On this fairly lengthy thread there is quite a lot of information on towed hydrogenerators, including some attempts at evaluating the performance of the Hamilton Ferris unit referred to by David Th. The standard spinner for the Ferris WP20 tow generator is (was) based on a Johnson/Evinrude propeller, Part number: 386587. Size 8-1/4x5.
Here is one example:
“I'm also measuring about 50% slip when extracting 60 watts at 5 knots with the same prop on a Hamilton Ferris generator.
<3.0 kts: zero output
3.5 kts: 0.2 amp
4.0 kts: 1.0
4.5 kts: 2.0
4.7 kts: 2.2
5.0 kts 2.5
5.2 kts: 2.8
5.3 kts: 4.0
5.5 kts: 4.5
5.6 kts: 4.8
The results above were obtained while charging a 400 amp/hour lead/acid battery bank with a terminal voltage of 12.5 volts”
Here is another evaluation of the Hamilton Ferris unit.
These figures would seem to match the requirements of a house battery bank while on an offshore passage or perhaps a lengthy coastal cruise, and I dare say there are a number of JRA members who know about these things and have used them in the past. Probably not going to add hugely to the challenge of maintaining the battery bank for an electric auxiliary motor.
However, part of the fun of that (going electric auxiliary) would be the challenge of grabbing free energy from wherever it is available by whatever means, whenever the opportunity presents itself, and storing it for when it is needed. Like collecting firewood in summer. In that case, hydrogeneration might still have a place, even though as a sailboat solution it is fatally flawed by the following simple paradox – a kind of catch 22 and a disincentive to improve its effectiveness: The better hydrogeneration is at producing electricity, the more it is going to degrade the performance of its own input source.
My feeling is that with today’s technology, the main source of energy for an electric auxiliary motor battery bank would probably have to be solar, but it would still be good fun to supplement this with other non-carbon sources whenever possible.
There is quite a lot to be found on the internet relating to the use of discarded Fisher and Paykel washing machine smart motors, converted to generators. (For example). Evidently they are especially suitable. This has always pricked my curiosity, because Fisher and Paykel is a New Zealand designed and made brand of washing machine, so it is one of those rare cases of a manufactured resource of which discarded examples suitable for repurposing ought to be readily available in New Zealand. I don’t have enough knowledge about electricity to evaluate if a converted washing machine smart motor would suit a towed spinner, but maybe that’s worth a look.
I wish I could get my hands on a discarded trolling motor which I imagine could convert very nicely into a retractable hydrogenerator, but I am afraid in New Zealand, that's not very likely.
Another thought, which to me is a more intuitive solution to hydrogeneration than a towed spinner, would be a reverse version of a Thai/Vietnamese longtail motor. Ought to be easy enough to assemble or dismantle and anyway no more difficut to stow than a yuloh. (Easier to change props too - perhaps it could also serve as an "electric yuloh"!)
Wind generators seem to have gone out of fashion these days, for a couple of good reasons, I suppose. But as we move beyond the need to merely feed a house battery, and into the era of sustainable electric power auxiliary propulsion (and I think we are now moving in this direction) perhaps wind power generation should be re-visited as part of a suite of complementary partial solutions. Vertical axis wind generators have not been found to be as efficient (yet) as the horizontal axis wind generators which used to be ubiquitous on long distance cruising boats – but that does not mean they are ineffective. I wonder if a slower turning, hi-torque vertical axis wind generator might better suit the geometry of a cruising boat deck layout, be safer and quieter, and perhaps be worth looking at as another possible source of battery power, along with hydrogeneration, to supplement and complement the primary source: solar panels. I wondered if the large diameter Fisher and Paykel washing machine motor might make a suitable partner for a vertical axis wind turbine. I wish I knew more. I am starting to find the forum comments of our electrically savvy JRA members of great interest now, and re-reading threads which I had previously not studied. Starting with Davd Ty’s Electric outboard drive for small cruisers thread which, by the way, I wish I had gone back and read more carefully before starting another one.