Wind power
Get continuous power where there is no falling water — which is most places.
Everything for this step
- Field guide
- Electricity → 8 sections Energy sources → 14 sections
- Builds
- Post mill
- Reference sheets
- 2 sheets on this page ↓ · 7 sheets in Electricity · 16 sheets in Energy sources
- Sciences it rests on
- Physics · Engineering
- Track
- Mechanical power, level 1 of 2 · next: The steam engine
Why this matters
Water power is better wherever it exists: steadier, controllable, and available day and night. It exists on a minority of sites. Wind is everywhere, needs no weir, no leat and no flood defences, and it is the only pre-industrial power source that works on flat, dry ground — which is where most grain is grown.
The price is that wind is intermittent and its power varies with the cube of its speed. Double the wind and you have eight times the power; the same machine must earn its keep in a breeze and survive a gale, and every feature of a working windmill is a response to that range. A settlement that has both water and wind uses water for the work that must not stop and wind for the work that can wait for weather.
The shape of it
- Measure the wind before building anything. Power is proportional to swept area and to the cube of wind speed, so a site with a mean wind of 6 m/s is worth nearly twice one at 5 m/s. Watch a site across a year, note the quarter the working winds come from, and remember that obstacles disturb the air for roughly twenty times their own height downwind.
- Pick the machine for the job, not for the picture. For grinding, a post mill — the whole body turns to face the wind. For lifting water, a multi-blade fan wheel: high torque in light winds, self-turning and self-furling. Where the wind blows from one quarter for months, a vertical-axis mill needs no turning gear and no bevel gearing at all.
- Twist the sails. A sail is a propeller blade, not a paddle. The tip travels five to eight times wind speed while the heel barely moves, so the blade must be built steep at the hub — 20–25° — and nearly flat at the tip. A flat board set at one angle works over a fraction of its area.
- Build the stopping before the starting. A brake, and a way to take area out of the wind: reefed cloth, shutters, or a rotor that turns itself out of the wind. Most mills that burned down were stopped with the brake alone in a strong wind, until band and rim caught fire.
- Gear for the work. Sails turn at 10–20 rev/min; millstones want 100–140. One step-up of 6:1 to 8:1 does it, and wooden cogs are cut to break before the wheel does.
- Expect to be idle. A windmill works perhaps a third of the hours of a watermill of the same output. Store the grain, not the wind: mill when it blows and keep enough flour milled ahead that a still fortnight costs nothing.
- Use it for what can wait. Grinding, pumping, drainage, sawing and the first stages of preparing fibre all tolerate stopping. Anything that must run continuously — a furnace blast, a clock, a hearth — should not depend on wind.
Doing it, step by step
The summary above says what has to happen. This says how, with the numbers that decide whether it works.
1 · Measure the wind for a year before building
Power rises with the cube of wind speed, so a site averaging 6 m/s is worth about 1.7 times one averaging 5 m/s, and siting decides more than the machine does.
Without instruments: record wind three times a day on the Beaufort scale from what you see — smoke, leaves, branches, whole trees. Note the direction the working winds come from, and how many days a month the sails would turn. Choose open, raised ground. Obstacles disturb the air for roughly twenty times their own height downwind — a 10 m tree line spoils 200 m behind it.
2 · Size the machine from the power you actually need
Power (watts) ≈ 0.6 × swept area (m²) × wind speed (m/s)³ × efficiency. Traditional sails reach an efficiency of around 15–30%.
Example: a 3 m multi-blade wheel — swept area about 7 m² — at 6 m/s gives roughly 0.6 × 7 × 216 × 0.2 ≈ 180 W: enough to pump water, not to grind for a village. A post mill with 20 m sails, swept area over 300 m², is working at several kilowatts. Double the sail diameter and you get four times the power. Size to the job before cutting timber.
3 · Pick the machine for the job
- Grinding grain: a post mill — the whole body turns to face the wind — or later a tower mill with a turning cap.
- Lifting water: a multi-blade fan wheel — high torque in light winds, simple, and it can be made to turn itself into the wind with a tail vane.
- Where the wind blows from one quarter for months: a vertical-axis mill behind a wall that funnels the wind, needing no turning gear and no bevel gearing.
Build the simplest machine that does the job; complexity is paid for in maintenance for its whole life.
4 · Twist the sails — a sail is a propeller, not a paddle
The tip travels several times faster than the wind while the heel near the hub barely moves, so the wind meets each part of the sail at a different angle. Build the sail with a twist: steep at the heel, around 20–25°, and nearly flat at the tip, a few degrees.
A flat board set at one angle works well over only a small fraction of its length. All sails must be set identically — measure the angle at the same points on each — or the rotor runs rough and shakes the structure.
5 · Build the stopping gear before the starting gear
A brake — a band or block acting on the brake wheel on the windshaft, at the top of the mill — and a way to take sail area out of the wind: cloth that can be reefed, shutters that open, or a tail that swings the rotor edge-on.
Most mills that burned were stopped with the brake alone in a strong wind until band and rim caught fire. Turn the mill out of the wind or reef the sails first, then apply the brake to hold it. Never apply the brake hard against sails at full speed.
6 · Gear up for the work
Sails turn at about 10–20 revolutions a minute; millstones want around 100–140. One step-up of about 6:1 to 8:1 does it — brake wheel to wallower, great spur wheel to stone nut.
Cut wooden cogs from apple or hornbeam to break before the wheel does, and keep spares. Grease the bearings — the windshaft neck bearing takes the whole weight of the sails and wears first.
7 · Furl ahead of a gale, not during it
Stop and secure the mill before the wind reaches a near gale — about 14 m/s, when whole trees are in motion and walking into the wind is hard. A runaway mill throws its sails or destroys its own gearing, and a brake applied in anger sets it alight.
Watch the sky and the barometer or the wind records; a falling glass and a rising wind mean furl now. Furled: sails reefed or shutters open, rotor turned out of the wind, brake on, and the sails chained or lashed so they cannot turn.
8 · Store the product, not the wind
A windmill works perhaps a third of the hours of a watermill of the same size, and calm spells can last a fortnight. Mill when it blows, and keep enough flour milled ahead that a still fortnight costs nothing.
Use wind for work that can wait: grinding, pumping, drainage, sawing. Anything that must run continuously — a furnace blast, a hearth — should not depend on wind. Where a settlement has both, water for work that must not stop, wind for work that can wait for weather. See water power.
What it looks like
Reference sheets
One drawn sheet covering the whole subject at once — the breadth a photograph cannot give, bought at the cost of accuracy. Read them for range and vocabulary, never as the thing you identify from.
Build it
Dimensioned drawings, bills of materials and construction procedures for the things this step asks for.