To improve fuel efficiency in a ship propulsion system having a waste heat recovery system.

A waste heat recovery system 11 includes a turbine 18 and a generator 20 for supplying electrical energy from waste heat to an electrical network 5 of a ship. A shaft generator/motor 10 is coupled to a propeller shaft 3, and is electrically connected to the electrical network 5, the electrical energy from the electrical network 5 is converted to mechanical energy for driving the propeller shaft 3 in a drive motor operation mode, and the mechanical energy from the propeller shaft 5 is converted to the electrical energy and is supplied to the electrical network in a generator operation mode. A controller 12 is connected to the shaft generator/motor 10, and the frequency of the electrical network 5 is controlled by adjusting operation of the shaft generator/motor 10.