An overshot wheel is fed from a flume above its axle: water drops into buckets around the rim and the weight of the filled buckets on the descending side turns the wheel under gravity. Because almost all of the water's potential energy (ΔPE = ρgQH) ends up doing useful work, overshot wheels reach roughly 85% efficiency — but they turn slowly (a few RPM) with very high torque.
An undershot wheel sits in an open channel with paddles dipping into the current below the axle. It is driven purely by the water's momentum (kinetic energy transfer, like a very crude turbine) with no vertical drop to exploit, so most of the flow's energy is lost downstream: typical efficiency is only 25-30%. It spins faster for the same flow, but produces far less power per unit of water than an overshot wheel of the same size.
- Power ≈ η · ρ · g · Q · Heff (ρ = 1000 kg/m³ water, g = 9.81 m/s², Heff the effective head each design captures).
- Torque = Power / ω, where ω is the wheel's angular speed — the slow, high-torque overshot wheel and the faster, low-torque undershot wheel can produce very different power from the same flow.
This is the flat 2D companion to the 3D water-mill simulation: the same hydropower mechanics, viewed as a side-on cross-section so the bucket-filling and paddle-driven behaviour is visible at a glance.