Combined diesel and gas

Pioneered by Germany with the Köln-class frigate, a CODAG system consists of diesel engines for cruising and gas turbines that can be switched on for high-speed transits.

Some ships even have three different gear ratios for the diesel engines — one each for single-diesel and double-diesel cruises, and the third when the gas turbine is engaged.

Such installations avoid the use of a complicated switching gearbox, but have some disadvantages compared to real CODAG systems: CODAG Water jet And Refined Propeller (WARP), a system developed by Blohm+Voss as option for their MEKO line of ships, also falls in this category, but avoids the above-mentioned problems.

[1] The idling water jet does not cause drag, and since its nozzle can be placed further aft and higher it does not affect the size of the propellers.

The Swedish Stridsvagn 103 utilizes a diesel engine for slow cruising and aiming, and a gas turbine for additional power.