I've been reluctant to write about my hydrogenerator for two primary reasons. First, it's hardly a new idea. There are commercial versions (that cost a grand or two) and hundreds of sailors have built DIY versions for decades. Second, I hadn't tested my little creation yet so it seemed a little presumptuous to wax on about something that might not work at all.
But today, halleluiah, I sea trailed the whole mess in the dinghy with great results.
Essentially you toss the black prop (connected to the stainless shaft there) into the water. It's connected by a shackle to some single braid line, and then goes taught, the other end of the line being secured to a shackle on an Electro-Craft E722 permanent magnet motor's axle.
At 300 RPMS, which seems to happen around ~4.3 knots, the motor makes about 12.5 volts. The faster it goes the more amperage and voltage it creates. I'm not worried about a voltage regulator because the amperage is low (about 5 amps at 12.5 volts, to maybe 12 amps at 90 volts), but to reach the upper threshold of voltage we would be doing double our hull speed so at that point over charging our batteries will be the least of my concerns.
I only lost about 1/4 of a knot in my testing, so if I double that (to be a pessimist) and do some math it means a 20 day passage would now take 22 days. But that's 22 days of making 5 amps continually, which means we don't need to haul as much gasoline or diesel. And as anyone who's sailed downwind knows, it literally stinks to run the engine because the exhaust fumes blow into the cockpit and cabin.
For the motor I had a few criteria. One, it had to be a permanent magnet motor. Two, it had to be fairly low horsepower because I wouldn't have a lot of rotational torque (generated by the prop) to work with. Three, it had to hit charging voltage (> 12.5 volts) at a low RPM. The Ametek models are definitely the most popular, but I re-read the specs on the Electro-Craft E722 and felt it would do the job as well if not better. Both of these motors were originally designed to spin large disk drives around and as such are frequently found on Ebay. The good news is that a lot of people are building wind generators these days, the bad news is that it's raised the price considerably on the motors. I was lucky and got mine for $50 but $100-$200 seems a bit more common.
For the prop I'm sure there are better options than what I got, but I was pressed for time and saw a $400 prop on sale at West Marine in San Diego for $40. I figured that was better than whatever else I'd be able to find so I tossed it in my checked baggage and called it good.
I had a machine shop drill a hole through the motor shaft that would accept an 8mm shackle so I could secure that to a thimble in the tow line.
I also had the machine shop use a solid stainless rod, thread it, and pop a nyloc nut on for the prop. Also, they put an eyelet on the end that I could likewise shackle to a thimble on the end of the tow line.
When testing I was happy to see that the line doesn't kink at all: the rotational twist builds up in a few seconds and then starts happily spinning the motor. The drag on the prop causes the line (at least my single braid) to not kink, and any twist is transferred as kinetic energy to the motor's shaft.
The only other thing you want to make sure to add is a blocking diode. Otherwise when there's more voltage in the batteries than the motor is making, either because the boat is slow or possibly because your solar panels are putting out a lot of juice, the motor will start consuming electricity rather than generating it, and you can watch the prop as it tries to spin itself away from your boat.
All in, rounding up, I spent $50 on the motor, $8 on a blocking diode, $40 on the prop, $100 at the machine shop, and $40 worth of line and thimbles. So that's $240 and possibly another $30 for a soft shackle to make a fairlead, call it $300 all in to hopefully have a sizeable heap of clean energy for our passages. I'll report back with more info but I've got high hopes that in addition to being quieter and more environmentally friendly it will also end up being cheaper as $300 worth of gas and diesel really doesn't go that far.
Regarding fish eating it. Personally I think that's cruiser-folklore (a.k.a. "sea stories"). The stainless steel shaft is two feet long and it's 1/2" single braid which is pretty tough to slice unless you hack it to death in multiple attempts. I'm of the personal belief that the loss of towed props is due much more to common and less dramatic reasons like forgetting to mouse a shackle. Either way though, the electrical connections and line that secures the motor to the boat will be sufficient but intentionally the weakest link so that if a school-bus sized sea monster bites hard it wont rip the transom off.