Grain mill with undershot waterwheel
The water only drives the lower blades of the waterwheel. The waterwheel only turns as a result of the pressure caused by the current. The term “undershot” refers to this aspect.
Since the flowing water is the only driving force, only the kinetic energy of the flowing water is used. If the water flows rapidly, it drives the wheel faster.
The inflow is often controlled by means of small weirs that channel a greater or smaller amount of water into the millrace or stream, depending on the water level in the stream or river.
If the waterwheel is operated in the main canal and not within a millstream or millrace of its own, pulleys with chains and rollers are often used so that the waterwheel itself can be raised or lowered.
Having a separate millrace is an advantage with respect to maintenance work, since it can be drained.