Using prop to generate?

[JeremyFowler]

Should I consider installing a pulley onto the new prop shaft? This would presumably enable an alternator to be installed later and thereby to charge batteries while the gearbox is in neutral when sailing in adequate wind. Yes? No?
Engine: Yanmar 3JH4E 29.4KW/3000RPM.
Gearbox: KM35A Gear ratio 2.64
80 Amp Hitachi alternator installed on engine currently.
Fixed prop, 14 inch.
It will be a big job to install afterwards but could be done relatively easily while I have the engine out and installing a new prop shaft. 1.25 inch dia.
Would the side loading on the gearbox be an issue? (as caused by the belt tension).
Has anyone successfully done this? Maybe I've been looking in the wrong places and it is already a well-researched thing...
Cheers!

[stormalong]

The issues are: Can your transmission be allowed to freewheel without damage? Can you fit a lay shaft with a pulley ratio that gets the alternator up to operating speeds?

I knew a sailor, boat builder that did this. Had loads of power under way. You can't just belt an alternator to a pulley around the prop shaft. You need enough of a ratio to get the alternator to operating speed. There is usually not enough room around a prop shaft for a pulley large enough to get the speed up. You have to place a shaft near it with two pulleys on it with a sufficient enough ratio to get the speed up. Commonly called a lay shaft.

[roverhi]

You would need to get an oversize prop to generate enough rpm/torque to spin an alternator up to a speed that'd give you much benefit. Bigger prop, bigger drag and seriously hurt sailing performance. Unless you are 'true believer' in alternate energy at any sacrifice wouldn't bother.

[goeasy123]

See here.... LIFE: Part 2: Prop-Shaft Driven Alternator

[seandepagnier]

you would do better to invest in a purpose tow generator. I built one for < $50. A propeller is simply the wrong shape for generating power, so the amount of drag is much higher for the same power generated.

This was also more common 30-40 years ago, but today solar panels are very potent and probably a better investment overall.

[valhalla360]

Originally Posted by seandepagnier

you would do better to invest in a purpose tow generator. I built one for < $50. A propeller is simply the wrong shape for generating power, so the amount of drag is much higher for the same power generated.

This was also more common 30-40 years ago, but today solar panels are very potent and probably a better investment overall.

Wow, I agree 100% with seanpagnier.

[JeremyFowler]

Thanks for the wise input!
I breathe a sigh of relief knowing that for once I'm doing the right thing by doing the easy thing (just putting a new shaft in without hassling about pulleys etc.)
Cheers!

[Uncle Bob]

Originally Posted by valhalla360

Wow, I agree 100% with seanpagnier.

Ditto.

[tkeithlu]

Yeah, that would be a very low efficiency set up. Lots more drag than power stored. Even with the trailing impeller, there is no free lunch. You will experience more drag than you will gain later from your batteries. If that were not true, you get a Nobel for inventing perpetual motion.

[GrowleyMonster]

First, all power taken from the boat's forward motion subtracts from the power creating this forward motion and potentially reduces that forward motion. However, with a fairly powerful rig or in strong winds, you are very likely overdriving the boat at times. In other words, at or exceeding the true hull speed of the boat. In such case, some of the power provided by wind and sail is being wasted, and you may as well harvest it and put it to good use. When the boat is significantly below hull speed, it is a zero sum game and creating more drag on the prop by loading it with an alternator that is placed under an electrical load will reduce your speed by some small amount.

As others have pointed out, a common alternator does not operate well or even at all below a threshhold speed in RPM. A large pulley on the prop shaft and a small one on the alternator will give you an RPM increase over same size pulleys. As Stormalong suggests, a lay shaft might be necessary. Not all transmissions are good to go, with long periods of freewheeling, so check with your mechanic.

You can also use a PMAC or BLDC (essentially the same thing) motor and controller such as is commonly used for electric propulsion, in regen mode. The required pulley size ratio would be very small, probably around 2:1 or at most 3:1 and there is a LOT of wiggle room. By the way, if you switch to a 48V DC system you can also use that motor for propulsion, especially at very low speed when you want to just ghost into a harbor or slip without bumping in and out of gear, or having to deal with a minimum idle speed. Also, power is instantly available with no startup, no warmup, etc and with a decent size bank, you always have enough power for docking even if your diesel craps the bed. 12V calls for too much current to be practical, and even 24V is pushing it, unless you have a very small boat. 48V is the standard for EP boats under about 36'. The PMAC motor and controller will cost multiples what an ordinary alternator will cost but will be more efficient and as I said, also can provide emergency power to the prop shaft.

A freewheeled prop produces power more efficiently with a high pitch. A low speed (such as a sailboat) vessel is usually more efficient with a lower pitch prop when using the prop for propulsion. And so, you have a compromise to make that will certainly affect efficiency either charging, pushing, or both. You could of course go with a variable pitch prop. $$$$$$$$$.

You can of course reduce the drag of the prop by allowing it to spin more freely, and you do that by reducing the electrical load on the alternator or brushless motor. For an alternator with a fixed regulator, simply increase the sense voltage so the regulator switches field current "on", less, and "off", more. Brushless motor controllers have a regen power adjust input usually controlled with a potentiometer.

A towed hydrogenerator would definitely be a better choice than coupling to the shaft, unless you want it to push your boat as needed, and likely, you don't. With a towed rig, you deploy it when you have more wind power than your hull can use, and keep it aboard when you want to squeeze every itsy bitsy fraction of a knot out of her. You can DIY your own drogue generator easily enough if you can't find one that you can live with. Unless you are quite the shade tree engineer though, I would look for a turnkey system that you just connect, splash, and go.