Solar power?

I think I am getting a little obsessed about solar power at the moment. I keep looking for photo voltaic systems (the off-grid homemade variety) and finding many ideas from this very simple, one battery with 50W solar panel , (incidently this guy has also built himself a nifty gas bottle wood stove and water heater system, and you can see many photos on his Facebook page, just follow the link on Tiny House Swoon)
 to solar cars and finally a solar powered tractor.

For some reason that solar tractor really appeals to me. We will be  using solar power for our boat (solar panels are being delivered on Tuesday) But finding this tractor made the cogs go overtime! Solar powered propulsion for the boat? If you can do it for a tractor that presumably require a lot of torque, surely I can do it for my boat! I am trying to think how this could work. I still like the idea of a stern wheel (I know they are out of fashion, lack efficiency,  as if efficiency should overide everything) and perhaps a electric motor (or two on a double wheel) would do the trick. Like every thing, it would have it's limitation, the range may not be in the hundreds of miles, but that's ok, we don't intend to go on for days without stopping, and the speed is limited anyway on waterways...Price?? Well, yes, but there again, a diesel engine or a couple of outboards would cost money too anyway! More thinking to be done on this.