An overshot water wheel receives water near its top, so the weight of water in the descending buckets drives rotation. Calculate the gross gravitational power available from head and flow, plus wheel circumference and bucket spacing. Useful shaft power also depends on losses and wheel performance.
Gross Water-Power and Declared Wheel Geometry
Enter declared gross head, flow, wheel diameter and equal bucket count. The worksheet reports the ideal water-potential rate before losses plus exact circle geometry; it is not a site or machine design model.
Equation Used
- H and Q are declared gross water quantities and are not surveyed net head or measured usable flow.
- Water density is fixed at 1000 kg/m3 and gravitational acceleration at 9.81 m/s2.
- The wheel is a perfect circle of declared diameter D with N equally spaced schematic buckets.
- Capture, entry and exit conditions, bucket volume and fill, spill, leakage, tailwater, losses, efficiency, torque, speed, gearing, generator, structure, permitting and safety are excluded.
Head and flow determine gross water power. Diameter and bucket count change the wheel drawing; the water markers and playback speed remain illustrative rather than responding to hydraulic conditions.
Water enters buckets near the top and gravity turns the loaded side
Library of Congress Historic American Engineering Record documentation for Simpsonville Mill describes the overshot wheel as a large wheel with continuous perimeter buckets. Water from the headrace passes through a trough or flume over the top into those buckets, and the water's weight turns the wheel.
National Park Service material likewise distinguishes top-fed overshot wheels and explains that water-filled spaces make one side heavier so gravity rotates the wheel. The animation keeps the wheel, spokes and bucket positions connected while schematic water markers move from launder to descending side.
Historical mill power with site-dependent operation
NPS records document overshot wheels at Peirce Mill and Cable Mill, and Library of Congress surveys document overshot mill arrangements and bucket construction. These sources establish topology and historic context, not a universal modern performance curve.
Actual hydropower work requires measured hydrology, environmental review and machine-specific engineering. This page does not select a wheel, generator or site.
Gross water potential and declared circle identities
The U.S. Army Corps of Engineers identifies head, flow and efficiency as distinct power variables. This worksheet deliberately stops before efficiency: Pg is the gravitational energy rate carried by the declared water, not shaft or electrical output.
Declared gross-water example
For H=4.0 m and Q=80 L/s, gross water-potential rate is 3.139 kW and potential energy is 39.24 kJ per cubic metre. For D=4.0 m and N=24, ideal circumference is 12.566 m and equal spacing is 15.00 degrees.
No shaft power, torque, daily energy or operating point follows from these gross and geometric identities.
What gross potential cannot determine
The Army Corps notes that efficiency is a separate, nonlinear function of head and flow in real hydropower facilities. For a water wheel, capture, fill, spill, leakage, wheel speed, tailwater, bearings, gearing and load further separate gross water power from useful output.
A shaft-power estimate needs a suitable wheel-efficiency model, while shaft torque also requires rotational speed. Neither is determined by gross head and flow alone.
Overshot-wheel questions
Is gross water power the wheel's shaft output?
No.
Does the animation show bucket fill physics?
No; water markers and blue bucket fills are qualitative.
Does bucket count predict efficiency?
No.
Can the calculator size a generator or gear train?
No.
Can it establish usable site head or flow?
No.
Overshot-water-wheel references
- Library of Congress HAER, Simpsonville Mill: federal historical survey defining the top-fed bucketed overshot-wheel topology and weight-driven rotation.
- National Park Service, Water Wheel, Hopewell Furnace: federal explanation of top-fed overshot wheels, water-filled buckets and gravity-driven rotation.
- U.S. Army Corps of Engineers, Appendix D: Hydropower Analysis: federal treatment of the head-flow-power relationship and efficiency as a separate nonlinear variable.
- National Park Service, History of Peirce Mill: documented restoration and operation of a historically appropriate overshot wheel.
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