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.[1] 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 and .
The also used it.