Working out the numbers as a first time efoil builder

efoils – Electric Hydrofoil Surfboards
1

Hey guys, I’ve been lurking in this forum for quite a while now and I’m slowly getting ready on building my first efoil. I’ve built FPV drones and a couple of hobbyist-level microelectronics in the past (working as a software engineer), so the process is not intimidating. However, I got a few questions that I couldn’t answer by googing.

It seems that building an efoil is pretty straight forward; there’s only 1 motor and a big battery.
Please correct me if I’m wrong, but It should go like:

  1. Calculate how much weight (your body)
  2. You need to push, for how much time
  3. Under what kind of conditions

… and start working out your build

Well,

  1. I’m at 95kg
  2. I hope to get around an hour ± 15m of good time
  3. And I’m located at Greece. So there might be a few < 0.5m waves, nothing crazy (for now… we got a lot of kitesurf spots which I think I’ll avoid) I’m just doing SUP until now.

So, now that we got those requirements, it boils down to selecting a capable board w/ foil, then selecting a motor capable of producing enough burst thrust to get us up in flight mode and that can sustain enough load to keep us going for about an hour. Building/Picking a battery should be pretty straightforward. Cells in series to meet the power output X cells in parallel to meet capacity. (And hopefully 5kgs won’t destroy the dynamics of the rest of the system).

So, is there any recommendation on what kind of board w/ foil to use for those circumstances?

  • gong allvator rise (XL) that I’ve seen getting mentioned a lot doesnt support >85kg if you look at the infographic, but provides a lot of thrust for just wing foiling.
  • I’ve also read about rules of thumb like “> 1750cm^2” wings for >95kg surfers, so It’s just hard to select some thing for sure. Not having any formula involved makes the thing way complicated and empirical
  • Plus, how big of a mast?

Okay, let’s say that we’ve selected our surfboard and our foil. Got to select a motor and everything else would follow. I see that there are a lot of flipsky builds around here so I took a look around that. I see people using 130-200+ KV motors and I got a few questions:

  • For the very big 200+ KV motors that some of you guys use, WHY do you do it? It’s nowhere near the max voltage range of the fattest 14-15-16s battery packs and you should be unable to go above 60-70% sustainable load!
  • For the very small 130ish KV motors (down to 6s! 4s was my drone, man!), I understand that it’s sufficient for assistance (to get up) or sup. Why is it mounted higher up on the mast on some builds though.
  • Let’s say you get like the 65111 160KV motor. It’s IP68 rated! They’re being sold as waterproof, but are there any steps to take vs. waterproofing and salt corrosion?

That’s a big thread already, and I should be able to make enough progression towards picking every component after getting those questions answered! Thanks In advance guys :pray:

PS. Those maytech remotes are kind of expensive, aren’t they? I’m surprised there’s no easy-to-make open source (software+hardware) alternatives out there…

1 Like
2

I started on the gong rise xl, 135kg total weight with board. The rise xl moves and turns like a truck - i don’t think it is better to learn on actually. With the rise xl and 80100 motor i could throttle straight up from standstill just by the power and lift available so it’s more lift than you’ll ever need! I use a curve LT now and like it much better, it tells you what is going on in a more direct and intuitive way.

I’ve used both 80100 direct drive and the 65161 motor both from maytech and flipsky. I like the flipsky shaft seal design better since the screw hole pattern in the shaft end is more inboards, good for if you mount a prop duct or use a prop cone.

There’s a clear sound difference between inrunner and outrunner, to me it’s enough to prefer the 65161, the loudest sound while riding with it is the sound of the water spray from the mast, it’s really quiet.

3

About motors, in my experience voltage isn’t all (there is also current ahahhaha), kv tell you nothing about the actual specs of the motor (torque at specific rpm, efficiency, ecc) so for example you can use different kv rating to match better your battery pack (or vice versa) in order to archieve better efficiency/torque at certain rpm. Unfortunately for most of the motors around on the hobby market there is little to nothing documentation so estimate the efficiency / torque based on kv is more a rule of thumb rather than real math :sweat_smile:
About battery, again 4s/6s tell you little to nothing about the output power. So for example your drone in 6s config will draw 3 Amps ((3.7V x 6) x 3A =66.6W nominal) but for example the 65161 motor in the same 6s config will draw aroud 40A so 888W (for the 65161 since there is graph about efficiency / input power at a certain rpm you can actually calculate this parameter and if you know the exact parameter of you prop you can also estimate the thrust) so maybe those who use big 200+ kv motor with low voltage pack don’t care about speed but instead efficiency of the system in order to increase the ride time even with relatively small battery pack (or maybe they are just afraid of speed :rofl:).
For the controller… i saw some threads about it but i didn’t spend much time since in my build i will probably build one myself with some custom features (maybe unique talking about existant controllers :joy:)
I hope that i answered some of your questions,i didn’t dig much on the math in order to keep as clear as possible the reply, also sorry if i made some errors but my main language is not english.

4

I don’t think you clarified anything there :grinning:
People who use high kV motors probably have high current controllers and want to go really fast, have small props and don’t give a rats ass about cavitation. Who knows? Most of us run 12-14S battery, 100-120kV motors and run roughly 5-6” props with 5-7” pitch and it works well. There are many other combinations that will work well too if you want some other kV, voltage and current, just balance them.

5

well maybe you know this but i replied to ntakouris since he had some question about choosing motor (kv, voltage ecc). When you know what are you doing its easy to say “just balance them” but if you are new to this world maybe a little bit of theory behind “just balance them” is a good starting point :roll_eyes: