Propeller with the duct integrated vs propeller + a duct?

Hello everyone!

As nobody uses such a propeller with an “integrated duct” instead of having the prop + the duct, there must be some good reason but I’m curious about them. Could somebody explain me a bit?


I’m no hydrodynamic expert, but I noticed when I was running one of my motors under water with no prop that it generates a lot of turbulence at high speed.

My guess is that the integrated duct, being at the outer edge of the prop and therefore moving very quickly, would generate a huge amount of drag and decrease efficiency significantly.

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I think the advantage of a static duct is to protect legs and avoid injuries too.

I’m no Hydro expert either and as you, I was thinking that the rotating surfaces had to be kept as low as possible to decrease drag. Then, I thought about the outrunner motors (see @Mat video) being tested successfully where the motor rotating outer shell surface is very important.
So now, I think both (with and without ring) deserve to be tested + It would be interesting to compare both drawn powers at given speeds.
What is sure is that with this ring setup, you don’t have the drag of the 3 or 4 struts holding the duct.

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I also think this is an interesting concept, especially for outrunners, where it is a bit more inconvenient to mount the duct struts.

I already tested this idea

The consumption is a nightmare! I was really surprised on this.

It is a modified hiorth propeller. Without spinning duct the hiorth propeller needs 38A @ 20 km/h on my setup.

With the spinning duct it is 100A @ 20 km/h.

I think there is no chance having an efficient propeller with spinning duct if it is 3D printed.


Probably it needs to be aluminum or similar. Bit still a sad result.

Did you try with the prop like on your image or being polished? I can totaly imagine that having a rough surface like unpolished 3d printing that is rotating can only give bad results, but if it’s perfectly polished we must have the same effect as the rotating outrunner motor Soefoil mentionned. I find all that interesting as I saw such a big difference in term of speed between having a duct and not having one, that maybe we can find something in between in terms of efficiency and safety…

It was slightly sanded. I have found an interisting paper called:
A viable approach to propeller safety for small craft: Ringed Propellers

Conclusion: It can work!!


I just can’t imagine the braking forces involved at high rotation at the link between the prop and the guard…

At least the end of the prop should be right at the border of the guard to give a chance to the water to escape from the prop without brakes

How ducting a propeller increases efficiency and thrust :


Is the torqueedo solution good for this by having an rpm of 1400 it would slap away a limb entering the zone but be less likely to sever a large limb like an ankle or femur (just in case the emergency stop fails)
Just curious about people s thoughts on this

Along these lines i have been investigating this. Not a very large increase in efficiency in air but under water might be a thing. I have yet to build it as I am still conceptualizing it. This might be a way to get up on the foil at lower speeds.

I would try again with a duct that is a third or less of the size. You definitely don’t need it that big. Aluminum guard would keep it rigid at a smaller size, as the issue would be how thin can the 3d print be and still survive the forces.

Something more like this.


But isn’t it the same with the non rotating duct in terms of “escaping flow”?