Combined gas or gas

Having previously pioneered the combined diesel or gas (CODOG) system, in 1968 the Royal Navy converted an old frigate, HMS Exmouth, to COGOG propulsion as a test bed for use in later ships.

Cruising power was provided by two Rolls-Royce Proteus gas turbines, originally designed for turboprop airliners, each rated at 3,250 shaft horsepower (2,420 kW).

[2] However, they were beaten into service by the Canadian Iroquois-class destroyers, which were powered by two Pratt & Whitney FT4A2 gas turbines creating 50,000 shaft horsepower (37,000 kW) and two Pratt & Whitney FT12AH3 cruising gas turbines creating 7,400 shp (5,500 kW), giving a maximum speed of 29 knots (54 km/h; 33 mph).

[3] The operation of large gas turbines on ships produces a high volume of very hot exhaust gasses, which can hinder onboard helicopter operations, and also greatly increases a ship's infrared signature making it more conspicuous to enemy sensors and guided weapons.

The ducting and filters required take up a considerable amount of space in a ship, and the volume of air being drawn in can exacerbate an internal fire.