I built some outriggers for my kayak for mounting solar panels (18V, 50W each). I also put together a small electric outboard motor using a motor and planetary gearbox from an 18V cordless impact driver.
The university (NMU) held its second annual solar boat race through the canals in St Francis a couple of weeks ago. It was also the first opportunity to test the whole shebang.I did most of the race on only two active panels and a substitute prop but at least managed to keep going for the full four hours without anything smoking or melting. Average speed was 2.2kts (4kph).
I have since had it out on the river for another 2hr test using a larger prop. Speed is up to 3.2kts which means propulsive power has tripled. The new prop is much more efficient but it also loads the motor more and gets it running closer to its own max efficiency rpm. The only trade-off is on the panels' side. Since it draws more current, it is pulling the voltage down so I am not getting rated power coming in.
Thus far I have wired the panels directly to the motor. No battery and no other electronics in between. I will be installing a dc voltage converter next. It will feed the motor more amps at lower volts while keeping the panels pushing out max power.