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Old 08-07-2012, 14:02   #1
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Alternator on the Prop Shaft?

Years ago I saw ads for towable turbines or props that turned an alternator while under sail. I don't see those kinds of ads any more.

But the other day I saw where someone said they had an alternator attached to the prop shaft (didn't say HOW)... so that under sail, the spinning prop generated electricity.

Does anyone here have such an arrangement? Happy with it? Would appreciate all comments!
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Old 08-07-2012, 14:07   #2
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re: Alternator on the Prop Shaft?

I have seen a boat with it. You need a lot of room to attach a sheave to the shaft and a bracket for a big alternator.
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Old 08-07-2012, 15:33   #3
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re: Alternator on the Prop Shaft?

It might work, but it isn't very feasible. First you would either need a big alternator with lots of poles or a 10:1 pulley system to speed up the alternator to a usable rpm. I don't know of any sailboat that has the room for a 30" diameter sheave on the propshaft or room for a multiple sheave jackshaft system behind the transmission.

To produce 1 kw (about 80 amps DC at 12 V) due to small alternator inefficiencies you would have to use about 2 kw of prop shaft energy or about 3hp. And that spinning prop is going to be very inefficient with lots of slip.

It would have the same performance effect on your sailboat as putting a 3 hp outboard in the water and running it at full throttle backwards as you sail.

David
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Old 09-07-2012, 00:57   #4
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re: Alternator on the Prop Shaft?

Quote:
Originally Posted by djmarchand View Post
It might work, but it isn't very feasible. First you would either need a big alternator with lots of poles or a 10:1 pulley system to speed up the alternator to a usable rpm. I don't know of any sailboat that has the room for a 30" diameter sheave on the propshaft or room for a multiple sheave jackshaft system behind the transmission.

To produce 1 kw (about 80 amps DC at 12 V) due to small alternator inefficiencies you would have to use about 2 kw of prop shaft energy or about 3hp. And that spinning prop is going to be very inefficient with lots of slip.

It would have the same performance effect on your sailboat as putting a 3 hp outboard in the water and running it at full throttle backwards as you sail.

David

David,

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.

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Old 09-07-2012, 05:45   #5
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Re: Alternator on the Prop Shaft?

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)
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Old 09-07-2012, 05:59   #6
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Re: Alternator on the Prop Shaft?

The French-built Amel offered this as an option on their sailboats. I believe it was called "Saildrive."
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Old 09-07-2012, 07:37   #7
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Re: Alternator on the Prop Shaft?

I would have thought a wind generator would be much more effecient at these lower RPM.
There is a lot off stuff on the web about homemade wind generators using things like washing machine motors that are rewired. These could be adapted to a propshaft generator.
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Old 09-07-2012, 11:18   #8
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Re: Alternator on the Prop Shaft?

Greetings and welcome aboard the CF, PlumBob.
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Old 09-07-2012, 13:45   #9
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Re: Alternator on the Prop Shaft?

Thanks, GordMay. It's good to be here

Thanks to everyone for the responses, and atoll for the pix.
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Old 09-07-2012, 14:36   #10
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Re: Alternator on the Prop Shaft?

I had one on my previous boat (35' with a 17"3 blade prop).
It would start producing 4 amps just below 5knots and increase from there.

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Old 09-07-2012, 14:56   #11
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Re: Alternator on the Prop Shaft?

Quote:
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)
A friend built very similar, he had the alternator modified to excite at extremely low rpm, he also disconnected the shaft. Engine was a beautiful 4LB Gardner, system worked well for him he was a sailor.....
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Old 09-07-2012, 15:47   #12
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Re: Alternator on the Prop Shaft?

As a power trawler owner, I was under the impression that it was preferable to stop the prop shaft "windmilling" when it is not being turned by the engine. The risk is damage to the transmission, not to the engine. Perhaps with a slow boat it is not such an issue though we rarely exceed 8kt ourselves. Just curious whether this is thought to be a concern.
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Old 09-07-2012, 15:53   #13
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Re: Alternator on the Prop Shaft?

Exactly it depends on the oiling arrangement of the box, Hank disconnected the shaft when he put to sea for that reason..
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Old 09-07-2012, 16:12   #14
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Re: Alternator on the Prop Shaft?

Quote:
Originally Posted by chrisjs View Post
As a power trawler owner, I was under the impression that it was preferable to stop the prop shaft "windmilling" when it is not being turned by the engine. The risk is damage to the transmission, not to the engine. Perhaps with a slow boat it is not such an issue though we rarely exceed 8kt ourselves. Just curious whether this is thought to be a concern.
some gearbox mnfg reccomend you do others that you dont lock in reverse.

my view is since there is no load being transmitted through the gearbox,and most marine boxes are oil bath type the risk of overheating or excessive ware is very low when freewheeling.
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Old 09-07-2012, 16:36   #15
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Re: Alternator on the Prop Shaft?

Your French built aluminum boat companies offer a high end prop shaft alternator as an add on. We are putting a German made alternator on the shaft. The owners of Boreal put one on their boat last year and crossing the Bay of Biscay had an output of between 5 and 11 amps while sailing 5 to 8 Knots. Between solar, wind and the prop shaft alternator we hope never a need for a genset.
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