Increasingly warships such as frigates and Offshore Patrol Vessels (OPV) are being used at slow-speeds either in constabulary duties or as part of a “poise and persistence without commitment” role to support other military activities. When operating in this way for long periods the main engine (ME) can be deleteriously affected and may require occasional loading-up to avoid coking etc. To avoid this, auxiliary motors are often provided to allow main engines to be rested to avoid maintenance related running hours. Sizing the motor is a trade-off between parameters such as: available Diesel Generator (DG) set power, together with the operational needs of the vessel, the fuel consumption, and emissions. This paper explores the solution space for the 90m Venator frigate design by studying a range of motors and converters of different ratings to see how these parameters are affected and to identify those which are most important to the ship’s roles. Military issues such as the crash-stop, lower noise, and heat signatures, loiter with persistence and redundancy are
also explored together the vulnerability benefits. Operating in the littoral with a motor drive leads to less noisefrom main engines and gearboxes and makes better use of the ship’s DG sets which can be acoustically isolated. The upkeep benefits and the greater range of reversionary modes for propulsion should individual equipment, such as the controllable pitch propellers (CPP) are also addressed.