Combined gas turbine and gas turbine (COGAG) is a type of propulsion system for ships using two 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....

s connected to a single propeller shaft. A 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...

 and clutch

Clutch

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

es allow either of the turbines to drive the shaft or both of them combined.

Using one or two gas turbines has the advantage of having two different power settings. Since the fuel efficiency

Fuel efficiency

Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is...

 of a gas turbine is best near its maximum power level, a small gas turbine running at its full speed is more efficient compared to a twice as powerful turbine running at half speed, allowing more economic transit at cruise speeds.

Compared to Combined diesel and gas

Combined diesel and gas

Combined diesel and gas ' is a type of propulsion system for ships which need a maximum speed that is considerably faster than their cruise speed, particularly warships like modern frigates or corvettes....

 (CODAG) or Combined diesel or gas

Combined diesel or gas

Combined diesel or gas ' is a type of propulsion system for ships that need a maximum speed that is considerably faster than their cruise speed, particularly warships like modern frigates or corvettes....

 (CODOG), COGAG systems have a smaller footprint but a much lower fuel efficiency at cruise speed and for CODAG systems it is also somewhat lower for high speed dashes.

List of COGAG ships

• Type 22 Frigate
  Frigate
  A frigate is any of several types of warship, the term having been used for ships of various sizes and roles over the last few centuries.In the 17th century, the term was used for any warship built for speed and maneuverability, the description often used being "frigate-built"...
  
   (Batch 3) (Royal Navy
  Royal Navy
  The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...
  
  )
• Invincible-class aircraft carrier (Royal Navy
  Royal Navy
  The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...
  
  )
• Hyūga class helicopter destroyer
  Hyuga class helicopter destroyer
  The are a type of helicopter carrier being built for the Japan Maritime Self-Defense Force . Two ships of the class were built to replace the two 7,000-ton Haruna-class helicopter destroyers. The new ships are the largest combatant ship operated by Japan since the Imperial Japanese Navy was...
  
   (Japan Maritime Self-Defense Force
  Japan Maritime Self-Defense Force
  The , or JMSDF, is the naval branch of the Japan Self-Defense Forces, tasked with the naval defense of Japan. It was formed following the dissolution of the Imperial Japanese Navy after World War II....
  
  )
• Neustrashimy class frigate
  Neustrashimy Class frigate
  Neustrashimy class frigates are the most modern large frigates in the Russian Navy. The Soviet designation is Project 1154 Yastreb .-Design:...
  
   (Russian Navy)