This paper aims to discuss the factors which led to the selection of the Spey as a marine gas turbine, and the market forces which have made an uprating of the simple cycle unit necessary, and which are now leading to the conception of an advanced cycle version of the gas turbine at even higher power and efficiency. The operational characteristics of warships using these Spey units are compared with those of ships using other propulsion systems. While this paper stands on its own, it can also be regarded as the first part of a series of two; the second part, to be published in Beijing during the September 1985 ASME Gas Turbine Exposition, will deal with that further possible uprating of the Spey to about 22 MW using intercooling and heat exchange to improve its thermal efficiency to about 43%.