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Old 31-01-2013, 06:36   #16
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Join Date: Sep 2011
Location: Barcelona (Spain)
Boat: Menorquin Yachts
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Re: Alternator on the Prop Shaft?

Originally Posted by atoll View Post
here is one i built a few years ago on a 52 ft yacht with an 18" prop.

lower pulley is 30 cm,upper is 5 cm dia giving a 6:1 ratio
alternator is a std 24v 80 amp.

it started producing power after 5 knots of boat speed,putting out a constant 15 amps that kept up with all fridge,freezer and instument demands.

alternator was machine sensed,so just trickle charged the batteries whilst satisfying equipment demands.

no appreaciable drag ,when running on the vessel.

the alternator needs a minimum 1000 rpm to excite,so 6:1 is adequete though 8:1 would have been better.

(top pulley in the picture was replaced for a smaller 5cm one)
the pressure pump that appear in the picture is not working well, the head always down!!

Regards from Barcelona

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Old 01-10-2016, 13:15   #17
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Re: Alternator on the Prop Shaft?

Originally Posted by Viking Sailor View Post

Your numbers are a bit high.

Shaft driven alternators are intended for use on long passages of many days.

A shaft driven alternator only needs to but out something like 5 amps to be useful. Five amps per hour is 120 amp hours per day. Which is a very useful amount of power. And, this would be achieved with less then a tenth of a horsepower worth of drag.

I have seen a suggestion that using a 24 volt alternator with a 12 volt regulator, and a 3:1 pulley system might make a practical 12 volt charging source. However, there were no details about prop size or shaft rpm requirements. So, some experimentation would naturally be required to get the proper voltage and current levels.

I have been trying to soak up as much info on this subject and one thing I have not seen discussed is what happens when the motor gets turned on and does that create too many rpms for the alt that's hooked to the shaft. Also...other than the built in regulator in the alt, would there need to be some other kind of controller to guard against over charging

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Old 01-10-2016, 15:53   #18
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Join Date: Nov 2006
Location: In Mexico, working on the boat
Boat: Hallberg Rassy 35 14ft.Whitehall pulling skiff.
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Re: Alternator on the Prop Shaft?

No one has mentioned yet that not all transmissions can run as free-wheeling. In cases, it can damage the transmission.

"Life is not a journey to the grave with the intention of arriving safely in a pretty and well-preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming: Wow - what a ride!"
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