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.
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.
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 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
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.
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.
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)
Here is a video from a guy who built electrical jet surfboards, he talks about the challenges, might be interresting to watch. https://m.youtube.com/watch?v=dT80hJUREjY
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?
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?
Check the following project on thingiverse : it is a printed “dive-jet” but the main body could be clamped to anything from efoil to paddle through kayak and windsurf boards.
Now the US naval institute says “A propeller moves a large volume of water at a slow speed, while a waterjet moves a small volume of water at a high speed. Waterjet propulsion is therefore able to propel a ship faster than a conventional screw propeller can.” Only concerns would be that “the jets are efficient at high speeds, but inefficient at low speeds—the opposite of propeller efficiency.”
Now my question is : what is considered high speed or low speed in the foil zone ? My understanding is that the more you reduce the output the more speed you get. So it would “just” be a mater of finding the right size of output for the water intake…
Don’t you think that plugging a 12S2P and a 6384 to the bottom of this “Dive-jet” could make some marvels for a foil assist ?
The jet drive is powered by… a Parkside 20V 4Ah on-board battery, which even doubled would not give much autonomy. The idea would therefore be to separate the battery from the module in a FauxDrive gen2 under the board (or on top of it).
Actually, I was willing to put a 12s2P into this body, a 7070 flipsky as motor and clamp the all thing to my mast as a stokefoil warhead equivalent but with much more autonomy…
I’ve designed RC jetboats and jet units for them over much of my life and I can say that a lot of the designs you will find are very poor efficiency. A jet can produce a huge thrust in a small size but you must still be cognizant that intake ducting will cause significant drag if you are trying to just coast unpowered where as propellors can have blades that flip out of the way when unpowered. A jet pump must have a well designed stator to be of much use, that will add a short length to the unit. A jet unit does have internal drag on all it’s surfaces so it needs to be very well designed to become close to prop drive efficience but it’s biggest weakness is to objects getting stuck inside it. A coarse intake grill can certainly help. I think you would be looking at a very different motor seup with higher RPM and a lot less diameter, that comes with more noise and less low end grunt.
1k€ for now is the goal.
