The Kaplan Turbine: A Propeller in a Pipe, With Blades That Turn

Print a four-bladed runner 70 mm across whose blades are separate parts, each on a short pin in the hub, with a set screw so each blade's angle can be fixed at 15°, 25° or 35°. The runner fits inside a 75 mm PVC pipe on a shaft carried by a printed spider bearing, and drives a hobby motor used as a generator, sealed outside the pipe through an O-ring gland. Upstream of the runner, fit a ring of fixed guide vanes printed to give the water a little swirl — the wicket gate of a real Kaplan.

Feed the pipe from the submersible pump in a bucket, with a ball valve to set the flow; time a measuring jug at the outlet to know the flow. Load the generator with a 330 Ω resistor and read the voltage; power is V² ÷ R. At full flow, try each blade angle and note the power. Then close the valve to about half flow and try each again. The best angle at half flow is flatter than at full flow — exactly what a Kaplan runner's blade-turning gear does automatically.

**Viktor Kaplan**, professor at the German Technical University in Brno, patented the propeller turbine with adjustable runner blades in Austria in 1912–13; the first installation went into service in 1918. His US patent number is not asserted here — it could not be read from a primary source — and the mechanism is fully documented. Kaplan turbines and their fixed-blade relatives run most low-head hydro plants, and the bulb turbine — a Kaplan runner with the generator in a pod inside the water passage — runs tidal barrages. **Honest limits.** At low head the machine is large and slow for its power. Kaplan runners are prone to cavitation (the NPSH problem of the centrifugal pump rung, in reverse). And the blade-turning mechanism inside the hub must stay watertight for decades.