The article presents a combined system of a large power ship power plant. The system consists of a piston internal combustion engine and the steam turbine system which utilises the energy transported with the exhaust gas leaving the internal combustion engine. The analysed variant of the combined cycle includes a Diesel engine and a steam turbine with a single-pressure waste-heat boiler. The numerical calculations were performed for two low-speed internal combustion engines made by Wärtsilä and MAN Diesel \& Turbo, each of an approximate power output of 54 MW. The assumptions and limits which were used in the calculations done using a specially worked out code are included. The energy optimisation of the entire combined ship power plant was done taking into account only the thermodynamic point of view, leaving aside the technical and economic aspects