The article presents the results of preliminary tests concerning the determination of operating characteristics of the examined engine in steady states. The study concerned main propulsion engines of an Oliver Hazard Perry class frigate operated by the Polish Navy. The propulsion system of that frigate consists of two LM2500 marine gas turbine engines, reduction gear, shaft line and a controllable pitch propeller (CPP). The tests were conducted in the ship's engine room when the ship was at sea. Operating characteristics were determined on the basis of the measurements recorded over the entire load range of the engine, from minimum to rated load. During the tests, the load of both engines of the ship was simultaneous and balanced. In order to facilitate the comparison of the operating characteristics of the tested engines, as well as to make a reference to various weather conditions, the data obtained was reduced to standard day conditions. For the aforementioned characteristics approximation relations were determined using statistical analysis. The relations obtained may be utilized in technical diagnostics and for further studies concerning the engines examined in the present research study.