Direct drive outrunner with direct water cooling


This is a solution for keep it simple stupid. I started in late 2016 as i saw e-foil vids. This was my new hobby. I dealt with electric vehicles, motor design and control, sailing, flying and designing and building modelplanes, windsurfing before and this was my thing. Get these things working under water. In the most simple way possible. Do not mess around with tightenings, they will all fail. We as end users do not have the time and experience to deal with such things. I wanted a prototype within 4 months, 1000$, using existing windsurfboards.
The new core idea came from vids trying to seal outrunners for his rov. He faced a ball bearing problem and switched to sintered bush bearings, i did not understand fully why. I was looking for good ball bearings, backed by some experienced buddies. I use Stainless with ceramic balls and seals type simmerring. There are a lot of different types, some with abec classification.
I searched for Brushless outrunners, because they have a higher torque constant per diameter than inrunners. The active volume of a motor is proportional to the possible torque by the means of the possible given magnetic flux density given by the materials used. If you overload a motor with torque, so the magnetic field is overriden, the efficiency sinks dramatically. Outrunners on the other hand are harder to cool effectively. If you stay conventional. It is easy to open the front and back, so air can circulate. That helps a lot.
But you could also change the medium which is circulating through the cavities of the windings. Wouldnt that be great? At least if there is enough space between them. So the motor should be designed in a way that easies cooling.
So Ampere is proportional to torque and thrust.
If we can overload all components which are under water if we almost contact the electric resistors with it, only with a small, short barrier in between, we could solve the cooling with water where it is needed most. At the surface of the stator made of copper, isolation and highly corrosive plated iron, 0.35mm thick i think. This stator has a greater surface than a simple cylinder, and the medium inside is moved. Dust and sand and microbes are in the cooling water. They all get milled and shered! What a mess! This will not last for 500 hours!

What we need to do is to plastify this conglomerate. We have to wet all surfaces.
This can be done e.g. by vacuum with epoxy. You should change the pressure frequently, this helps much to get out all air out of cavities and bring in more epoxy into it. Continue to apply changing pressure until there are no more air bubbles coming out.
I use special epoxy “wasserklar” with 4-5h curing time for this special item.
When the epoxy starts to gel, take the stator up and let it drop off. Wrap it some times to get a better average coverage and warm it with e.g. hairdryer to accelerate. Latest after 10h warm it up to 70°C for 3h to cure everything.
Clean the bearing seatings after removal of the bath and a second time some hours later. Make it look nice.

The electrical interface is unclear to me up to now. The ends of the internal motor windings are too short to seal them completely. On the other hand they are only 2-3cm long. With the custom motor from alien we will find a way, make them longer, so there is no additional interface needed.

Belt Drive Concept
Propulsion System - Motors, Gears, etc
Propulsion Type Overview - Wiki
Propulsion System - Motors, Gears, etc

Hi @PowerGlider! I like that and experimented here too. See the post here. Is that a special heat conducting epoxy you are using? Lots of info in the ROV world on that. Also important to coat the magnets, they corrode fast. Keeping everything spinning without touching with that even tighter air gap (actually water gap) is tricky. Curious to see your progress. Post most more info on your tests and results so far please.


Idea is cool, but we wanted to build a motor like this, but we were told the friction of the fluid instead of air is that much more that it does not make sense at all?
What are your values sofar?


To make it clear:

This is definitely unwanted.
By such an application of plastic around the complete winding package, the thermal resistance is only worsened and can be worse in water than without anything in air.
Also this is formulated very euphemistic:
"Outstanding Thermal Conductivity: our filled epoxy materials excel at transporting and dissipating heat from motors and other coil windings, a reduction in core temperature is always achieved."
Only if you compare it to conventional epoxy, maybe. I have found little evidence, that epoxy or other plastics could be enhanced significantly by mixing with copper or silver dust. The thermal capacity rises and conductivity as well, but it is still magnitudes less than the metall itself would have. Epoxy has 0.2 W/mK, pure copper around 400. So if someone tells that the conductivity is 70% or 300% better than epoxy it is still very very bad. Arctic Silver Epoxy is sold for terrific price and they do not even tell you what the thermal conductivity is.
I had frustrating results with this motor design:

I filled the epoxy with masses of copperdust and the windings were burnt after short time. Additionally i closed the bell to have less hydrodynamical losses, i suspect the water was cooking inside the bell.

Lessons learned: 1. Do not care about the hydrodynamical losses, keep the water going through and 2. keep the epoxy as thin and as large area as possible, use very thin epoxy with long curing time and vacuum and let the epoxy dripp off before it is geling too much.

The hydrodynamic power loss you can easily estimate. Measure the no load current in air before and in water afterwards without a propeller. My motor spins with 5700RPM, which is not very much. I think it added 90W to the no load losses, could be slightly more with the slots left open. Water has low viscosity, motor oil has 3 times higher viscosity.
If you plan very high RPM, the losses will be higher, i cannot tell you how much.

From my last motor i have no pictures, sorry, it was built in a hurry and since then not disassembled. It is working fine so far after several hours testing in the sea.

The magnets in the rotor should be covered with epoxy, i had it done in my lathe. Fix a tightening rubber ring inside the bell where the magnets end. Fix a disc with a center hole to apply the epoxy. Use 40 minutes curing time or even less.

Use more tape than suggested by this foto to fix the disc. The disc is made from a solderwire bobbin.
Let it spin for some hours and warm it only slightly, otherwise the tape looses contact and whooosh…you have a decorative memory on walls, clothes, lights.
After curing warm it some hours in an oven at 70°C.
Be careful when lathing the abandon epoxy not to hit the magnets surface.
Be careful when clamping, my lathe has very bad tolerances when clamping the drive shaft, it is better to clamp the bell at the aluminum head directly.

Electric surfboard - dual motor direct drive. Few Questions

Currently i am in contact with Bruno, to make the outstanding 6384 even better for our purposes.

The goal is to drive such propeller directly:


or similar
with around 7.25 x 6"
which can be cut down to work in a smaller shroud or Kort jet or used in original shape.

  • My plan is, to have no sensors and have a longer stator instead, the magnets are long enough to do this without any change on the rotor. 10% more stator length brings 10% more torque, thrust and power.

  • Bring the kv slightly down, i estimate 80-100kv instead of 130 might be optimal for me, please discuss this here.

  • Longer phase wiring outlets from the hub/stator. This will enable a newbie, to make a watertight coupling to the wires.

  • Hub and stator shall not be glued to ease the application of Epoxy only to the stator, not the hub and thereby the bearing seating. Stator and hub shall be disassembled for epoxying. Makes it easier to handle the process.

Please discuss her other items, Bruno and his team could improve on the motor manufacturing side and what we could want to do on our own, so this thing does not become too special.

With 12s and 100kv this motor would have 4400RPM max.
Already now i am getting very good thrust with the 130kv and the motor does not seem to be overloaded, only the cables and controller.

To get a smaller duct, the propeller can be cut further to a diameter of 12cm. The efficiency of the propeller sinks under high load condition, though the duct adds less drag at higher speeds.

I think a 63mm diameter is still rather slim, and the length of the complete drive including nose is only 200mm.

This system has a high efficiency, as the motor windings stay rather cool, the hydrodynamical losses inside the motor are overseeable and it can drive a large rotor without gearbox.
The complexity is the lowest i can find at the moment and the cost is low.

You will have to change the bearings from time to time, its 20€ per 3-set.

Please discuss the kv and further improvements, I need some input.:wink:


@pacificmeister @PowerGlider
Hey guys, I have the option to upgrade to SS bearings or ceramic bearings in the motor.

What would last longer or be stronger for long-term use in damp conditions like we have?


I’d go ceramic. Most stainless steel will start corroding in salt water at some point. Also ceramic bearings can be run lubrication free.


Awesome, great to know. Thank you Merten!


yes ceramic, and low friction value is incredible!


Hi pacificmeister, thanks for makeing my life worth living again, you are a big inspiration:star_struck:
Ok i have found this on youtube about stainless steel vs ceramic bearings. Maybe you find it usefull.


Wow thx, a full 20min video about that :smile: I fast forwarded to the conclusion, will try to watch more later… so far I am not discouraged, I assumed that hybrid ceramic/steel bearings are not ideal, found that full ceramic one. Will see. And it seems like he evaluated it in context of a bicycle, low speeds.


What do you think of a Nylon bushing instead of a ball bearing at the propeller end of the shaft? Water lubricated Nylon should have low friction and it would be salt water tolerant.


I like it. IGUS seems to have a good selection of these: Worth a try.

So I just talked to the IGUS guys. There are 2 high speed choices for our use case: L500 material and L350 material. They will send me a couple L500 samples (8mm ID 10mm OD, 10mm length for $5.89 each) for testing. Guess I will have to change the design one more time, but just very minor :slight_smile:


A few watts is not the end of the world, the key here that ceramic we don’t need to worry about rust, saving a few watts is just an added bonus. Great video, thank you for sharing @Flex


I just realized i only use simple stainless steel bearings. Wanted to have ceramic balls but did not succeed with size 61800 or 6800 to find any including “tightenings”. Anyhow, bearings in/at saltwater are wear parts like tires for a car. Do not spend too much money for them. They all do well in the beginning :wink: so find a solution to maintain and exchange them without disambling every screw your design comprises, thats the challenge.


I just realize, there is only one rotating tight in my design if i easily optimize it. So rear side needs rotating tight and the other gets some kind of cap mounted to the hub.


If the front cap would be elastic, it could take over the volume displacemet of the inner volume between the bearings, caused by pressure changes so there is no pressure difference between inner and room with the bearings and the ambient.
By this design sea water is not pressed through the rear rotating sealing, because of missing pressure difference.
The front bearing is only confrontated with air and grease pressed through it by ambient pressure changes.
This design reliefs the rotating thightening from static pressure from different medias.


That ceramic bearing video was great. I reviewed it again and those WATT savings look tiny, but really they are % wise quite a bit… Considering he was using biking RPM for this of only 90 RPM, I have a feeling the watt savings would be dramatically larger for high RPM speeds of 4000 - 20,000 rpm on our motors. Ceramic bearings are designed for HIGH speeds, not so much for skateboards and bikes as proven in the video above.

In Short: Ceramic bearings saves power, reduces heat/friction, and do not rust.


I purchased 3 bearings from here:
Warning: These are normal steel bearings with maybe ceramic balls in.

Do not buy them. I cannot measure any bearing correctly, they all show Overload on my Ohm-meter, ceramic or not. If i press them together they show all possible values, ceramic or not.
In short: Ceramic bearings cost a lot of money and have no benefits for our purposes. If they are “cheap” it is a cheat. Save your money for something meaningful.


Some more pictures i found in my vids.
This is latest configuration with alien 6384 130kv, for all details please read above, as a windsurfboard drive.

I believe you need this compactness of the drive unit. Its just the motor.
Plastic parts are made from nylon and milled from ring, plate and cylinder.