Combined diesel and gas (CODAG) 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.
CODAG system consists of diesel engines for cruising and gas turbines that can be switched on for high-speed transits. In most cases the difference of power output from diesel engines alone to diesel and turbine power combined is too large for controllable-pitch propellers to limit the rotations so that the diesels cannot continue to operate without changing the gear ratios of their transmissions. Because of that, special multi-speed gearboxes are needed. This contrasts to combined diesel or gas (CODOG) systems, which couple the diesels with a simple, fixed ratio gearbox to the shaft, but disengage the diesel engines when the turbine is powered up.
Such a propulsion system has a smaller footprint than a diesel-only power plant with the same maximal power output, since smaller engines can be used and the gas turbine and gearbox don't need that much additional space. Still, it retains the high fuel efficiency of diesel engines when cruising, allowing greater range and lower fuel costs than with gas turbines alone. On the other hand, a more complex, heavy and troublesome gearing is needed.