Using spinning prop to charge batts under sail - SailNet Community
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post #1 of 25 Old 06-04-2008 Thread Starter
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Using spinning prop to charge batts under sail

Does anyone know if anyone makes a generator that can somehow connect to the driveshaft and use the freewheeling prop to generate electricity to charge the batteries while under sail? I know Lagoon now has some cats that are series hybrids (actually powered by electroc motors also used as generators under sail), but why could all sailboats not just use this wasted energy to charge our batteries? I'm sure someone has thought of this already, but I haven't come across any products to buy!
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post #2 of 25 Old 06-04-2008
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Some gearboxes don't like being turned continuously when the engine and raw water cooling is not running.

Then there is the noise, the nice thing about turning the engine off, is the relative quiet thereafter. A spinning prop-shaft makes a surprising amount of noise. It would make more with an alternator attached.

But the technique has been used.

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post #3 of 25 Old 06-04-2008
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My friend's boat has an alternator hooked up to the propshaft to be used as a generator. Don't see why you couldn't do the same thing on yours. In fact, one brand of wind generator does basically the same thing.

As the seagull says.... you need to make sure that leaving the prop spinning won't fry your transmission. Also, you need to make sure that you set it up properly, so it doesn't side-load your propshaft and cause misalignment problems.

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post #4 of 25 Old 06-04-2008
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I considered this, but prefer the idea of a dual-mode wind generator that can be towed, because of access and because my transmission prefers circulating oil.

Most of the "tow-able" wind generators seem to be at best average amp-producers when used with the wind, but work very well indeed when towed.

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AquaGen Marine Wind / Water Generators

I like the AquaGen model because the alternator is easily accessible at the base of the pole, meaning less weight aloft in the head of the unit as is the case with most wind generators.

For me, wind/water generation is the third of four methods of making amps, as listed in the following likely order (the system is designed but not yet installed): 1) 4 x 130 W solar panels; 2) 75 amp alternator with spare; 3) Wind/water genset; 4) Honda EU2000 as "spot charge" or so I can charge the separate anchor windlass battery or so I can run heavy-draw appliances or hand tools.

This will go into an approximately 840 Ah battery bank and through a Xantrex RS 2000 charger/inverter. The biggest aspect will be the rather complex combining switches, the monitorings of input/output and the decision between AGM or lead-acid batteries. I was committed to AGM but I am beginning to see the logic of just getting a horde of easily serviced and widely available "golf-cart batteries" of the Trojan T-145 Plus variety or something similar.
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post #5 of 25 Old 06-04-2008
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I mentioned this briefly in the solidnav thread, I don't think anyone actually produces a solution but it certainly can be done.
It would not be as efficient as a purpose designed prop such as the the aquagen.
My boat has a unique arrangement, a lifting leg made by Stillete coupled to a (IIRC) 2.3:1 transmission, 15x12 prop and Westerbeake 30 hp diesel. I doubt I could work something to generate amps with the system as is (the prop doesn't spin while sailing leg down and engine in neutral that I've noticed).

My Hunter 31 with a Yanmar 21 was supposedly 'jump' start capable by sailing fast with the compression lever disengaged then popping it. FWIW while sailing with the prop spinning the altenator (which takes 3-5 5hp off the output) was not spinning.
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post #6 of 25 Old 06-04-2008
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An alternator will do the trick. A large sheave on the shaft and a good mount for the hi output alternator. A buddy has one on a boat with a hydraulic transmission.

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post #7 of 25 Old 06-04-2008
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Has anyone considered the extra drag of having to pull the spinning prop thru the water while sailings. There is going to be more drag with that prop spinning than with it locked. May or may not be noticable, depending on the boat.

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post #8 of 25 Old 06-04-2008
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can't lock the prop with a hydraulic transmission without a brake and lock on the shaft. On my own boat I always sail with the prop spinning, three bladed prop. Never put it in gear.

Rick I
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post #9 of 25 Old 06-04-2008
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While you would think the 'feathering' prop would have less drag its been proven that a still prop causes less drag. Not certain why. I leave mine locked in reverse while sailing along.

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post #10 of 25 Old 06-04-2008
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No Free Lunch

Even if the prop is "free wheeling" it is not imposing as much drag as it would if you loaded the shaft with a Generator. Not to say this isn't a good idea, but the energy isn't "free"
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