Propellers and Ducts


Thanks! 3d printer should arrive this week but my motor is a few weeks away. I’ll keep you posted.


Yes sanding does help. Hard to say how much but I believe I can feel the difference. My neighbour works in the jewelry business and he has some great sanding tools, give him my props and he brings them back super smooth :smile:


Hi,here is some explanation…

“To understand why most waterjets have the nozzle above the on-plane waterline, let’s take a look at where the thrust of a waterjet comes from.
Ignoring for the moment the internal components, a waterjet is a classic example of Newton’s laws at their simplest - for every force, there is an equal and opposite reaction force. Force is the product of mass times acceleration, or put another way, the rate of change of momentum.
To apply a forward force on the boat, the waterjet accelerates a large mass of water in the opposite direction. In early prototypes, with the output nozzle underwater, this posed a problem. Water is dense and incompressible, so the outlet stream quickly encountered a lot of back pressure from the surrounding water. It’s very hard to get a high flow rate at a high speed with the surrounding water resisting the output stream (momentum = mass * velocity). Thus it quickly becomes very difficult to give the water leaving the jet a significantly greater momentum than the water around it.
When we move the outlet above the water, this picture changes. Air is compressible and about 600 times less dense than water, and so presents very little back pressure. Now it’s not nearly so difficult to get the outlet flow to have both a high volume and a high speed, thus the change in momentum of the water through the pump is higher, thus the force is higher.”



Thanks for that explanation Simon!

The first time I saw an electric jetboard I though how fun it would be to put a foil on one. The design in my head was to suck the water up through the mast and out the back of the board. I noticed on my Waverunner that the water shoots out above the waterline so I figured it would work to have the water shoot out the back of a foilboard. Then I ran across this video which confirmed the idea would be possible:

It wasn’t long after that when I saw a video of the Jetfoiler and that concept looked more practical. But according to Simon there may be some benefits to shooting the water out above the waterline.


Cool did you try the hiorth duct with this prop?


-und momentum schon getestet? Ergebnis wäre interessant…:wink:


Hi Simon, my current values and speed come from my own personal arduino hacker set up and should be taken with a grain of salt. I’m pretty confident that the numbers I’m recording have alteast +/-10% accuracy. The prop was designed for me and my own setup to minimise slip and maximise efficiency. It will vary setup to set up.



Tunnelvision, where i buy this propeller? Thanks.


Which propeller? Mine? I used existing CAD and modified it to my needs then 3D printed it. Happy to share the file. Here it is.


Home Made Flume

Hey Guys, A backer of my Kickstarter Project found a really good program for wing foils and props both air and water. It helps you designed and test everything on the spot with JAVA based program. I have not yet read through the entire site but looks interesting.

You can even select the prop surface finishes…:
NACA Standard
BUGS AND DIRT… yes, this is an option

3D design of foil

@pacificmeister What pitch prop are you currently using with your 5:1 ratio model? Thank you in advance!


Hi @Graystorm44. I didn’t calculate pitch. Maybe you can? :slight_smile: Have a look at the CAD here. I have successfully used the 3 and 4 blade props created with this parametric prop setup and the default parameters. The Fusion setup was since improved by @Taylor here. You can quickly adjust all the curves to modify size, shape, angles… resulting in pitch changes.


Hi VeFoil,

I’ve played around with that program in the meantime and find it very helpful. I would be really glad if someone with some propeller experience could have a look at it and confirm that for all of us. :slight_smile: Would be very cool to be able to reliably calculate the efficiency and thrust of a setup before buying and testing various parts.




yea, having similar thoughts, I’ve be at marine shops looking at small 3 to 8 hp outboard motor propellers plastic and aluminium, if we are aiming for around 3000rpm and maybe to have a amp current meter and a few temperature reading sensors, then it could help on the calculations,
i have played with different props on my 70hp outboard, and its just to not risk to over rpm by to light of a pitch (but it will get on plane faster) or to have a to aggressive pitch and risk stressing the motor (and for us electronics), i think trying different props is the only true way, and a way to read the data in real time


in an ideal world a servo controlled variable pitch propeller would be perfect, gentle pitch for start up, and then aggressive pitch for when up on the plane,
so its finding the sweet spot of compromise


I have a concern that some might automatically think that having a duct will make it safe. Nothing against any of the other comments but I have as many concerns about a duct setup as without.

  1. The duct it self can have many sharp edges if not designed right. Rounded trailing edges are very inefficient so we tend to make them thin
  2. A large duct is inefficient so we tend to make it as small as possible. This can create a scissor effect between the duct and the prop.
    I personally plan on running with no duct but will take other precautions to make the propeller as safe as possible.
  3. The leading and tip edges will be rounded.
  4. I want to use an out runner motor to keep the assembly as short and close to the mast as possible.
  5. I will have a disconnect with a lanyard. I have already accidentally bumped the throttle when falling.
    Regardless, there should be great caution from the rider about where their feet are. The mast and especially the trailing edge of the wings can injure a foot. Having flown model aircraft all my life I have see how little or much damage a spinning propeller can do to fingers and they are spinning at 30,000 rpm.


Thank you sir, You are an awesome resource! I enjoy watching your progression!!


yea i hear you, safety all the way, i am thinking of trying a 5 to 8 hp outboard engine propeller, or maybe the aliexpress thruster propeller assembly with the duct, trying to work out the best option,
so far i have the sss 360 kv motor a 240amp esc, and the neugart ple 60 5:1 (the ple 60 has a higher Nm range)
so do you think the open propeller is the best option?
its just rapid cutting off the power if (when) you fall off might make it safer


I don’t know what the answer but Dan and I plan on doing some safety testing this winter with different prop and duct configurations. I just want more open thinking and discussion on the subject.


Anyone try this software?

Looks better than the javaprop hah