Propellers and Ducts


Let’s talk about propellers and ducts. Unless we have a jet, we need a duct for safety. I hope everyone agrees with me here. Duct is the “guard” around the propeller, also known as Kort Nozzles or Shroud. There is some good overview info on ducts in this video here.

I have tried many propellers, starting ducted, then un-ducted, now back to ducted. I still see a big drop in efficiency above 12mph with a duct and want to understand how to improve that.

Here is my prop history:

  1. First prop, found it online somewhere, blades were dull without foil shape, round blades
  2. Went to straight bladed simple prop with thinner foil shaped blades, no duct. That had plenty of power and efficiency. Broke several blades, fragile design.
  3. These were my direct drive out-runner experiments. The props wrap directly around the can of a 60kv out-runner spinning wet under water. I’ll post something separate here. I see some builders are experimenting here too. The ROV community is too. I’d love to see us all explore this further.
  4. Prop for a test running a 149kv direct drive out-runner dry inside a housing. Totally overheated aver a minute and burned up the motor. Maybe I’ll post something separately here too some time.
  5. My current props, see the parametric prop video and design part of this Fusion, CAD assembly, or here a direct link to the prop Fusion file. Duct and prop inspired directly from @Hiorth. Works in the duct but as mentioned above, I am not super happy with the efficiency yet, especially after riding without duct. I need to get this optimized.

When I look at the blade shape of Lift and Fliteboard I see round blades with a pretty low pitch and huge surface area.

Lift prop/duct:

Fliteboard prop/duct:

What are your thoughts on these large, round and low pitch blade designs?


Big storm killed my attempt to test out 4 different props this weekend, hoping this week I get the an opening in weather and available time…I favor smaller props currently but as I keep leaning my direction alters course. The smaller prop allows for slower acceleration and smoother coasting when you ease off on the throttle VS larger prop acts like water brakes nose drops as you weight shifts forward harder.

You can see photo the smaller props I am using currently:


We used long time to read about different ducts/props/stators pros cons etc. They can increase thrust and efficiency at low speeds (they are used a lot on thug boats). But will give more drag than help above a given speed.

The kort nozzle was a early duct type, where they had to cut some corner to manage to produce it (round oddly shaped hollow steel cylinders are really pain to produce).

A time when the oil price was high some guys wanted to improve the kort nozzle, as they knew it was not optimal. A new type of nozzles emerged, nozzles with airfoil cross sections all the way around, and they are much better than the kort nozzle

As the nozzles are designed for low speeds (thug boats) we need to modify them. One way is to lower the AoA so there is less drag from the profile, altho this will reduce the efficiency at low speeds.

Twistes Stators (especially downstream of the propeller) are proven to increase efficiency. The stators takes some of the rotational energy from the water and converts it into additional thrust. I belive the upstream stators could be made much thinner (without twist) to mimimize drag, but they will have to be reinforced if 3d printed on a table top printer.

The prop also needs to be great… talked with some hydrodynamic people, seems like the easiest way is just to try different designs. Most prop efficiency literature are on large ships, and not that easy to convert into our hybrid solution (Ducted high speed vessel).


Has anyone entertained the idea of using a variable pitch prop or some sort of gear shifting so you can get more power to get out of the water and then use the power for speed once up? I realize it may be a bit over complicating things and hard to implement but curious if anyone ever thought about that.


Might as well publicly list the video now. Especially since it’s so cool @pacificmeister!


Bendy blades, with more sweep back outboard on the blades. At lower speed, the trailing area outboard lowers the pitch. ?


Just on the topic of propellers. I have been testing this one with amazing success.I’m 105kg and my set up is 22kg. I’m pulling ~50-52A at the start to get up on the foil and around 32-35A cruising at ~26mph. Its a 148mm OD propeller with a 200mm pitch. I’ve made a custom square 10mm 316SS propshaft to manage the torque and made a custom seal housing with 2 608 sized seals. All CAD is available for free.


Home Made Flume

What rpm is your prop turning? motor kv, gear box?


530KV motor, 6.67:1 planetary gearbox, prop rpm depends on how charged the batteries are, but it spins around ~3,500rpm.



@tunnelvision are you running this prop inside a duct? We need to figure out high performing prop/duct combinations. Prop alone is too scary.


@tunnelvision That is a nice prop!

Curious if anyone out there has been trying to model the performance of their prop. I’ve got an excel sheet that gives me an idea of thrust / torque where I am putting rough in CL / CD values from The results don’t mean much to be though because I don’t have a model for how much thrust is required to push my board out of the water and start foiling. And then, once you’re up on the foil it only takes a fraction of the thrust / power.

Here is a picture of my prop. Its an H105 hydrofoil profile that is designed to have a 3.5deg angle of attack at 2750 RPM and 14 knots. I found it interesting how much the drag drops off once you get to your on design condition.

For example, compare the Coefficient of Drag at 0.01 at on design vs 0.14 at stall for an E817 hydrofoil. 14X more drag when you are operating off design.


Thanks @tylerclark. There are still a few things to improve. I looped in Fusion expert @Taylor and we want to improve it more. Not sure if anyone tried the parametric prop yet. When changing the blade count the computer sits there for a minute, something in the model I set up in a non efficient way. Stay tuned.


Yeah, something like that would be interesting to take full advantage of your amp draw at the higher speeds


I run this propeller with a duct. I have two designs, one is a perpendicular hydrofoil (kort nozzle: i.e it does not taper in at the back like a traditional nozzle), this taper results in excessive drag. It is simply a aerofoil or hydrofoil profile which generates lift towards the propeller. I also have another design which is a pure foil (i.e. same shape on both surfaces) which also works. Both don’t seem to generate a lot of drag, however I haven’t had the guts to test it without it :wink:



I didn’t bother to reinvent the wheel with props because there are so many tried and true designs already. What I did was used this very simple calculator to figure out what diameter and pitch I needed for my fat ass to scoot along at 25mph given the kW I have available and the rpm of the system. Then I found and downloaded a mercury racing propeller design (which was far to big) and simply scaled it down with CAD to the correct OD, made some modifications like the square prop shaft and a few other things. That’s it. Printed it in PC-Max and bam!


This is awesome performance!Thanks for share!
But at 3500rmp on the prop,148mm dia and 200mm pitch with 26mph onboard this is no slip propeler :open_mouth::grinning::ok_hand: ,is this even possible? :thinking:.
Didn’t understand you with the duct… do you have duct with airfoil and without any angle?

best regards!


guys maybe time to think in a complete different way? Put the propblade inside and the engine outside? Gives you incredible power and no sealing problems anymore…:innocent:Expeller_explosion


Personally I use a solas one


Yep, a rim driven thruster should work very well. Only challenge is to build it from scratch, with a decent budget.

Is this your design? Does it work with 3kw?



@tunnelvision, I am very impressed that you got up to 26mph with that prop and with a duct. You said your CAD is available, you post a link to your prop and duct here? Thank you.