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.

Principle of a COGAG propulsion system

Using one or two gas turbines has the advantage of having two different power settings. Since the fuel efficiency of a gas turbine is best near its maximum power level, a small gas turbine running at its full power is more efficient compared to a twice as powerful turbine running at half power, allowing more-economical transit at cruise speeds.

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.

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