Question regarding motors


Hi All !

First of all, thanks for this forum and for all the topic already opened, it helps a lot !

Like many people I guess, I’m going through the question of the motor.
We are oposing 2 thinks:

  • Inrunner with Gearbox
  • Outrunner with direct drive

The only real question for us I think is the torque.
My question is: How to estimate the torque ? I know the basic physics behind it but struggle with one question regarding the 56104SS motor.
This motor’s specs tell 500kv, 11kW, 116A max and 88V max BUT, we are all using 12S (44V) batteries.
Does this means that the actual power with 12S batteries is not 11kW but half (5,5kW) ?

This is a pretty big deal for me since it’s change my equation by a factor of 2 !

If I’m correct, I’ve calculated the following torques:

  • 56104 SSS Inrunner motor:
    Max speed at 44v: 22 000tr/min (2 303rad/s)
    Max torque : 5 500/2303 = 2,4 Nm (If my assumption regarding the Power at 44V is true !)
    With 5:1 gearbox:
    Max speed: 4400 tr/min
    Max Torque: 12 Nm

  • 6384 100Kv - 4000W
    Max speed at 44V: 4400tr/min (460 rad/s)
    Max Torque: 4000/460 = 8,7 Nm

As we can see, the difference between the 2 setups is not that big, especially if we take into consideration the many loss that the Inrunner + gearbox setup implies (gearbox loss + more bearings).

Do you guys agree with the maths and the “Power theory” ?
This would help me take the right decision a lot

Thanks =)


You are correct, but: in general inrunners are more efficient than outrunners.
And the outrunners runs either wet => losses because of water pushing around in windings, or dry but then you need the same bearings and seals (without the gearbox but still 0,9x (efficiency inrunner) * 0,96 (efficiency gearbox) is more than ~0,85 (which is already pretty good for outrunners).
And thats the point your missing in your equations. The SSS can draw 116A at 44V => 5,1kW but will give you maximum only ~4,5kW at the prop.
Same for the outrunner: 44V*80A => ~3,5kW drawn from ESC, but only ~3kW at prop.
So there might still be quite a difference.
And you can adjust the needed torque by using different size and pitch prop.


Inrunners are more efficient than outrunners if you compare power and weight, because an inrunner can spin faster, so the gravimetric power density is higher. But to get this higher power density you need to spin the inrunner with really high RPM. This is the reason, why these low RPM/Volt motors have a high voltage, so to benefit from this higher power density you have to use the full RPM capability. We do not only need high power, we also need rather low RPM to be able to use more efficient props, so we need torque. So if you would use the full RPM range of the inrunner you end up with a 2-stage gear and a rather small motor. Unfortenately this is costly and complex. And it also becomes unpractical, your drive system will become longer, which increases the lever and so on.

I agree, that inrunners have a higher power density if used in the optimal way, but to reach this optimum is not easy, especially if you look for efficient large props. On the other hand we have some examples here in the forum, where rather high efficiency seems to be generated with jet-like setups using inrunners without gear. As we want a duct around the prop for safety reasons, we generate drag, as we need a kind of grid at least at the front of the duct to prevent fingers, arms or head to get in touch with the prop, this added drag might not be proportional to the diameter but to the area the prop covers. The larger efficiency of the prop might be nullified by the larger drag of the duct and grid. The optimum prop and duct for a given power output and velocity has to be found first, we dont know it yet.

Another question is, how much power do we need? For foiling it seems to be rather low, for classic surfboards it will remain higher with raising speed.

And in the end its about cost. I can buy a 6384PG 100Kv for around 100-120€ and i have to put some epoxy which creates some work. But than my drive chain is finished. Compared to inrunner + gearbox + sealings + extra bearings + sealed housing + thermal grease +++ this is really cheap and simple. But there are also drawbacks, like you should not use a watercooling for the ESC (e.g. the YEP HV WC makes a connection from Battery to water), or at least you have to make absolutely sure there is no leakage from the motor connections and windings.

The 80A specified is for intense air cooling, water cooling will allow for higher amps, maybe i go up to 90A if needed. The 44V i mentioned in my chart is 12s at 50% battery charge *0.95 because the VESC only puts this out as a maximum dutycycle. I am not sure if other ESC put out higher dutycycle. There are different approaches to explain why, but they all do not convince me so far.

There are a lot of things to think about when choosing between these basic concepts.


Can anyone help?.What reduction gear and ratio for the following setup…Im running 3000 watt aquastar 4084 620kv brushless inrunner coupled with a Kenzen RC 110mm prop kort nozzle and a Turnigy AquaStar 200A Watercooled Sensorless High Voltage (ESC) with 6s lipo …I know its on the low power side ,But I plan on using a large homemade 30 by 11 inch sup wing for lift…Any comments on which reduction gear would be ideal?any comments…thanks


Thaks a lot for your answers Giga and PowerGlider !
Your emails confirmed my choice to go for an outrunner with some modifications.
I’ll start designing something and get back to you regarding new ideas to come =)

PowerGlider, If I’m correct you are not planning to start with a Foiler board but with a Windsurf one ?
Have you got an idea on how much power you are losing with the epoxy on the stator ?


I cannot help with dimensioning gear box BowenEfoil. Basically you take the pitch by 0.75, the wanted velocity and the resulting RPM.

millesth: I am focused on motorizing windsurfboards but also like to try foils. But first i would like to learn it before building one.

Not sure if i get this 2nd question right, you are asking about the effect of epoxy film on the thermal resistance?
Hard to estimate, maybe you try it yourself? As it is essentially needed i do not care. As water has much higher cooling effect, i do not care but assume that overall cooling is much better than in air.


I’m more asking about the impact of the epowy film on the power transmitted from the stator to the rotor. Does it decrease the performance of the motor ?
I haven’t received the 6384 motor yet but I’m not planing on using epoxy =)
We’ll see how it goes, I’ll keep you guys updated !

PowerGlider, I think I read somewhere you are doing your trials in Hyeres - France ?


I am not aware of a measurable effect of epoxy coating on the performance of a motor. What is your concern about using epoxy?

I went to Hyeres several times in easter and autumn to sail and windsurf. Last two times i had with me the electric surfboard. Conditions were rather good. I live in the wrong place, in Frankfurt. Maybe next easter again for 10 days.


Powerglider, next time you come to Hyeres, please let me know, it is an hour drive from home/