Is there a way to estimate the thrust of a setup?

EFOIL.builders Propulsion System (Motor, Gears)
#1

Hi, here again with the janky, cheapo E-surf build.

I’m again considering different motors and batteries to buy, and have come here for a bit of insight. Right now my plan is to buy 4 Turnigy 20,000mAh Li-Po batteries, and wire them in 2x2 for around 44V, with 40,000mAh of charge. First of all, is this viable? I’d like smaller batteries wired together so I can use them in other projects aswell, therefore I’m not a big fan of a huge Li-Ion pack.

But the real question I have is about the motors. I’m looking at either a Flipsky 65161 120KV 6000W with the prop, or the huge 85135 140KV 12000W one. Any recommendations? Note that a surfboard will require much more thrust than an E-foil.

Is there a formula or a calculator online to estimate the thrust on one of these badboys running at 44V, maybe 120A?

Cheers!

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#2

To be honest i think you would need 5 of those Turnigy Lipos in parallel and then series them for 12S and that may not even be good enough lol. They’re more like 1 or 2C not 12C. For anything half fun you would probably need 16KW+ of power and at 12s thats over 300 Amps assuming you’re using a Jet Pump

#3

The batterys are more like 3-4c based on some tests ive seen from an old esk8 form

#4

To answer your question about thrust. There is a very nice web tool that can somewhat do this: This propeller generation website run by Caeses (well known propeller design software) is free to use. You just input your motor parameters, your wanted propeller size, and it simply generates a propeller geometry for you, and calculates the efficiency and thrust. Depending on your vessel speed, the propeller will have a different pitch, which is mainly what limits your thrust.

You can also do some math. Most propellers are around 50-70% efficient (source).
If your motor power is P, and your vessel speed is v, the thrust T would be equal to:

T = propeller efficiency * P / v

For example, let’s assume 60% prop efficiency. If your motor power is 6000W and your desired vessel speed is 6 m/s (12 knots) the thrust would be 600 N or 60 kg of force. If your surfboard needs more thrust, you must use a lower pitch propeller and your top speed will be lower.
Static thrust is harder to approximate. Hope this answer will help you.

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#5

If/when you have a working setup, to get the static thrust, you can apply the board nose pressure against a bathroom scale (not necessarily waterproof) against a pontoon or a swimming pool wall.
This static thrust or Bollard push is given in kg, @Frage’s in 2018 measured 42kg of static thrust 117Amps 4200W.

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#6

I agree totally, thought about if i even should post this but maybe it can help someone to understand something i can myself get lost in:

Often the question is ”which prop do i need” and then we go to caeses propeller generator (or the example) which is calculating power and energy balance - under the given conditions. It means that if you input irrelevant data into caeses it will give a prop, efficiency and thrust data that is wrong in your use case - it’s not even sure you can replicate the conditions while on the water. It’s easy to think ”i need some margin, i’ll set power in caeses to 6000W since that’s what my system can output” For an efoil we will probably never consume 6000W at 6m/s so a prop made for those conditions will be wildly inefficient on an efoil.

I guess one way to truly know what are your inputs to caeses is to measure drag@velocity with your board, your weight and your wing when towed behind another boat. Apart from that you could pick a starting point from something known to be close to your scope and iterate from there…

(I know above were only example numbers so i am not in any way trying to derail your post @Slewentogzz)

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#7

I´ve got Flipsky 65161 120KV and 4 x 8ah 22v Turnigy Lipos. series and parallel. 16ah x 44,4v. i weight 88 kg and use a slingshot 76 infinity. board is small. 128 x 58 x 15. around 70 liters. The battery setup is more than enough, getting 100a with ease. i only use 66% throttle. My ride time is 25 min.
so 2 x 20 ah 22.2v - 20ah x 44.4v will for sure get you going. no doubt.
There are a lot of guys using this set up. Good Luck!

#8

This page has a decent explanation of propeller thrust and related equations:
https://www.mh-aerotools.de/airfoils/propuls4.htm