Undershot

Undershot refers to a type of water wheel in which water flows beneath the wheel and pushes against its lower portion, causing rotation. The wheel is placed in a stream so that its paddles are submerged and water is directed to strike the underside as it passes by. This simple configuration often uses a flume or millrace to deliver water to the wheel and a tailrace to carry it away.

Compared with other wheel types, undershot wheels rely more on the velocity of flowing water than on vertical head. They are well suited to rivers or streams with strong flow but little ambient elevation change. However, they are generally less efficient than breastshot or overshot wheels, especially when head is available, and they can be more susceptible to debris damage and wear due to direct water contact with the lower wheel surface.

Historically, undershot wheels were common in medieval and early modern Europe for tasks such as grinding grain and driving saws. They were easier and cheaper to construct than higher-head alternatives, making them practical in many rural settings. With the rise of steam engines and, later, hydraulic turbines, their use declined, though some undershot wheels have been preserved in mills and open-air museums for demonstration and heritage purposes.

Design considerations for undershot wheels include paddle shape and spacing, wheel size, the depth of submersion, and the management of debris in the inlet and outlet channels. Their performance depends on steady, abundant flow and low head, which define their place in the range of traditional water-wheeling technologies.