[Jets vs Propeller] pros and cons, practicality

efoils – Electric Hydrofoil Surfboards
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#1

Hi, relatively new to all manner of electric vehicles, but I’ve been reading a lot on this stuff, and been planning on making my own motorized surfboard.

I’m now trying to design the board and plan out the build process, while still staying in a student budget, and I started to wonder how pretty much all of the DIY motorised surfboards online use water jets with impellers, while practically every Efoil design uses a slower rotating propeller attached to the mast. Is it a question of useability? Do waterjets not work well underwater? Why dont surfboards use larger propellers?

I’d like to know more about the reasoning for choosing either one as the method of propulsion on a surfboard or an Efoil. I was going to buy a cheap used surfboard online and buy a beefy waterproof motor and propeller to propel the board, but decided to ask you guys first. My goal is to go very fast on a cheap, ugly board.

Cheers!

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#2

There is at least one electric surboard that uses a propeller, see here: Propellers instead of water jets on electric surfboards - #4 by Sliman_O

There are some efoils with sort of a jet, like the Audi etron: https://aerofoils.de/

If you want to go fast on a budget, better build an efoil, as you will need very high currents for a fast jetboard. This is challanging as you might need to cool your batteries. Motors for big jets as well as the jets are expensive to.

#3

Thanks for the quick answer sat-be!

I had heard of one brand making the propeller propelled ones, but I forgot the name. Waterwolf, thats what it was! I wonder what are the advantages of them using propellers… watercooling made easy? Also, if Efoils dont need as much current, wouldn’t they be also slower than a surfboard? Or does the foil really reduce the drag that much…

Efoils dont interest me quite so much, as I’d like to really feel the water and the surface under the board when im zooming around.

As for the budget, I’d still be happy to spend around 300€ on a good motor, 200€ on the ESC, and another 200-300€ on the battery setup, not even mentioning the board itself :slight_smile: 1k€ for now is the goal.

I did some crude calc’s in my head, as I’m not looking for anything pretty or safe, just results and speed:

  • Ideally I’d want to go atleast 40km/h
  • It seems the boards available right now accelerate to their top speed at around 1 to 2 seconds, so the acceleration would be around 7m/s^2 for that top speed
  • My weight + the board would be around 100kg, so for the acceleration I’d need something like 1kN, or 230lb of thrust
#4

I use both jet and regular prop and neither perform that different. The jet is better when you breach the water and recovery vs a prop which needs to gain RPM’s again. But overall about the same with the jet being safer of course.

#5

Yes the drag of the board is much bigger than that of a foil. On an efoil I can reach 40km/h@around 4kW. I don‘t know how much you would need on an esurf but it will be more.

#6

I see, very interesting.
I’m also struggling to get a grasp on how much thrust any of the available propellers would give me.
I found this guy online, at 6000W, but I can’t find any thrust data or a calculator online to tell if running it at around 50V would give me anywhere near enough thrust (~1kN)

#7

Here is a video from a guy who built electrical jet surfboards, he talks about the challenges, might be interresting to watch. DIY Jetsurf Build | featuring Christian Green, a creator of his own DIY electric surfboard - YouTube

#8

You built a jet drive efoil that performs the same as the prop version? This seem remarkable to me, what jet drive are you using and how comparable is run time?

#9

Unfortunately I cannot disclose anymore because it’s a commercial product in development. Just know it works! :wink:

#10

Is there a general formula for the thrust produced with a certain motor setup? Lets say for example:

2x 5kW waterjets that were lets say 60% efficient
and a 10kW propeller motor unit with a theoretical 60% efficiency

Assuming thats all there is to know, I’d say you’d get the same thrust on both setups. I hear waterjets are a lot more efficient than propellers though… Is that true? To what extent?

In reality I’m sure the math isn’t so simple, but is it still possible to estimate the thrust from the voltage, KV rating, prop size, torque, and mass of the rider?

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