I was involved in
electric vehicle development for a Chinese company -
marketing development, not technical.
Naive ("sophomoric" - great word!) suggestions from time to time would be along the lines of placing wind generation in the roofracks, handlebars etc etc to recharge the power supply.
The Professor of Aeronautics response (apart from disguised eye rolling) would be that such generation devices were (in the late 00's) around 30% efficient. There is always a cost of generating power. For these, a 70% loss in normal operation. Its OK to lose energy when going downhill or braking, but these times are relatively few. More importantly there were more efficient, lower cost, lower
maintenance ways of harnessing that that loss than little windmills.
It seems all this is analogous to the OP's thinking. It makes sense that a similar 70% or so inefficiency percentage would apply to
hull or tow hydro generation on a boat.
IMO any loss percentage at all puts paid to suggestions of hydro generating power for use in
propulsion. But most
boats would be very happy to sacrifice a tiny bit of forward speed on a long trip for the comfort of
charging batteries to run lights, make a bit of ice. And where I am from there's normally too much wind, and the boat often has wind and water energy to burn. But I wouldn't want an in hull system that is permanently engaged, and therefore susceptible to weed growth, breakage, difficult to
service.
The OP, as a
novice, may consider how the Log on a boat (to
record boat speed), works. It's a little water
wheel, 6 paddle blades about 1cm sq, on the end of a through hull
plug. To keep mine accurate, (in low weed growth fairly cold water), I'd have to pull it out every month, have a brief water spout inside the boat, replace the
plug with a blank plug, scrape the weed off, put it all back, and then mop up from the water spout. Just another of the myriad of five minute jobs.
A similar but larger device could generate usable accessory power from water flow over the hull, in conditions where the inefficiency doesn't matter. But the
maintenance? And my guess is there is more water flow away from the hull, not right next to it (this thought supported by the necessity to calibrate the Log). So the
windvane system of a power generating
outboard would be attractive to me. (I was reluctant to get a tow generator, as I was damn sure there'd be a day when I'd be too knackered and forget it, wrapping the thing around the prop when berthing (as done to the
tender a few times).
There you go.