Overshot Wheel
Part of Hydro Generator
The most efficient water wheel type — water enters at the top, fills buckets, and the wheel turns by gravity as the water-laden buckets descend.
Why This Matters
The overshot wheel is the classic water wheel of history — the image most people conjure when they think “water mill.” It’s also the most efficient simple water wheel, typically achieving 65-85% efficiency compared to 25-35% for undershot wheels. For high-head sites with modest flow (where water falls 10-30 feet but isn’t a torrential river), the overshot wheel extracts maximum energy from every gallon of water.
The mechanics are beautifully simple: water is delivered to the top of the wheel through a chute (launder), fills buckets built into the wheel rim, and the weight of water-filled buckets on the descending side unbalances the wheel, turning it. The water weight does the work, not the water’s velocity. This gravity-based operation means overshot wheels work well even with slow streams — the flow rate needed is much less than for an undershot wheel of equivalent power, because each gallon of water does much more work falling through the full wheel height.
Overshot wheels were the backbone of pre-industrial milling from ancient Rome through the industrial revolution, grinding grain, driving hammers, pumping water, and running textile machinery. Building one is demanding but achievable with intermediate woodworking and masonry skills.
Design Parameters
Diameter: Should be slightly larger than the available head — typically 1.0 to 1.2 times the fall height. For 12 feet of head, a 12-14 foot diameter wheel. Larger diameter means slower rotation (requiring more gear step-up) but higher torque and more efficient water use.
Rotational speed: Peripheral velocity of the wheel should be about 2-3 feet per second for maximum efficiency. At this speed, the water enters the buckets smoothly without splashing, the centrifugal force retaining water in the buckets is just right, and the water empties cleanly at the bottom.
N (rpm) = (Peripheral Velocity × 60) / (π × Diameter) For a 14-foot wheel at 2.5 fps: N = (2.5 × 60) / (3.14 × 14) = 3.4 rpm
This very low speed means significant gear ratio is needed to reach generator speed — typically 200:1 to 500:1 from wheel to generator.
Width (face): Determines how much water the wheel can handle and therefore its power. Width = Flow Rate / (Bucket Volume × Wheel RPM × Number of Buckets × Fill Factor). Typical widths: 2-8 feet.
Number of buckets: 36-48 for medium-sized wheels. Too few: gaps in water coverage, uneven torque. Too many: buckets are small and difficult to fill completely from the launder.
Bucket shape: Curved (parabolic or circular arc) to hold water through the full rotation arc. The bucket must hold water from about 2 o’clock position (as it rises over the top) down to about 5-6 o’clock (where it releases into the tailrace). A curved bucket pointing backward relative to wheel rotation holds water through this arc without spillage.
Launder and Water Delivery
The launder is a trough that delivers water from the millpond or diversion channel to the top of the wheel. It’s a critical component — poor launder design wastes much of the head and reduces efficiency dramatically.
Launder height: Water should enter the buckets when they are at exactly the top of the wheel (12 o’clock position) or slightly past (1-2 o’clock). Entering too early means water is in the bucket on the ascending side (works against the wheel); too late means water falls past the top and doesn’t fill the buckets efficiently.
Launder geometry: The launder end should curve downward, directing water into the bucket tangentially. The water should “fall into” the bucket rather than “pour onto” it — minimize the drop height from launder tip to bucket bottom to avoid turbulence and splashing.
Flow control: A hinged gate at the launder inlet controls flow and therefore power. Raise the gate to reduce flow; lower to increase. A simple wooden lever with a weight or counterweight makes adjustment easy.
Launder construction: Wood (planked or sawn box section) is traditional. Line with sheet metal or tar if leakage is a problem. The launder must be level or very slightly tilted toward the wheel to ensure flow. Support every 4-6 feet along its length.
Wheel Construction
Traditional wooden construction:
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Axle: Square timber (or round iron rod) running through the wheel center, supported in two bearing frames on the mill walls.
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Shrouding (rim boards): Two parallel circular rings of planked wood, like large barrel hoops, mounted on the spokes. These form the sides of the buckets.
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Spokes: Heavy timber spokes running from axle hub to rim, typically 12-16 spokes for a 14-foot wheel. Mortised into the hub and tenoned into the rim.
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Bucket boards: Curved planks spanning between the two shroud rings. Each bucket board, combined with the neighboring boards and the shroud rings, forms an individual bucket.
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Sole boards: Flat boards at the outer rim (between bucket boards) completing the bucket enclosure.
Modern approach: Weld steel frame (angle iron or RHS) in place of wooden spokes and rims. Fabricate steel buckets from bent plate. This is more durable but heavier and requires metalworking skills.
Maintenance and Common Problems
Leaking buckets: Water escapes from bucket seams before completing the descent arc, reducing power. In wooden wheels, swell the wood with water before running (the wood expands and seals joints). Caulk persistent leaks with oakum and pitch, or sheet lead hammered into gaps.
Iced tailrace: In cold climates, the tailrace can ice up, raising the water level and drowning the wheel bottom. Clear ice regularly. In very cold locations, design the wheel so it can be easily stopped and the tailrace can drain completely.
Imbalance: Unequal bucket fill or uneven construction creates vibration. Add counterweights (metal strips bolted to the rim) or remove material until balance is achieved. A severely imbalanced wheel at 3 rpm seems innocuous, but the forces transmit through the gear train and cause bearing wear and structural fatigue over time.
Bearing wear: The axle bearings of an overshot wheel carry very high loads (the weight of the wheel plus the weight of water in the buckets). Check bearings monthly; rebabbitt or replace annually on working mills. Dry wood bearings (traditional for vertical mill axles) wear faster than bronze and should be checked more frequently.