Combined gas and gas
Combined gas turbine and gas turbine (COGAG) is a type of propulsion system for ships using two gas turbines connected to a single propeller shaft. A gearbox and clutches allow either of the turbines to drive the shaft or both of them combined. Marine usage of COGAG systems are similar to those found ashore. Description
A COGAG system consists of two gas turbines, each connected to a reduction gearbox. These are each attached to a coupling with both connected to larger gearbox and then to the ship's propeller. Advantages and disadvantages
Advantages of the system include a large degree of automation along with quick startup time, they are easier to silence and protect from shock. Compared to combined diesel and gas (CODAG) or combined diesel or gas (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. Issues with COGAG systems include their complexity and gearbox issues and high fuel use. List of COGAG ships 
 guided-missile destroyer (Indian Navy)
 (aircraft carrier) (Indian Navy)
Type 22 frigate (Batch 3)  (Royal Navy)
 (Royal Navy)
 (Italian Navy)
 (Japan Maritime Self-Defense Force), and subsequent destroyer classes
 (Japan Maritime Self-Defense Force), helicopter carrier
 (Japan Maritime Self-Defense Force), helicopter carrier
Type 055 destroyer (People's Liberation Army Navy)
 (Russian Navy)
 (Republic of Korea Navy)
 (Royal Norwegian Navy)
 (United States Navy)
 (United States Navy)
 (United States Navy)

Citations

References
 
 
 

Marine propulsion