The efoil crew using a model air propeller in water

Bouncing on @Toto44 video link and comment.


Yes, the video is older than this forum… April 2017 and I didn’t really notice the prop at the time. Since LIFT have published pictures of their drive train (late 2017), we are all obsessed by the 140-160mm with large blade props in the @Flying_Rodeo style. That’s why I am all the more impressed by Don Montague efoil reaching such a high speed with such a not so short but elongated air propeller in water.
So I was wondering if we had missed something or if members of this forum had investigated the trail and gave up for good reasons.
How many Kv’s for this 240mm (?) prop x 3 inch pitch (guess) ? Geared or direct drive ?
@pacificmeister, you have observed this efoil closely but have you ever thought about investigating such high AR propellers rotating at probably high rpm (200-300kv ?) possibly not geared ?


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This is a cut down apc prop I think pitch will be in 8-12 range. This is typical pitch range for these props

Had the exact same tought after reading an old post yesterday :

Their motor + prop combo looks really close to this one. Really really close.

After that I checked on Lehner website for some infos : this is a 375kV motor with 16:1 reduction. If they run it at 48V (which seems legit because of the solar powered boat and used inverter), they should get around 1125 rpm unloaded.

I don’t think its been overlooked, designs have just advanced to something better.
A prop in that style has a few negative aspects:

  • you have to foil lower down on the mast so that it doesn’t breach the water
  • You have to run lower RPM which means higher and heavier gear units or direct drives that have lower KV and hence much lower power ratings.
  • Marine props are different to “slow-fly” props for good reason. As you increase speed, you want more rake in the prop etc.
    I’m not sure what speed Don hit with that prop, but I doubt he hit 55kph like FR did with his prop.

In fact these must be run at higher rpm not lower.

Higher in comparison to what?

To what is mostly used right now

That prop won’t work at 4000-5000rpm. Happy for someone to prove me wrong though.

When i compare (think about) it to few of my older tri/quadcopters, without thinking i would say this wouldnt work in the water over idk 1k rpm lol. I guess it does but…

Why you think that? In air these run up 25 - 30.000

Based on the Reynolds number you’d need to divide the rpm by around 15 to get to what the rpm would spin at in the water. Hence same thing with the speed. So if the prop is spinning at 30k rpm in air, it’s going to be at 2k rpm in water. Hence why you move to smaller 3 blade props etc.

Well the video shows something different. To get this speed. I estimate from the video it is around 35-40. This prop must be in the 5000-6000 range or more.
This is all theoretical talking. If I would have received my waterproof remote I would have tested already. I have many of these props in my workshop

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Which video? Any link. What speed did he record?

I think a bit of further proof that there’s not much point in reinvestigating this type of propeller should be the fact that Don and jetfoiler use the newer designs now instead:

Video link top of this Thread

Why not follow? Did Don and jetfoiler investigate this airplane prop shape?

Unless you have some real numbers to show us I think this path deserve to be investigated.

And just a photo of jetfoiler does not prove anything, I made maaaaany different kinds of propulsion’s systems (direct inrunner, geared, outrunner… since 2017)
I made about 25 propellers, you can find me one day testing a propeller and make a photo of it, it won’t mean it is the best propeller and other models need to be forgotten.
This kind of comment is dangerous and will kill the forum.
I remember starting myself my first motor with a 6374 out runner when the forum started and seeing people saying this setup would not work because of this and that. Finally it worked like a charm and I derivated two other units from this.
Back to the topic few weeks ago. This forum I remember seeing a two human powered hydrofoil going 35kmh+ with this kind of propeller which is I think great efficiency evidence.


It’s been pretty solidly investigated in the ROV community and they’ve gone in a different direction. There some forums with all the mathematics for the ROV’s etc. But it’s also important to remember that they are also using slower speeds and lower rake props etc.
I investigated these props when I was designing waterproof drones that could potentially be used both out and in water. I found that the props that worked well in air worked worse in water and visa versa.
Then of course there’s the endless boat forums for fast, slow and electric trolling motors. When you read through them you will eventually end up back to similar props like FR is producing for the speeds and applications we need.
The photo of the jetfoiler board is their latest iteration they ride with pacificmeister. The first photo in this thread was their first version. If that first prop outperformed the latest versions then that would become their standard.
Riding a prop of 240mm also puts you 10cm lower into the water. That more than 10% mast length wasted.
I’m not stopping anyone trying to prove it’s better. But I don’t think you will. I think the conclusions will just come to the same ones as the other various forums.

There was (is) one guy in the forum (like almost 2 years aga) who did the first tests with outrunners for efoils. He also build up a test basin for measuring the thrust vs consumption. He wrote that with the air propeller he had the best results. I can’t find him in the forum. He had to quite because outof personal reasons.

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I found it:

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So when you read that whole thread it came down to the prop was efficient for slower speed foils and you had to use a big board etc.
What is really interesting is that his best performing boat style prop was the Solas which I already have a 3d printed prop that’s outperforming it. Then the the FR prop way outperforms both of those.