Propulsion System - Motors, Gears, etc

Nice idea, could you give us a link to such motor which is better designed than RC brushless motor? Why do you think low Kv means increased ohmic loss?


I could not give you a link to such a motor datasheet, due to intellectual property reasons, but I have seen som of the work done by my friend, who designs PMSM motors, ranging from a couple of Nmm torque at some thousand RPMs, to the couple of hundred Nm torque, at the speed of around 10k RPMs…
…About low KV: As far as I know, a DC/BLDC/PMSM motor is pretty much the same from the behaviour point of view. The same motor produces torque (ideally, as if commuted well, and no field weakening is applied, or as RC guys call it, timing) proportionally with current flowing in the windings, and generates backEMF against the voltage trying to generate the current, proportionally with RPM. Consequently, If you want more torque (once again, in the same motor), you will need more turns of windings, and in the same volume, this means longer wire, with less diameter. Of course there are “tricks” just like changing winding pattern, or pole/slot count, but that is not the same motor.

or run it in FOC mode

All in one?

New to the hobbie and wondering why not? Take it apart, few O Rings, grease and plough on?


Sure would be nice, but there is no way a 90mm turbine blade will turn 59,000 RPM (or any rpm fast enough) with a small motor like that in the water, and your throttle response would be all or nothing at that rpm, no feathering. Plus the motors torque would be maxed out all the time and likely burn out quickly.

Running direct drive you more or less need much lower KV motor, larger diameter motor for more torque, and you’ll likely be running with 56-70mm impeller with 3-4 blades.

No problem with IP, but it makes the discussion meaningless in an open source forum.
On the other hand, like a collegue told me: All patents about electric motors have been done already. There is nothing really new.
But it is an interesting field anyhow, with many small optimizations.

and many more…
The cooling with low Kv direct drive is a challenge.
If you solve the cooling issue (e.g. by water inside), at least the ohmic losses do not destroy the motor.
The truth is, the lower the RPM, the lower the power, so gearboxes are often used.
So yes, we have a lot of compromizes and you can build a series of custom motors.

I will not reflect on the IP topic any more, but it became obvious that I need to clarify my intentions.
After checking out some of the videos you guys posted, it became clear to me, that if I want to build an eFoil thruster, I will need a motor and an esc specifically designed for the application in order to do things right.
In this case I’m risking tens of thousands of Euros spent on a system, which might as well be useless, because I have no experience with the application. On the other hand, if you help with the specs., and I decide to have manufactured a couple of pieces (1-2) for my personal interest, after that it will be significantly cheaper to produce quantities of couple of tens per batch. (And sell it to you guys with minimal-to-no margin, after having the initial costs returned)
Hence the suggestion: First of all lets find out some specifications, like required RPM, torque, current, voltage, and geometries of a motor. It also requires the decision to be made: large prop or jet.
After this, I can get you some estimation on pricing, and approximately plus-minus 5% accurate details of such a motor. (@PowerGlider: Yes that means opensourcing the motor design) . When it is decided, that the motor is ok, I can have some protos made, and get them tested (maybe by some of you in europe), and we can start mass production.
At the end I see this as a win-win scenario, because we split the initial costs, and we all gain some optimised power source, yet we have the chance to do things, that never would be possible with a stock motor (like integrate the stator itself to the external housing of the thruster unit, or use a single, but replaceable shaft from the rotor to the prop…

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You should look at my other post ive done the same thing and have sent out motors for the community to test. The motor will be all inclusive with shaft, seals etc built in. we are just tweaking the KV vs prop.


What is the maximum of the outer diameter of the thruster, you guys can live with? As far as I can see it is around 60mm. Is this correct? Could it be for example 70 or 80 mm? Every mm gain in diameter will help to give more torque.

Yes. We have seen people here foiling with 80mm direct drive outrunners. These have the much discussed cooling challenges and running them wet has other challenges like making them corrosion resistant. A bigger affordable low kv direct drive inrunner would be great. 70mm OD, no problem. The benefits of more torque at low rpm will outweigh the additional drag.


check out underwater iron man on youtube… this is what these guys did.

where did you make these ALU parts?

I think the same project but it is poor.
I was thinking an eletric smerigliatrice (in italiano) machine…it is multitool.

Anyway nice idea…cheaper and fast to ride the waves!

need your advice to choose my sss 56114 in combination with the reducer NEUGART PLE 40/5: 1 …
360 kv or 500 kv …
I admit to being a little lost, less Kv more torque certte but the reducer will not you have too much impact on a low kv engine … ??
thanks for your advices.

Good question … Not much impact on torque with kv with a 56mm and 1:5 i think… Focus on rpm and top speed you want, 500kv with a “low” pitch propeller or 360kv with a high pitch propeller , both will work i guess , which one is better , hard to said …

Propeller design gain is higher than " kv torque gain " i think

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Are you able to machine down a case-hardened shaft from 10mm to 8mm (or let machine down) ?
otherwise I would look for a motor with 8mm shaft, which might be non of the 56114 but the 56104 e.g. 500kv

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I did not pay attention that the 114 was in 10 mm … although I can rectify the tree … I will look at the side of 56104.

just curious if a higher rpm on a ceramic sealed shaft and sealed motor all the water flow will keep the motor cool and you could change prop to a less pitch prop

Yeah you can. I took my shaft down from 8 to 6mm in about 3 minutes with a mill.

Yeah, I did mine from 10 to 8 as well, but without mill it would have been impossible…