Part of Food Processing
Watermills convert the kinetic and potential energy of flowing water into rotational mechanical power, making grain grinding far less labor-intensive than hand milling. A well-designed watermill can process 50-150 kg of grain per hour, replacing dozens of person-hours of manual work daily.
For a rebuilding civilization, the watermill represents one of the most transformative technologies possible. It frees human labor for other tasks, enables larger populations to be fed from the same land, and provides a platform for other mechanical processes: sawing timber, fulling cloth, crushing ore. Every medieval European village that thrived did so in part because it had access to a mill.
Site Assessment
Before building anything, assess the water source carefully. You need three things: adequate flow volume, sufficient head (vertical drop), and a reliable year-round supply.
Head measurement: Head is the vertical distance water falls before reaching the wheel. Use a level and measuring rod to determine the drop available at your site. Even 0.5 m of head can drive an undershot wheel; 2-4 m enables overshot designs that extract more energy.
Flow measurement: Dam a temporary test weir across the stream. Measure the notch width and the water depth flowing over it. Use the weir formula: Q (liters/second) = 1.84 x L x H^1.5, where L is notch width in meters and H is head over notch in meters. For a small grain mill, you need at least 50 liters/second for an undershot wheel, less for overshot.
Seasonal variation: Check the stream in dry season. Many streams that look ideal in spring become trickles by August. Observe water marks on rocks and high-water debris lines. Build for the low-flow season, not the peak.
Soil and foundation: The millhouse site needs firm ground — bedrock or dense gravel is ideal. Soft clay or silt will shift under the weight of machinery and vibration. If bedrock is available, anchor directly to it.
Water Wheel Types
The wheel type determines efficiency and the required head.
| Wheel Type | Head Required | Efficiency | Best For |
|---|---|---|---|
| Undershot | 0-0.5 m | 20-30% | Fast streams, low banks |
| Breastshot | 0.5-2 m | 40-60% | Moderate head, most common |
| Overshot | 2-6 m | 60-80% | High head, most efficient |
| Pitchback | 1.5-4 m | 65-85% | Tight sites with reverse flow |
Undershot wheel: The simplest design. Paddles dip into the stream current and are pushed by water flow. Build from timber with flat paddles nailed to a central axle. Diameter: 2-4 m. Width: 0.5-1.5 m depending on stream width. Requires fast-moving water (>1 m/s surface speed) to work effectively.
Overshot wheel: Water is delivered to the top of the wheel via a millrace (channel) and flume, filling buckets that carry water around the outside of the wheel. Gravity does most of the work. This is the most efficient design and the one to build when head permits. Typical diameter: 3-6 m. Buckets should hold 15-25 liters each and be spaced 30-40 cm apart around the rim.
Breastshot wheel: A compromise — water enters at roughly the axle height (mid-wheel). Good for sites with 0.5-2 m of head. More efficient than undershot, simpler to build than overshot.
Wheel Construction
Materials: Oak or other dense hardwood for the arms and axle. Elm works well for submerged parts due to its resistance to rot when perpetually wet. Avoid alternating wet/dry cycles with softwoods — they degrade rapidly.
Axle: The central shaft should be 15-25 cm in diameter for a small mill. Hardwood works but iron is far superior. If you have access to a blacksmith, have the axle journals (bearing surfaces) wrapped in iron. The axle rests in bearing blocks — carved wooden cups lined with hardwood or bronze, greased with tallow.
Arms and shrouds: Arms extend radially from the axle hub. A typical overshot wheel has 6-8 arms per side, forming two spider-like frames. The outer ring (shroud) connects the arm tips on each side. Boards called sole boards span between the two shrouds to form the bucket floor. Side boards complete each bucket.
Assembly sequence:
- Fit and pin arms to hub mortises
- Attach shrouds to arm tips with mortise-and-tenon joints
- Fit sole boards between shrouds
- Attach side boards to divide the rim into individual buckets
- Install axle in bearing blocks before final assembly
Wheel diameter rule of thumb: For overshot wheels, diameter approximately equals head x 0.85. A 3 m head needs roughly a 2.5 m diameter wheel.
The Millrace and Flume
The millrace is the channel that diverts water from the stream to the wheel. The flume is the final wooden trough that delivers water precisely to the wheel.
Millrace construction:
- Dig the race to slope 1:500 to 1:1000 (gentle enough to maintain head, fast enough to flow without silting)
- Line with clay puddled to waterproofness, or stone and mortar if available
- Width: 60-120 cm; depth: 40-80 cm
- Install a headgate (sluice) at the intake to control flow and shut down the mill
Flume design: The flume narrows toward the wheel to accelerate water. For an overshot wheel, the flume should deliver water just past the 12 o’clock position so it falls into the buckets on the descending side. Angle of delivery: 15-30 degrees past vertical.
Tailrace: Water leaving the wheel must drain freely. A choked tailrace backs up water against the wheel bottom, killing efficiency. The tailrace floor should be at least 15 cm below the lowest point of the wheel.
Millstones
Grain grinding requires two millstones — a stationary bedstone on the bottom and a rotating runner stone on top. The quality of these stones determines flour quality.
Stone selection: The best natural millstones come from freshwater quartzite (buhrstone), which has a naturally porous, self-sharpening structure. Granite is harder and more common but produces slightly gritty flour. Sandstone is soft but workable.
Dimensions for a small mill:
- Diameter: 60-90 cm
- Thickness: 20-30 cm for bedstone, 15-25 cm for runner
- Eye (central hole in runner): 10-15 cm diameter
Dressing the stones: The grinding faces must be cut with furrows (channels) radiating from the eye to the perimeter. These furrows perform three functions: they cut grain, convey meal outward by centrifugal force, and ventilate the grinding zone to prevent heat damage to flour. Cut furrows with a mill bill (a steel chisel). Furrow depth: 6-10 mm, width: 8-12 mm. The lands (flat areas between furrows) do the actual grinding.
Balance: The runner stone must be balanced precisely or it will vibrate destructively. Rough balance by tapping wedges under the stone while it rests on its edge. Fine balance by rotating slowly by hand and noting which side drops.
Gearing and Drive Train
Converting wheel rotation (typically 5-15 RPM) to millstone rotation (80-120 RPM) requires a gear train.
Pit wheel: A large gear attached to the wheel axle, inside the mill pit. Typical diameter 1.5-2.5 m. Teeth are wooden pegs (cogs) or cut wood teeth.
Wallower: A small gear on the vertical stone shaft that meshes with the pit wheel. The gear ratio between pit wheel and wallower provides initial speed multiplication.
Stone nut: A small gear on the stone spindle, driven by a larger gear above called the great spur wheel. The stone nut can be disengaged from the drive to stop the stones without stopping the wheel.
Typical gear ratios: For a wheel turning at 8 RPM and stones needing 100 RPM:
- Pit wheel to wallower: 3:1 yields 24 RPM
- Great spur wheel to stone nut: 4:1 yields 96 RPM
- Adjust tooth counts to hit your target speed
Wooden teeth: Traditional mill gears used hardwood pegs as teeth on one member of each gear pair. The wood absorbs shock and acts as a sacrificial fuse — a broken wooden tooth is easier to replace than a broken iron or stone shaft. Apple, hornbeam, and dogwood are traditional choices.
Millstone Gap Adjustment
The gap between bedstone and runner determines flour fineness. This gap is controlled by raising or lowering the bearing that supports the stone spindle — a mechanism called the tentering gear.
Standard settings:
- Coarse meal: 1-2 mm gap
- Fine flour: 0.1-0.5 mm gap (the stones nearly touch)
- Too tight: stones grind together, heat flour, score faces
- Too loose: grain passes uncrushed
A skilled miller adjusts gap by feel and sound — a rumbling means stones are too close; a hissing means too open. Flour temperature should not exceed 40 degrees C; touch the meal at the chute regularly to verify.
Output and Maintenance
Typical output: A 1-meter diameter runner stone spinning at 100 RPM will process approximately 50-80 kg of wheat per hour into coarse meal. Finer flour requires slower throughput — reduce to 30-50 kg/hour.
Daily maintenance:
- Check bearing grease (tallow or vegetable oil) — replenish every 4-6 hours of operation
- Clear grain and straw from the hurst (the frame holding the stones)
- Inspect wooden gear teeth for cracks or wear
- Check the millrace for debris blockage
Weekly maintenance:
- Inspect wheel buckets or paddles for rot or loosening
- Check that sluice gate operates freely
- Test stone balance by listening for vibration at startup
Stone dressing: Every 100-200 hours of operation, the millstones must be re-dressed — the furrows deepened and the lands re-leveled with a mill bill. Allow stones to cool completely before dressing.
Safety Considerations
Machinery entanglement: Never wear loose clothing near gear trains. Keep long hair tied back. Install wooden guards over gear pairs wherever hands might reach.
Flooding: Design the headgate to open fully so the millrace can be emptied if the stream floods. Floodwaters carrying debris will destroy an unprotected wheel within minutes.
Fire: Grain dust is explosive in high concentrations. Maintain ventilation, sweep accumulated dust frequently, and never bring open flame near active milling.
Runaway: If the load is removed suddenly, the wheel will accelerate until it tears itself apart. Install a brake — a curved wooden beam pressed against the wheel rim by a lever — that can halt the wheel in 10-15 seconds.
A well-built watermill, properly maintained, can operate for decades. For a community of 100 people consuming 1 kg of grain per person per day, a modest mill operating 4 hours daily provides more than adequate milling capacity.