A method for operating a ship propulsion system. A setpoint rotational speed is determined for a propeller shaft and a setpoint pitch angle for an adjustable propeller on a control side based on an adjustable propeller characteristic diagram and an operator. An engine is determined based on a ship's engine characteristic diagram and the setpoint rotational speed for the propeller shaft. An actual engine operating point is determined as a function of a measured actual rotational speed and a measured actual torque, so that when the drive power is constant, the set-point rotational speed for the propeller shaft and the pitch angle for the adjustable propeller can be varied while reducing fuel consumption of the ship's engine and, when this is possible, the setpoint rotational speed for the propeller shaft, the setpoint pitch angle for the adjustable propeller and the setpoint operating point of the ship's engine are adapted.