CODAG stands for Combined Diesel And Gas and refers to a type of propulsion system for ships. Usually, it is used in very high end yachts or (mostly) small to medium sized warships. It consists of two types of engine, both Diesel and gas turbine engines as the name suggests. The reason for having both is to allow the ships to cruise economically on Diesel engines, and when high sprint speed is required, engage both the Diesels and the high-power gas turbine. The difficulties of combining the power output from Diesels (high power, lower speed) and the turbine (high power, high speed) are usually solved by using reduction gearboxes on one or both types of engines which have differing power ratios and ratings. Sometimes the two engines are used to turn two different sets of shafts and propellers. The Queen Mary 2 uses CODAG, and uses a different system whereby the engines are used to turn generators, with electric motors driving the ship. See also: CODOG, COGAG, COGOG, COGAS, COSAG, COSEG, CODAD.