Amberwolf, I'm pretty sure a toothed belt would work, but it would make it more complicated to play with the gear ratio. The bigger top chainring on the 2nd video is hacked together with various part, and it does not run true at all, that's why it is so loud. Also the chainline is not good on the 2nd video.
So it can be quiet, if everything is right.
Still thinking about the prop.. I'm pretty sure the prop is not usable for the speed I want to achieve. On the other hand, 6kph is OK, but for 6kph, I could have just bought one of these Minn Kota clones, build a decent Li-Ion Battery, and maybe add an ESC.
My ESC went for a swim after I did the test, and it runs reverse now
I hope I can fix this.. already gave it a flush with alcohol.. didn't help.