Water power

Power is proportional to head times flow.

• Head — the vertical fall available: run a clear hose full of water from the intake point to the wheel site, or use a water level, and measure the height difference in steps.
• Flow — for a small stream, time how long it takes to fill a container of known volume at a spout or a temporary dam. For a larger one, measure cross-section area × surface velocity × 0.8 — time a float over a measured 10 m and correct for the slower water near the bed.

Measure flow at the driest time of year. That is the power you can count on.

Power (watts) ≈ 9.81 × flow (litres per second) × head (metres) × efficiency.

Efficiency: overshot about 60–75%, breastshot about 45–65%, undershot about 20–30%.

Example: 3 m of head and 100 L/s through an overshot wheel ≈ 9.81 × 100 × 3 × 0.7 ≈ 2 kW — continuously, the work of dozens of people. Half the flow in summer means half the power; a stream that dries up gives nothing. If the number is under a few hundred watts at the dry-season flow, look for a better site before building.

• Overshot — water enters at the top and fills buckets, turning the wheel by weight. For high head and low flow, and by far the most efficient. The wheel diameter is a little less than the head.
• Breastshot — water enters around axle height. Moderate head.
• Undershot — paddles dipped in the flow. Lots of water and almost no fall, and inefficient.

Run an overshot wheel slowly — a rim speed of about 1–2 m/s — so the buckets fill properly and do not spill. See the overshot wheel.

The wheel is the small part. In order:

1. Weir — raises the water level at the intake.
2. Leat or headrace — carries water along the contour to above the wheel, falling very gently — around 1 in 1,000 to 1 in 2,000 — so it keeps almost all of its head.
3. Sluice at the leat entrance and a spillway to return excess water to the stream.
4. Penstock gate at the wheel.
5. Tailrace — takes water away fast so it does not back up and drown the bottom of the wheel.

Line the leat where it leaks, and inspect it after every storm.

A wheel that cannot be stopped cannot be maintained safely, and someone will eventually be caught in it. Build before the wheel runs:

• A sluice or penstock gate that cuts all flow to the wheel, reachable from outside the wheel pit.
• A brake on the wheel or the first shaft.
• A lock-off — a pin or bar that holds the gate shut while anyone is working on the machinery, with the key held by the person working.
• Guards round every exposed gear and belt.

Wheels turn slowly — a few to perhaps 15 revolutions a minute — and millstones want around 100–140. Take power off with a pit wheel and a lantern pinion or a wallower, stepping up in one or two stages.

Cut gear teeth with a dividing plate, never by eye, and use hardwood cogs — apple, hornbeam — designed to break before the wheel does, with spares made ahead. Distribute power with line shafting and belts to several machines. Grease every bearing on a schedule.

1. Grinding grain — the largest continuous labour in the settlement.
2. Sawing timber.
3. Driving the bellows for smelting — continuous air that people cannot sustain for long.
4. Trip hammers for forging.
5. Machine tools.

Plus fulling cloth, pumping, and pressing oil. Record hours run and output per day for each use, so the next wheel goes where it pays most.

The flood will come and it will be larger than planned. Design the weir to be overtopped without destruction: a stone or timber crest with a protected apron below it, and banks armoured where the water returns. Set the wheel and machinery above the highest flood mark you can find — look for debris lines in trees and ask anyone who has lived there longest.

Close the sluice ahead of a flood, and expect to rebuild part of the leat after a large one. Keep timber and stone for repairs stored above flood level.