Gray-B-Gon wind-powered evapotron for graywater disposal

Step 23: Propeller: make a blade template

Picture of Propeller: make a blade template


  • card stock (manila folder) or thin cardboard (cereal box)

Wheel sizes, spoke numbers, and spoke patterns vary. The exact layout described here may not work on your wheel, but something similar will. The goal is to attach aluminum blades that angle outward on the front side, and are restrained by spokes from extending at all from the back side (the side with the pulley wheel.) Strong winds can blow the stocking drive belt against the back of the wheel. Hitting the spokes is harmless, but hitting blade after blade coming edge-first isn't.

Measure from the rim to a spoke crossing about 2" from the hub and add 1/2". You will probably choose this as the blade length -- if blades extend past the rim, the propeller is more awkward to transport. But if you like the look of larger blades, as in the Step 6 image, then cut them longer.

The image below shows how two blades for a 27" or 26" wheel can be cut from a 12 x 9" sheet of aluminum. For a 24" wheel, make the blades shorter and use a 12" x 8" sheet of aluminum.

You will need nine blades for a 36-spoke wheel, or eight blades for a 32-spoke wheel. For a 24-spoke wheel (rare and unusual), you may want a shape that's about an inch broader.

Draw and cut a blade template of cardboard, try it in Step 22, and when you're satisfied cut a trial blade from aluminum. You don't need tin snips; good-quality scissors will do the job.

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