The Three Types of Waterwheels
There are three basic types of waterwheels, each with its own advantages and disadvantages. Each type has been in use since at least Roman times, and remained remarkably stable from AD500 to the mid-1700s, when a new type, turbines, began displacing them, but their heyday is beyond the time period of this lesson.
The three types of waterwheels are the horizontal waterwheel, the undershot vertical waterwheel, and the overshot vertical waterwheel. For simplicity they are simply known as the horizontal, undershot, and overshot wheels.
The horizontal waterwheel is the only one that rotates around a vertical axle (confusing!). The undershot and overshot waterwheels rotate around a horizontal axle, like a car tire, but as you can see, receive their driving force from the water at the bottom and top, respectively.
A hybrid of the over- and undershot wheels developed some time in the later Middle Ages or colonial period as well, but it used the basic principles of the other two. This was known as the breastshot wheel that worked somewhat like the undershot wheel, but the water came into contact with the breastshot wheel at mid-height (i.e., as if it were hitting us at breast/chest height), and flows down a closely-fitting housing around the wheel. The wheel therefore partially gets the advantage of the moving water like the undershot and also the weight of the falling water, partially like the overshot.
Oliver Evans, considered the father of American millwright work, showed the three types in his 1848 treatise, The Young Mill-Wright and Miller's Guide..
For more on vertical waterwheels, see Pond Lilly Mill Restorations page.