Direct drive outrunner with direct water cooling



Finally found some time to make a little progress. I covered all the iron and magnets with epoxy hope this is enough. Looks like a lot of people trying to run a outrunner direct underwater. Any good results yet?


What motor & housing is that?


Well, i reached 16.5km/h with a cut down 7.5X8" Yamaha propeller on my windsurfboard with a battery power of 2.1kW using the 6384 PG 100Kv with 12s and a VESC 4.12 with modified cooling with heat pipes and forced air.
At 16km/h it is gliding, becomes little instable and the turnes are tighter.
I have the possibility to swap props now without disassembling the motor and another prop with 7.5X7" is on its way.


Were the STL files helpful?
Does anyone else need them, i can provide them per mail?


Yes, that would be really helpful, thanks!

Btw, has anybody the Solidworks files from the user tunnelvision? He uploaded it on Dropbox but the link expired.


Hi powerglider, still tuning the 3d printer and waiting for few props. Will update once will run in water. Great news. I think with a foil it will get much faster.

Do you know your current at start and 16.5km/h?


The motor current i can actually only measure with my computer and the VESC connected to it. I am not sure, if there was a motor current limitation with the 8" pitch prop. I am very sure there was no current limit with the 5" pitch prop. So i wait for the 7" to test. In the meanwhile i am building a 5 blade guide vane from PLA. Additionally i am rebuilding my battery holder which was made of carbon now with glass fiber. The carbon is highly conductive and i already had two small incidents leading to small fire. Luckily without much damage. This time i am using GFK plate and nylon threads, no more steel rods. There are such a lot of cables everywhere, one small plug on the Tf02 was shorted when i turned the whole board upside down to mount the old drive system, when the battery holder out of carbon was pressing right in the middle between the contacts for B+ and B-.:roll_eyes:


I appreciate that you share so many details. This really helps out others to not make the same mistakes.


Where are you getting this equation from? Are you converting 5.25" in your computations? RPM/V = Kv? V = volts? Kv = ?


Hi MaB,

Did you ever post your results from these props? I’m interested how the pitches of the 2 props compared



Its a Rotomax 80

I only need to install it on my board, so soon i`ll get some results.



Not enough, the 143 x 200mm pitch seems to push well but shaft failed too soon. I’ll try again this weekend


sounds good. I’m waiting on parts and then will start testing as well.
Fooling around with javaprop this weekend and hopefully printing out props next week.
Good luck!


136mm diameter, cut down from 7 or 7 1/2"
5" pitch was 13-14km/h at 1.7kW
8" pitch was 16.5km/h at 2.1kW
old setup with 6384 130Kv i ran last weekend
~170mm diameter, pitch 5" estimated 16km/h at 2.4kW


This looks beautiful, tell us, how did you test it?


Thank you,

Build goes a little bit slowly, because of work, but maybe a couple of hours away before testing. First I want to test how the motor holds up, when running underwater, and if the esc stays cool. Esc is mounted on the aluminum 5 mm plate that I also use for mounting the foil on the board. I also bolt I trough the board. I quite heavy 90kg, so I’ll hope the Chinese sup foil has enough lift?
Maybe the prop is still to big, but I can always trim it down. The motor is 195 kv, and my esc is 200A running on 8s 45Ah, so I hope this will work. In my old setup, without foil, My throttle is a bike brake, and the cable is connected to a linear potential meter inside the board, this has proven to be a solid waterproof throttle. Let’s see what happens, I’ll keep you updated


Is that a drive shaft that runs through the mast and connect to the prop?


no guess same board same motor , just put on the foil :wink:

@Johannes: this is a very nice built, do you have any problem with water and your bike throttle?

i think with you KV your propeller might be a little big , but you can always switch for the torqeedo ultralight 8*8 … waiting for the test !


Awesome looking setup, but are you going to put a duct around that Sushi maker? :slight_smile:Open props are so deadly and its SO easy for a wrong fall or accident to happen. Just be EXTRA careful Johannes!