Feedback/pointers on prop design for outrunner

Hi folks,

I’ve started to dig into propeller design, since my outrunner has these four mounting holes. Initially I designed an adapter for a 2.5HP propeller, but the propeller I bought turned out to be quite heavy (about 400g) and made of aluminium. I don’t think the 3D printed adapter would fare well with such heft, and suspect the printed shaft to shear pretty quickly.

My solution is to try to make a propeller that “wraps” around the outrunner. The only prop I’ve seen on this forum that does this is this one from FR.

Down the line I’ll design a duct, but first want to get the geometry of the propeller right. My approach is to specify three cross-sections of one propeller blade, and then loft around them. The MIT prop pdf I’m using illustrates these as circular cross-sections, but I figure my approach should be close enough?

Here are some calculations:

6S = 22.2 V
Power = V * I = 22.2 * 30 = 660W
RPM = V * Kv = 22.2 * 140 = 3100 RPM

Say I want to go 10 mph (goal is to plane, not foil),

pitch = speed / (rph * (1-slip))
pitch (in) = 10 mph / ((3100 * 60) * (1-0.5)) * 63360 (in/mi) = 6.8"

My prop ended up with a smaller pitch at 4", and diameter is 6.6".

Here are some cross sections, not sure how else to talk about this…

I have no idea how to estimate slip, and have no idea how to know how fast the prop will actually spin in the water, or how to incorporate the total weight of the boat and rider (~100kg).

Any feedback on the profile of this prop?


For what I measured on different king of setup
Prop slip: 5-12%
Rpm drop under load : 10-35%

Prop design going over outrunner already tested on a big 80-100 : not the best results

Planing will take more power than foiling … ( tested by taking of my wing, but using a « small board »)

Thanks. Can you tell me more about the results on the 80100? Maybe a thread or something about it? What was the problem?

Also, how are you measuring power? How do you measure RPM? I was thinking of maybe using a light-based RPM meter?

Data was recorder by the controller vesc
Tests were done by @superlefax

I have a sensorless motor though, how can VESC know what the RPM is?

The vesc drive the motor so it knows the switching frequency , you get the erpm and by knowing the number of pole of your motor you get the rpm
No sensor needed

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