The main engine on Challenge is a three cylinder 1150 hp triple expansion reciprocating steam engine. It was built by Alexander Hall and Co Ltd, Aberdeen. It drives a 10 ft 6" or 3.2 meters diameter propeller that weighs 2 tonnes.
The development of this type of engine was important for its use in steamships as by exhausting to a condenser the water can be reclaimed to feed the boiler, which is unable to use seawater.
Additionally by maintaining a vacuum in the last stage and in the condenser more work can be extracted from the steam and efficiency improved over land-based steam engines where they exhausted their steam to atmosphere. Land-based steam engines could exhaust much of their steam, as feed water was usually readily available. Prior to and during World War II, the expansion engine dominated marine applications where high vessel speed was not essential. It was however superseded by the steam turbine where speed was required, for instance in warships and ocean liners. HMS Dreadnought of 1905 was the first major warship to replace the proven technology of the reciprocating engine with the then novel steam turbine.
An animation of a triple expansion engine. High pressure steam (red) enters from the boiler and passes through the engine, exhausting as low pressure steam (blue) to the condenser.