A propeller turbine suits sites with only 1 to 5 meters of head and a large flow, because a propeller spinning inside a tube lets a great volume of water pass while taking a small drop. The carrier’s example of 200 L/s over 2 meters gives 200 × 2 × 7 = 2,800 watts, and the result depends on blade angle and a tight tube clearance more than on any other detail.
Part of Micro-Hydro Turbine
A propeller (Kaplan-type) turbine is a reaction machine in which water flows axially through a propeller held in a tube, used where the available fall is only a few meters.
Low Head Means Water By The Tonne
Power is proportional to flow times head, so a small head is made up for with a large flow. The carrier’s third sizing example is 2 meters of head and 200 L/s, which is 200 kilograms of water every second. The shortcut gives 200 × 2 × 7 = 2,800 W; the hydraulic power is 9.81 × 200 × 2 = 3,924 W, and the carrier’s quoted output of 2,000 W is 2,000 ÷ 3,924 = 51% of it. Compare that with the 71% implied by the factor 7, and the gap shows how much of a first build’s output depends on the runner.
The carrier gives the efficiency as 70 to 85% for a good runner, and notes it is harder to build because blade angles are critical, with boat propellers or pump impellers the suggested salvage.
Method: Size The Tube And Choose The Runner
Step 1: Find the tube area. Flow = area × water speed. For 0.2 m³/s and an assumed speed of 3 m/s through the runner (an assumption chosen for the arithmetic; check it against your propeller): area = 0.2 ÷ 3 = 0.067 m², so an unobstructed tube diameter of √(4 × 0.067 ÷ π) = 0.29 m. The hub and blades take part of that area, so the real tube must be wider, by an amount that depends on your hub; a hub half the tube diameter blocks a quarter of the area.
Step 2: Choose the runner. A boat propeller is made to push water and give thrust; as a turbine it extracts, so its pitch and the angle at which water meets it rarely match without adjustment. Expect lower efficiency than 70-85% until you have tested it. A pump impeller of the axial type behaves similarly. Cast or fabricate a runner of known blade angle if you can.
Step 3: Guide the water in. A propeller wants the water to arrive with a slight swirl. A scroll-shaped inlet or fixed guide vanes ahead of the runner provide it; this is standard practice I am recalling, not a figure from the corpus.
Step 4: Keep the tube tight. Clearance between the blade tips and the tube leaks flow past the blades. Make the tube round and true, and keep the gap small and even.
Step 5: Submerge the outlet. The Head Measurement page notes that for reaction turbines the height from the runner down to the tailwater also contributes (suction head). A draft tube whose mouth is below the tailwater level uses that height, but a runner placed too high above the tailwater can starve and cavitate, which pits the blades.
Step 6: Gear up. A propeller does not run at generator speed. Check the shaft speed on test and fit a belt ratio as in Generator Coupling (Belt vs Direct).
When A Waterwheel Is The Simpler Answer
The carrier lets an overshot wheel work above 2 meters, though it is large and slow. Between 1 and 2 meters the Water Wheel Types page offers breastshot wheels at 50-70% and undershot at 20-40% on shallower drops. A wheel is built from timber and iron with ordinary tools, where a propeller turbine needs a true tube and a correct blade form.
Common Mistakes
- Taking the blade angle from an unrelated propeller — it was designed for thrust, not for extraction.
- A sloppy tube — the gap at the blade tips is a bypass for the water.
- Forgetting the trash rack — weeds wrap a propeller; see Intake & Trash Rack.
Safety
The water below a weir can drown a strong swimmer
The flow over a low weir creates a rolling current at its foot that holds a person against it. Never swim or wade below the weir or the tailrace outfall. Keep the propeller inside its tube behind a guard, shut the intake gate before opening the tube, and work with a second person on the bank.
What’s Next
- Learn next: Tailrace & Environmental Flow for sending the water back to the river
- Combine with: Overshot Waterwheel (Traditional) for a slower alternative at the upper end of the head range