DIY Foil Assist Gen 3 thoughts for optimization

Hello all,

On original FD Assist (+, max, slim) , Foildrive use 6374 130 KV 3200 W 5.6N.m to have 25kg of trust at 3000 rpm on the propeller. The system is limited to 1500W by the VESC (half motor power) so I wonder if we could use a smaller motor because I conclude it’s an overkilled size for this use.

I’m thinking about an APS 5065 90kV 1800 W 5.7 N.m at best to reduce drastically the drag


the APS 6355 100kV 1700W 3.8 N.m to reduce the weigh.

Any comment or test about that ? @Jan3 maybe


NB: 12S2P instead of 8S3P is a game changer on Foil Assist. Thanks FD for the Gen 2 coming idea !

Well that’s not how it actually functions at all…

The assist has max power for an initial period to get the rider up. Then they limit the various iterations of FD to a safe continuous power limit because of battery longevity and lifetime.

The smaller 56mm motors can’t generate enough torque to get the rider up easily even at max power.

Then you need to take into account that guys are riding anything from 3ft to 10ft boards with an assist so there’s a massive difference in what is needed power-wise. If they decided to support a smaller motor, then it’s just an additional bunch of costs for a company with zero real gains…

That again has nothing to do with foil drive. Foil drive use a 11s3p battery.
The higher voltage push on assist units came from other companies like E-sea Foil and then FD one upped them on series count. Manta is the first company to use 12s2p.

Ok, so let’s stay in the DIY philosophy. If I want to optimize my personal device:

My setup is 12S2P with 6374 130kV, limited to 40 amp battery on BLheli and runs like a charm.

I’ve a lot of power available until the end of my battery (2.8 V cut off) , so I consider I can still get up and efoil at 3Vx12x40= 1440W

Why it wouldn’t be possible to use a 1700 or 2000 W motor ?

I understand the smaller motor will have less torque, does it mean it will draw more current to reach the good torque ?

The Foildrive Power Sport is a 11S2P for your info so very close to my setup

Yeah it’s basically the smaller rotor gives you less torque that’s the issue. But that also comes down to each individual and the board size, weight etc…

You might be able to get going on a smaller motor. You might also just need that tiny assist…

Try it and record data and let us know the results.

What is considered required startpower for 70kg, 90kg, … takeoff weight for a given board type and volume/shape, anyone of you who has educated guesses from your testing?

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I’m 95kg and can start a 27L 4’6 board with a very inefficient prop at about 3kw…
I was meant to test some new props this week but instead decided it was a good idea to lift a CNC machine and put my back out.

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I’m 85kg, FD 3 blades aluminium prop, 45 liters 4’6 board, 900 high aspect foil, I can start with 1500 watts.

These examples makes one think you are onto something here. Finishing my first fd classic clone and already thinking what is next. Slimmer motor size sounds like a nobrainer candidate, along with subsurface/board battery and esc mount….

Regarding torque. It’s about saturated stator steel afak and enough stator/rotor length can also deliver required torque for a given diameter so the slimmer APS with 5.8nm but lower watts is the same torque design as any diameter motor with 5.8nm, so factors affecting takeoff and sucess would be more system overall design such as prop eff, proper rpm, stable voltage and getting enough current to the kv motor design to deliver required torque without heating etc?
Sound like lots of fun ahead.

Can’t trust rc motor specs, normally a fantasy number the seller just makes up.

A proportional comparison shows that a 56100 sized outrunner can be estimated to have equal max torque to a 6384 if the airgap flux is the same. In reality it’s unlikely to reach same flux density, with smaller diameter the area for steel in the stator teeth and room for copper between the teeth gets smaller. Efficiency will be also be lower at max torque.

What happens at smaller diameters is also that (for same power) the efficiency is shifted towards higher rpm so the smaller motors can have good power and efficiency but are not the best for direct drive where torque needs to be higher.

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A question to those of you who’s been here and built for years now. Is it pure wisdom and testing that established 63mm as the diy defacto choice?

Wisdom is a strong word😀
I’ve tried smaller motors without success, they burnt without reaching foiling speed even though i tried a few props. Torque and power just wasn’t enough.

Trial and error into a pre-existing range of motor diameters : 50 56 58 63 70 80mm

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