The undershot wheel is probably the oldest type of waterwheel, having been developed over two thousand years ago. Mounted vertically on a horizontal axle, it has flat boards, called floats, mounted radially around the rim. It is turned by the impact of water striking these float boards as it flows under the wheel.
Undershot wheels are not very efficient, but they are fairly simple to build and can be placed into a rapidly flowing stream with a minimum of site preparation. When placed in a carefully channeled raceway, however, their efficiency increases somewhat. Small diameter undershot wheels, known as flutter wheels, can run at over 100 revolutions per minute and were the most common type of wheels to run the thousands of "up and down" sawmills that built early America.
The overshot wheel is a much more efficient wheel than the undershot; it can harness over 85% of the potential energy in falling water. However, it is more difficult to build, requires careful site preparation, and will not operate in many locations.
Mounted vertically on a horizontal axle, it has angled troughs—also called buckets—mounted all around the rim. Water fills these buckets from above, making one side of the wheel heavy and causing it to turn as the water in the buckets falls. At the bottom the buckets are in an inverted position so that they spill out the used water, which flows gently away. While the water filling the buckets has a slight force upon the wheel, the overshot is primarily a gravity wheel in that it is the dead weight of water in the buckets that causes it to turn.
This large diameter wheel can generate a great deal of torque—twisting power. Its size means it cannot turn very rapidly, however, and so machinery that needs to run at higher speeds must use gears to increase the speed of rotation. But gears add cost, increase maintenance, and rob some power.
Overshot wheels cannot turn when the water cannot freely flow away from them, a condition known as back-water, or wading. Finally, many sites cannot accommodate an overshot wheel because there is not enough head, or drop to the water, to reach the top of the wheel.
The breast wheel, which was developed in the late middle ages, is somewhat of a compromise between the inefficient undershot wheel and the highly efficient but limited overshot wheel.
Like the overshot wheel it has buckets on its rim, but they face in the opposite direction. Water fills the buckets at the mid-point—or breast—of the wheel. The water's dead weight causes the wheel to turn. Often a concave shell, also known as a breast, is fitted near the underside of the wheel to keep water in the buckets until it reaches the bottom of the wheel, thereby increasing efficiency. The breast wheel can operate over a wider variety of water levels than can the overshot wheel, and does better in backwater conditions. Its large diameter requires gears to increase rotational speed when needed.
Versatility and moderate efficiency made the breast wheel the workhorse of American industry in the early 1800s.
The tub wheel in its simplest form is just a small undershot wheel mounted horizontally on a vertical axle. This configuration was developed in the early middle ages and was called a Norse wheel. Turned by the impact of a stream of falling water striking its paddles, its efficiency was increased somewhat by building a bottomless wooden tub around it. This tub harnessed more of the potential energy of the water before the water fell below the wheel.
The tub wheel is easy to build and maintain, and is fairly dependable. While it is not very efficient and does not generate a great deal of power, its relatively small diameter (usually less than six feet) allows it to operate at moderately high speeds, often eliminating the need for gearing. Sometimes it could be directly connected to the machinery it was to run. Small neighborhood mills often made use of tub wheels in the 18th and 19th centuries.
Outward-Flow Reaction Wheel
In the early 1800s many Americans were experimenting with different new waterwheel designs. One of these men was Calvin Wing of Maine, who patented this design in October 1830.
The reaction wheel is made of a hollow iron disk with a large hole on one side to allow pressurized water in from a penstock,and six angled holes on the rim to allow water to exit. The force of water squirting through these six angled jets causes the wheel to turn in reaction to the force of the exiting water.
The reaction wheel, in some ways the predecessor of the modern turbine, operates on water pressure. (The pressure is obtained by confining the water as it falls). It has moderate efficiency, can operate over a very wide range of water levels, and runs fairly well in flooded back-water conditions.
The wheel's cast iron construction makes it extremely durable; it will not rot like a wooden wheel. It is also compact, generating much power as well as achieving high speeds while taking up very little space and eliminating the need for costly gearing. It requires precision manufacturing and installation, and thus is somewhat expensive compared to a simple wooden wheel. The 1830s miller who bought one was taking a risk and making an investment in new technology.
Modern Hydroelectric Turbine
In the late 20th century, and probably throughout the 21st century and beyond, water power still has a place in America. Today, instead of generating mechanical energy to be used on-site, the energy of falling water is frequently converted to electrical energy to be transmitted for use elsewhere.
The turbine is a carefully engineered and highly efficient means of harnessing water power. Water is smoothly funneled into a restricted space where it turns the precisely designed impeller blades of the turbine, much as a breeze turns an unplugged electric fan. On the same axle as the turbine is a compact gear box and electrical generator. After the turbine has extracted most of the water's energy, the water flows gently downstream; very little energy is wasted by turbulence.