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.
- Pitch: how far the prop would move through a solid material if it was a drill.
- Slip: how different in practice moving this prop through water is vs. solid (see this calculator)
- Smooth vs. splined shaft: refers to this mechanical splining as a way for better attachment to the shaft. There are a shocking number of ways you can attach a prop to a shaft.
- Weedless design: keeps vegetation in water, off the propeller. For weed infested waters.
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):
- The more power the motor has, the higher the diameter should be.
- The faster you want to go, the higher the pitch should be.
- the lighter the boat, the higher the prop pitch, and the smaller the diameter.
In other words, for my specific case:
- Pitch should be high, because we want to have high speed
- Diameter should be low because boat is light.
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:
- vXX/pYYYY means XX km/h at YYYY W, for example v10/p350 means reaching 10 km/h at 350W.
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
- Illustrated guide to Blade Element Theory from Cambridge
- Marine propellers from MIT 2.016
- Simple flat propeller in Onshape
- Sketch & loft propeller in Onshape, with airfoil profile.
- Succinct explanation about prop pitch and top speed vs. acceleration.
- How to measure prop pitch.
- Props 101
- Understanding a Boat's Propeller Pitch