Remote motor location

Why not mount the motor In the upright and power the prop with a flex shaft , then just the prop would be in the water. Less drag , more efficient.

I suppose the flex shaft will go in the mast ?
Won’t handle 15Nm
Water is used to cool motor

How about in the board with a right angle gearbox and driveshaft?

Good concept it may work. Also see Propulsion Type Overview - Wiki - Wiki -
Lots of R&D $$$ lots of options to try.

has been discussed already: Use a grinder gearbox and torq-tube for a simple solution - #9 by Daniel_Hines - Propulsion System (Motor, Gears) -
But I don’t think it is a good solution. With all the direct drive options available today. Direct water cooling is a big advantage for motors with several KW.

I got pretty far into that design including a full mast the had the gear box in it, full hight prop drive shaft and everything functional. Ended up giving up on it because heat from the chain was too much. Even with a greased enclosure it was literally smoking hot withing moments of running at any reasonable throttle.

Sounds like more trouble than it’s Worth.

OK, twin low power motors/4" screws 1/4"flex drive.