A water wheel is a desirable water feature, so why not make some volts too

Depending on its size, a water wheel can grind grain for homemade flour, power a sawmill to make lumber, run an old fashioned machine shop, make a foundry, or process flax to make your own fine linen paper. You can even grind some grain for your alcohol still.

A water wheel makes a fine water feature. But you can also make a water wheel to do a lot neat stuff too. If you’ve got the room, and if there’s enough water, and if it’s big enough. Take a look at the following video.

In any case, I don’t have to explain what a water wheel is, you already know this part. The question is, which one do I build? That will be largely determined by the local landscape. But I do think it would be helpful to know some water wheel ABCs. Okay, first off, there’s two main versions: the vertical and horizontal. A vertical water wheel transfers rotational power through the wheel’s axel direct, or to pinion and ring gears or through belts and pulleys.

Horizontal water wheels usually drive the load directly as in turbine generators. Interestingly, the modern hydroelectric turbine of today is a direct descendant of the horizontal configuration. However, we’ll discuss vertical wheels because most horizontal wheels are mounted beneath the floor, and what’s the point in having a water feature like this if you can’t see it.

Anyway, a water wheel can be one of four basic types. And here they are:

1. The Undershot
2. The Backshot or Pitchback
3. The Breastshot
4. The Overshot
5. Trash Rake
6. Wheel Efficiencies

WARNING
But first, I would be remiss if I failed to mention the following to those who may attempt to build one of these devices. If you intend to construct a water wheel of any size, you should consult someone whose in the water wheel know, has experience, or understands a water wheels dynamics.

You should at the very least, find someone with mechanical engineering ability, has construction knowledge, someone who understands the principals of stress and pressure on materials; perhaps someone with hydraulics understanding. These are extremely important issues. Without adequate knowledge of such principles, and proper construction techniques using the correct materials, someone can be seriously injured or even killed.

For example, let’s consider metal verses wood. While a wood wheel is attractive, maybe even nostalgic, wood has inherent shortcomings. Simply put, a metal water wheel is superior. For instance, a wooden wheel will swell due to water logging, when this happens, it will go out off balance.

Also, in colder climates, wheel stoppages in icy conditions are much more likely. Simply put, wooden wheels will freeze up. Whereas, metal buckets and blades tend to be warmed by the water keeping them clear of ice.

The Undershot Wheel
Also known as a vertical stream wheel, it’s easy to understand why this water wheel design is called ‘undershot,’ because the water strikes the blades at the bottom of the wheel. However, it is the least efficient of the four. Nevertheless, several variations of this design have improved performance. The Poncelet, Sagebien and the Zuppinger. Each of these wheels produce greater shaft efficiencies. See the Undershot: i.e. the Poncelet, Sagebien and Zuppinger illustrations right.

For instance, the hydraulic dynamics of the Poncelet curved bucket design is more than double the efficiency of the standard undershot blade design. In fact, it may be the most practical water engine for low heads where ample water flow is available. The curved bucket design engages the water much the same as a marine propeller in water. This way there is zero impact as on flat blades. This alone accounts for its increased efficiency.

An undershot wheel’s impact on the environment is at worst; negligible, and therefore does not represent a serious change to the flow of a stream or river. The only drawback is their lower efficiency. But because they are the best suited for shallow streams in open country, they are simpler to build thus less expensive, they are most often used.
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The Backshot or Pitchback Wheels
A vertical system, the backshot wheel AKA pitchback wheel, is really a variation of the overshot wheel. Only here, water drops just behind the wheels crest allowing its weight and the velocity of the water flowing underneath to do its work. First, as the falling water strikes the blades paddles or buckets, energy is released. Second, the waste water continuing on increases the energy delivery. See the Backshot illustrations left.

Like the backshot, water dropped slightly behind the top strikes the wheel on the back downward rotation side; it is thus heavier than the empty front side. Hence, the water’s weight and velocity causes the wheel to rotate. Because the water drops jut behind the wheel’s top, it is said to be an backshot or pitchback wheel.

This type will continue to function even if the water level rises well above the axle. On the other hand, such high water could destroy an undershot, or at best, completely stop it. Consequently, the backshot wheel is well suited for streams that tend to experience flooding during rainy seasons. As a matter of fact, the greater the current, the more axel power will be produced.
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The Breastshot Wheel
Also vertically mounted, the breastshot is a water wheel that is forced to rotate as water stricks the buckets near or just above the wheel’s center. This type of wheel is preferred where a steady high-volume flow of water is available. Depending on the width of the stream, breast wheels of various widths can be made to take advantage of greater available energy.

This type of device is the most commonly used in the United States and generally considered the power behind America’s industrial revolution. See the Breastshot illustrations right.

It is much more efficient than the undershot type, but less than the overshot wheel. Be careful not to confuse the breastshot with the overshot, because they are different. And don’t confuse the flat paddles of most undershot wheels with the blades of a breastshot or an overshot. In the later, the blades similar to the Poncelot design and are actually buckets that slice through the water like a boats propeller.

The exception is the breastshot needs an efficient trash rake, and a properly positioned masonry penstock to channel the water to the wheel. An apron is positioned in the penstock in order to uniformly direct the waterfall onto the wheel’s buckets. Like the backshot, this water wheel is twice as efficient as; say the undershot, because not only the weight of the water falling in the buckets, but the flowing waste water under it catching the buckets spin the wheel.
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The Overshot
Unlike the backshot, the overshot directs the water slightly over the top of the wheel into the buckets in the direction of rotation. As the water is collected in the buckets, the wheel gets heavier and heavier on that side, then the next bucket is filled, and the next and the next, the great weight of the water rolls/spins in that direction. See the Overshot illustrations left.

The overshot wheel can actually use all of the water in a stream, unless some of the water runs elsewhere.A precisely designed wheel is essential as the mechanical power delivered to the axel is directly related to the material size of the wheel and the head flow. Consequently, careful engineering to maintain balance and structural strength is critical.

Overshot wheels are ideally suited for mountainous terrain. Hilly topography is ideal for a dam. In this case, an open flume water way or long penstock can be constructed to bring sufficient water for power. With proper damming, a water wheels full potential can be harnessed from the falling water. Turning a water wheel for maximum horse power takes sufficient quantities of water.

The down side is that considerable engineering and investment is required. The upside is that the full potential of falling water can be realized with the overshoot.
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Trash Rake
Simply put, a trash rack is a device or arrangement to catch flotsam before it encounters the a water wheel. It is strategically placed at the head of the penstock flume or waterway where the main current is apt to carry debris such as logs, animals and water borne weeds and assorted flotsam. This device protects water wheels, water ways, turbines, flumes, mill races, sluiceways and penstocks from damage and destruction during seasonal floods.

A trash rake can be as simple as a row of closely spaced wooden stakes driven into the stream bed’s bottom. This is rather crude and performance is usually poor. A water wheel can jam and with flooding - destroyed. Carefully positioned metal grating and wire screens provide the best protection against debris damage

Large trash rakes are clearly visible as part of the waterscape at commercial hydroelectric plants, such as Boulder dam. These devices prevent large objects such as boats from entering the penstocks and ultimately the huge turbines. Large metal screens set at an angle provide self-cleaning from the action of the currents flow.
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Wheel Efficiencies
The Undershot wheel's efficiency is only 20% and the least of the four.
The Backshot or Pitchback wheel's efficiency is 90%. It is the most superior water wheel of all.
The Breastshot wheel's efficiency while onlt 50% is a nice system.
The Overshot wheel's efficiency at 70% is second highest.
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Here’s a water wheel site with neat information.
A good Glossary of Water Wheel Terms

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