Moving water has a lot of energy. Just six inches (15 centimeters) of running water can wash away a small car. A water wheel is an early, successful attempt to harness the energy of running water and put it to human use. A water wheel is a wheel with paddles or buckets attached to it. Water flows over the paddles or buckets, causing the wheel to move and allowing energy to be transferred in order to do work.
Water wheels have been in use for millennia. In fact, there is evidence that they were in use as early as 4000 BC. A Greek writer, Antipater, mentioned a water wheel in one of his works. Evidence of water wheels can be found all over the world, including in Italy, India, China and the Americas. No matter where they are found, water wheels are commonly classified in three categories: the horizontal wheel, the overshot wheel and the undershot wheel.
A horizontal wheel is thought to be the earliest form of water wheel. The wheel itself is usually placed on the floor of a mill or wherever it is being used. Rapidly running water is then allowed to wash over it, hitting the paddles and causing the wheel to turn. The turning wheel is attached by a vertical axle to the upper millstone, which turns and grinds the wheel against a stationary lower millstone. This is the least efficient form of water wheel.
The undershot water wheel is more efficient. This type of vertical wheel can be used where the landscape is flat. The force of the water pushes the paddles below the wheel, causing it to revolve. A variation of this type of wheel, though sometimes classified as its own type, is the breastshot wheel. With this type of wheel, the water hits the buckets close -- or just above --the center of the wheel.
Using about 63% of the water’s energy, the overshot wheel is the most efficient type of water wheel. A backshot wheel is one varitation of this type. This vertical wheel uses the water pouring on top of the wheel to move it. Thus, not only does the wheel move with the power of the water, but it also takes advantage of the power of gravity. Oftentimes, the water used to move these wheels will have to be dammed or diverted in some way. Even though this type of water wheel is the most efficient, its construction can also impact the environment the most.