Combined gas or gas is a propulsion

Marine propulsion

Marine propulsion is the mechanism or system used to generate thrust to move a ship or boat across water. While paddles and sails are still used on some smaller boats, most modern ships are propelled by mechanical systems consisting a motor or engine turning a propeller, or less frequently, in jet...

 system for ship

Ship

Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

s using gas turbine

Gas turbine

A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....

 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

Clutch

A clutch is a mechanical device which provides for the transmission of power from one component to another...

 allows either turbine to be selected, but there is no gearbox

Transmission (mechanics)

A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...

 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 cruiser

Slava class cruiser

The Slava class cruiser, Soviet designation Project 1164 Atlant, is a large conventionally-powered warship, currently operated by Russia.-Design:...

s, and in the Royal Navy Type 42 destroyer

Type 42 destroyer

The Type 42 or Sheffield class, are guided missile destroyers used by the British Royal Navy and the Argentine Navy. The first ship of the class was ordered in 1968 and launched in 1971, and today three ships remain active in the Royal Navy and one in the Argentinian Navy...

s.

HMS Exmouth (F84)

HMS Exmouth (F84)

HMS Exmouth, pennant number F84, was a Royal Navy anti-submarine warfare frigate of the Blackwood or Type 14 class.Exmouth became the first major warship to be powered by gas turbine engines alone when, in 1966, she was taken in hand and was refitted with a combined gas or gas arrangement...

of the Royal Navy was converted to COGOG propulsion as a test bed for its use in later British ships.