Water Wheel

What is it?

A rotating machine that converts the energy of a stream or river into mechanical work taken off a shaft — which can drive a millstone, a saw, a forge hammer, an oil press, a pump, or even a generator.

What is it good for?

Wherever there is a steady watercourse with sufficient head or flow speed, a water wheel provides a continuous power source largely independent of the weather (unlike wind). Classic uses: grinding grain, sawing planks, powering trip hammers, fulling cloth, pressing oil, lifting water for irrigation — and at modern village scale, small-scale continuous electricity generation.

The physics behind it

The power extractable from water depends on the mass flow (flow rate, Q) and the head (H): P = ρ·g·Q·H·η, where η is the efficiency. There are two basic principles:

• Undershot wheel: the kinetic energy of the flowing water pushes the paddles dipped into the stream. It works on flat terrain with a fast-flowing, high-volume stream; the traditional flat-paddle version has low efficiency, which Poncelet improved substantially in the 19th century with curved blades.
• Overshot wheel: a flume carries the water to the top of the wheel, and the weight (potential energy) of the water sitting in the buckets turns the wheel down. This is the most efficient traditional type (efficiencies above 60% are achievable), but it requires a head greater than the wheel itself. The intermediate form between the two is the breastshot wheel.

The wheel turns slowly with high torque — most working machines need gearing (gears or belts) to step up the speed.

History

The vertical-axis watermill was described by Vitruvius in the 1st century BC; the Roman Empire already used it at industrial scale (the mill complex at Barbegal in Gaul ran 16 wheels). England's Domesday Book of 1086 recorded thousands of watermills — medieval Europe's first "wave of mechanization" was built on the water wheel: besides grinding, it powered fulling, sawing, ore crushing, and bellows. In the 19th century it was displaced by more efficient and compact water turbines, but the principle is unchanged.

Simple version

An undershot paddle wheel in a fast-flowing stream: a wooden wheel with flat paddles, a shaft in bearings supported on the bank(s), and the working machine driven directly off the shaft or through a simple belt drive. No damming is needed — in exchange, the extractable power is small and fluctuates with the water level.

Advanced version

An overshot wheel with a bucketed rim: a small dam or a longer headrace flume provides the head, with the flow controlled by a sluice gate. A gear or belt transmission raises the shaft speed for the working machine. Poncelet-style curved blading brings a large improvement even in the undershot configuration.

Industrial version

Water turbines: Pelton for high head, Francis for medium, Kaplan/propeller for low head with high flow — with closed penstock, governing, and generator. At village scale, ready-made micro-hydro units (turbine + generator + charge controller) feed battery charging or an off-grid island network.

Building your own

1. Measure first, design second: determine the flow rate (Q) and head (H) at the chosen site (see How to measure) — this decides the wheel type and the expected power output.
2. Choosing the type: if there is head available (even 2-3 m) → overshot; flat terrain with fast flow → undershot.
3. The wheel: two side discs/rims, spokes, paddles or buckets; waterproof glue, stainless or galvanized fasteners.
4. Shaft and bearings: keep the bearings above the water line, greasable and replaceable.
5. A trash rack ahead of the water intake, plus transmission and a clutch or coupling to the working machine.
6. Important: interventions in a watercourse (damming, diversion) require a water-rights permit in most countries — check the regulations first.

Common mistakes

• Wheel type and site do not match (e.g. an overshot wheel with no head, an undershot wheel in slow water)
• Skipping the upfront flow and head measurements — the finished wheel "somehow doesn't deliver"
• Bearings ending up underwater, greasing neglected → rapid wear and seizure
• No trash rack: branches, ice, and debris smash the paddles
• No plan for flood levels (liftable mounting, bypass channel) — the first high water takes the wheel
• Tailwater rising until the wheel "drowns" (backed-up water brakes it)
• Expecting too high a shaft speed without gearing

How to measure

• Flow rate (Q): for a small stream, the bucket method (how long it takes to fill a known volume); for larger ones, the float method (surface speed × cross-section × correction factor)
• Head (H): a clear water-filled hose used as a level, or a surveyor's level
• Rotational speed: a laser tachometer or simply counting with a stopwatch
• Power output: for electrical output, voltage × current; for mechanical output, a Prony brake (friction brake + lever-arm force measurement) to get the torque
• A water-level log to track seasonal variation

Videos

(TODO)

Downloadable PDF

(TODO)