Water wheels come in four basic types, with the final choice determined mainly by the head of water available.
With a head of 3m or more the overshot or pitchback wheel is the obvious choice.
Traditional overshot wheels could achieve about 70% efficiency, while the slightly more complex pitchback design could reach 90%, thanks to the exhaust water flowing away in the same direction as the rotation of the wheel.
With a lower head still the breastshot wheel was the answer. Traditional designs rarely achieved more than 50% efficiency, but towards the end of the age of the waterwheel improvements in blade design and more efficient use of the available water supply significantly increased that fi gure.
Undershot wheels are the least efficient, working off very low heads and needing to be sited close to large, swift-flowing streams or rivers. They rarely achieved more than 20% efficiency.