load conditions.