I also filled up the outer rotor magnets and reduced the holes at the top, to have less rotational drag:
Prop I 3D printed one out of the forum and attached a layer of epoxy (Volkers prop)
The board is also done. xps with a top and bottom sheet of thin wood to get forces not pressing into the xps. At the mast I doubled the wood and placed floor pvc parts, because that was for free at the store as an example and it is water resistant. I ued the additonal layer pvc because Iwas feared that the mast and screws on the other side are pressing in the wood and then the sealent epoxy layer at the top gets broken and water comes into the wood. One layer of carbon I placed from the mast side to the top side to eliminate that the different layers get separated by forces from the mast. More strengthening I dont think is needed, due to the thickness of the xps, the board is stiff. the wood I sealed with a layer of epoxy.
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The electronics inside is in a normal kitchen box, which I wrapped around such a cheap china silikon ball to make it super tight. The box I am holding down in the board with a frame which I can snap in ontop of the box. Wiring just attached with adhesive stripes. adhesives stripes also used to smoothen the surface of the motormount.
I also buildt a quick and dirty water detector.
I just opened an old damaged electronical scale and soldered the needed parts out. You cant see those small smd parts, but they are there 
1: 10kOhm resistor
2. buzzer
3. npn transistor
4. sensor cables attached to the basis of the transistor.
As soon as one of the cables are in water, current can float into the basis and the buzzer gets activated. It works fine. But I havent built in the board yet.
Coming to the first test run.
The board overall worked fine.
A bit of rework is needed, because the controll unit gets quite hot. I hope I can do something in the settings to reduce it.
Those 3 wire inlets into the box are not perfekt waterproof. small drops came inside.
Does anybody has a better Idea how to make the inlets waterproof, but still easy to disassemble.
The last thing is that the cheap china fuse can by far not withstand what is written on it (150A). Something in the inside melted and now it is not possible to trigger or close the fuse anymore. Is that issue know or did I just could have a faulty part? Anybody also using that part or another better one?
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Who wants to see what such a cheap efoil can do, see below.
I will improove the issues and then making a calculation, what it was overall. I assume it was more like 700€ but the battery is quite big.
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People used those in the early days 2019/20 but most cheap ones could not withstand more than 60A, no matter what they were rated.
Better use an antispark connector like xt90s or QS8S. If you want a fuse, midiOtto 150 - 200A is rated for 58V and is quite compact. Works well in my 2 boards.
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Here an update. I opened the fuse and the metal pins which are making the conection inside are held by cheap plastic which melted down. I excluded the fuse and did a second testrun:
(proper foiling skills ;))
Unfortunately the the ESC got to hot and is shutting down. I got a hint that maybe my prop does not have good efficiency. It is the prooven Volkers prop, but I printed with a bad printer and I deformed the prop maybe also a bit due to heat (I wanted to eliminate burrs at the edges and flamed it. The prop got very soft while that) and I also fixed an edge with UV-epoxy (I crashed the prop inside my door :D).
So I have bought a plastic flite prop and a 38€ flipsky aluminium folding prop (https://a.aliexpress.com/_ExFvH3u) and printed mounts for it:
I also bought a cheap china wifi amp/power meter and built it in the board to compare the new props (conection to smartphone works fine): https://a.aliexpress.com/_EzZ7fiA
The fuse I exchanged to 175A also from Ali: https://a.aliexpress.com/_EIVDPfu
So on the weekend I will test at least the flite prop. So let us see how it is going out.
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Nice build. Can you please share the 3d printfiles you used to mount the props to the 6384 and what type of bold head is that? looks familiar.
that adapter for the flipsky folding prop is from @seagull_nz (BDUAV 120kv Motor Flipsky Prop Void Fill by seagullnz - Thingiverse). I could not find his original post, so I hope it is also fine referencing him like that. The fit on my 6384 is excellent (perfectly centering and in line with the outer diameter). It fits also to the flipsky prop which I got, anyhow the outer diameter transition and centering the prop could get even more improved (maybe Seagulls prop has slightly other dimensions).
I ordered this 38€ alu folding prop: https://a.aliexpress.com/_ExFvH3u
The flite adapter I did myself. Centering on Motor is quite good (not as good as seagulls for the flipsky. He has here really 0 play). Centering on the Fliteprop side is perfect for mine, because I optimized for that. But it could be, that it does not fit to others, because there is also a flite prop measurement of someone else in the forum and that is deviating for several 0.x mm (3 blade prop file? - #3 by SoEFoil). So looks like there is quite a lot of fluctuation in the part.
Here is all of my data(creo, stl). I printed it with the motor side upwards to have a really smooth surface on the motorside to fit better to the motor.
https://www.dropbox.com/scl/fi/wgwijfsukrtipn820o3ka/Fliteboard.rar?rlkey=u11s3kgsfxi8x5u8ni76h43tk&st=2idw0cxx&dl=0
Screw which I used is in M8 x 40mm: https://a.aliexpress.com/_EzxgdmW
For the flite adapter attaching to the motor, I use quite short screws (10mm) to keep everything compact)
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Thanks a lot for the detailed reply! Ill post a picture when finished. First learning the 3d print proces. 
I tried today the flite prop. Seemed to be much better to me. Had more power at lower throttle. Current I do not know in detail because I do not have measurements from the self made prop. But now coming to the issue. After 400 meter of driving I got a complete shut down and motor started stuttering. I opened the motor and unfortunatelysome epoxy got lost and the wires got pressed out. that is strange to me why the wiring moved out between the rotor and stator. Does anybody have an idea?
I ordered now a new one (unfortunately a sensored one, but this was the only one I could find here in germany on ebay to get it fast. This time I will do it with corrosionX for eliminating corrosion inside. Let us see how this works.
Is it possible to remove the sensor easily or will I get issues with that?
Removing the Sensor is a quick and easy job, just pull the pcb out
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The same thing happened to my 6384 flipsky motor. I have two options: the epoxy layer is breaking down due to vibrations from bad bearings or poor propeller balancing, or it is heating up and breaking down because this type of resin is not thermoplastic.
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I assume the epoxy did not bond well to the stator and in use some parts came off and pulled the enclosed cables with it.
Did you prepare the motor in some way before potting it in epoxy?
If there was a residue of oil or something similar, that could explain it.
I am at a similar stage and my epoxy does not really bond to the magnets for example. Not really sure how to solve this.
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No, I did not prepare the motor. Also to my mahnets onn the outer rotor it is not sticking. So I removed it from the magnets with a screw driver. Now there it is just between the magnets. I put grease on the magnets. Let us see if this works. Next time for sure I will not don it myself and will pay double the amount for the flipsky
With the new motor I did the same coating again, because corrosionX did not arrive before I had to leave to lake garda. Unfortunately the same issue happened once again. But this time I kept the circlip off, that I can pull off the rotor. So I analyzed the motor immediately after first signs of same issue occured. This time epoxy was not removed but it smelled burned. So I assume my coating does not conduct heat enough. It also looks like that it burned at the end surface facing towards the propeller. So next motor I will for sure not do on my own. 3 options which I am thinking about right now.
-flipsky 6384 and using it with short cooldowns between starts
-flipsky65161/maytech65162 to get some more power. (starting against wind waves I was not capable of, only going with the wind waves)
-65220 with integrated esc (I am feared that I get overheating problems with my esc which is right now just placed in the batterybox without any active/passive cooling).
What are your thoughts/opinions on that?
Well on the risk of being wrong again.
I have a Flipsky 6374 which is working great so far.
Compared to your picture above, there is barely any coating on the stator.
The gaps with the wires are actually recessed, so they are not flush with the stator.
I assume your coat is way to thick, probably due to the white stuff mixed in.
Above you see visible rubbing marks.
Your epoxy is probably rubbing against the magnets and thereby creating the heat you are talking about. At least that is my guess.
On your motor I would try to sand away as much of the coating as you can and then try again.
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Thanks for your comments. Helps a lot for reflecting. With the thickness I had indead issues with the second motor in the beginning (there is grease on the outer rotor, so it gets black where it got in contact). The first one was thinner. On the second I grinded down the contact points and thereafter, there were no contact points (also not under load). Overheat of the motor happened after grinding. Due to the fact, that I do not want to loose more time on the water, I am giving up with the selfbuilt of the motor.
After some more thoughts on the burned motors, I think the option with the aluminium did not work. The thermal cunductivity I assume is to low. Bubbles in my epoxy did not improve the situation.
I dont know if someone else tried already the aluminium, but for me it is not working. I bought now the flipsky 65220 with inside esc in 120kv, uart mode and threaded shaft. Hope this will go better.