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Propeller design

Propellers are defined by a variety of parameters: diameter, angle of attack, and many others. Each propeller blade is a foil shape, which adds to complexity. Whole courses are dedicated to this subject.

Theoretical foundation

Definitions:

Basic equation: speed = rev_per_time * pitch * (1-slip)

For example, with a 3000 RPM shaft (my motor), what pitch do we need to go at 10 mph (= 16 km/h)?

pitch = speed / (rph * (1-slip))

pitch (in) = 10 mph / ((3000 * 60) * (1-0.3)) * 63360 (in/mi) = 7.5"

Practical attempts and notes

There is a trade-off between top speed and acceleration.

My theory (untested), based on torqeedo’s lineup(spreadsheet):

In other words, for my specific case:

Torqeedo

Torqeedo props are designed for planing: prop is rated to propel a 200kg boat at 30 km/h under 4000W of power, with 16.3” pitch, 11.8” diameter.

Torqeedo spin at 1500 RPM, which is pretty slow, and why their props are so large. My motor should spin at about twice that, so can have a smaller prop.

Good summary of Torqueedo’s propeller offerings. Breakdown of properties:

Prop ducts for safety

Ducted propellers are sometimes used in ships for efficiency reasons:

Advantages are increased efficiency at lower speeds (<10 knots), better course stability and less vulnerability to debris. Downsides are reduced efficiency at higher speeds (>10 knots), course stability when sailing astern, and increase of cavitation

But for efoils, having a duct is a safety feature. Unfortunately it also introduces a lot of drag generally speaking. The real solution is to build a well tested kill switch.

Resources I found useful