Combined gas or gas
Combined gas or gas (COGOG) is a propulsion system for ships using gas turbine engines. A high efficiency, low output turbine is used for cruising speeds with a high output turbine being used for high-speed operations. A clutch allows either turbine to be selected, but there is no gearbox to allow operation of both turbines at once. This has the advantage of not requiring heavy, expensive and potentially unreliable gearboxes.
The reason that a smaller turbine is used for cruising is that a small turbine running at 100% power is more fuel efficient than a bigger turbine running at 50% power.
The system is currently used in the 3 ships of the Russian Navy's Slava-class cruisers, in the Royal Navy Type 42 destroyers, the Japanese Maritime Self-Defense Force's Hatsuyuki-class destroyers, the Royal Canadian Navy Iroquois-class destroyers, and the Dutch Navy Kortenaer-class frigates (on which the Greek Navy Elli-class frigates are based).
HMS Exmouth of the Royal Navy was converted to COGOG propulsion as a test bed for its use in later British ships.