waterwheel

A waterwheel is a device for transforming the energy of moving or falling water into rotational mechanical power. It consists of a large wheel fitted with paddles, buckets, or blades arranged around its rim. When water strikes the paddles, the wheel turns, driving a shaft through a gearing arrangement to operate mills, pumps, or other machinery.

In operation, water is directed to strike the wheel in a mill race or headrace. The resulting torque turns the wheel, which can be connected to a vertical or horizontal shaft. The power output depends on water flow, water head, and wheel design. Overshot wheels use water entering at the top and are typically the most efficient with significant head. Undershot wheels are driven by water flowing at or near the bottom and are simpler but less efficient. Breastshot wheels engage water at mid-height, offering a compromise between head and flow.

Waterwheel designs are commonly categorized as overshot, breastshot, or undershot, based on where the water acts on the wheel and how energy is imparted to the rotating drum. Overshot wheels tend to deliver high torque with higher head; undershot wheels rely more on the kinetic energy of flowing water; breastshot wheels balance these factors and can work across a wider range of conditions.

Historically, waterwheels have ancient origins and were widely used in Greece, Rome, and medieval Europe, as well as in Asia, for grinding grain, sawing wood, and pumping water. With the development of the water turbine in the 19th century, their use in large industries declined, though they remain in use in micro-hydro installations and are preserved at many heritage mills.

Today, waterwheels are valued for simple, durable operation in suitable sites, but they require adequate head and flow, are generally less efficient than modern turbines, and can be affected by sediment and debris.