Combined steam and gas (COSAG) is a propulsion system for ships using a combination of steam turbines and gas turbines to power the shafts. A gearbox and clutches enable either of the engines or both of them together to drive the shaft. It has the advantage of the cruising efficiency and reliability of steam and the rapid acceleration and start-up time of gas. This system was mainly used on first-generation gas-turbine ships such as the Royal Navy's County-class destroyer and Tribal-class frigate.
The Spanish aircraft carrier Dédalo also used it.