Prop shaft driven alternators

Malo37

Malo37

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What happened to prop shaft driven alternators? I remember a few years back the idea seemed to be the ultimate solution to power generation while sailing. Assuming a fixed blade prop and a gearbox tolerant to freewheeling, a belt pulley mounted on the shaft driving an alternator seemed to be a neat idea. Does anybody have one that works?
 
Some boats have the prop size/ underwater configuration to generate considerable power under the right conditions - I heard of a boat that inadvertently "bump started" running down a wave in gear.
But - you don't get anything for nothing. The horsepower required to give a charge will become drag, won't they?
Ask the folks who use aquagens (other types of generator are available) we hauled ours inboard if the boatspeed dropped to 4kn as percentage drag-vs-amps got too high.
 
I remember in Lisa Clayton's book about her circumnavigation she mentioned having a shaft alternator. I remember it because it cam in for a lot of comment, almost entirely unfavourable.

Efficiency levels are bound to be pretty low as the blades of the prop are going to have reverse camber when used as a turbine. Also the output shaft of the gearbox will always be turning which can't be ideal.

The drag on a prop is zero when freely rotating at the right revs to give zero slip. as it slows down through friction etc the drag increases to a peak value then drops as the blades start to stall. When the blades are stationary the drag is higher than the free-turning prop but lower than one turning slowly (hence the debate on whether to lock or freewheel while sailing).

A shaft alternator is likely to push the prop into maximum-drag mode.
 
I expect the electric propulsion boys have got solutions. Personally, I don't like the grundling sound of the rotating prop when I'm sailing.
 
Killed off by a number of factors. Inefficient, increases drag under sail, move toward saildrives, very little space or exposed shaft on many recent designs of boat, popularity of folding/feathering props, improvements in other power sources - solar panels and wind generators on smaller boats and diesel generators on larger ones.
 
Tranona said:
Killed off by a number of factors. Inefficient, increases drag under sail, move toward saildrives, very little space or exposed shaft on many recent designs of boat, popularity of folding/feathering props, improvements in other power sources - solar panels and wind generators on smaller boats and diesel generators on larger ones.
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But shouldn't we be hearing from people who actually have them rather than people who don't and are making assumptions.
That would be more informative than just an opinion with nothing backing it up.
 
fuss said:
But shouldn't we be hearing from people who actually have them rather than people who don't and are making assumptions.
That would be more informative than just an opinion with nothing backing it up.
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Perhaps the deafening silence (so far) answers the question!

Also as far as I know nobody is marketing such a device commercially. Not sure what "assumptions" are being assumed. All the factors stated are facts. Therefore the only assumption is that these facts have lead to the use of prop shaft driven alternators being uncommon.
 
The reason why they havent gained ground, and theres no commercial marketing, is because they must be customised to each and every single boat due to the shaft/engine room contingencies (unless youve got a piece of standardised plastics, but they dont really tend to worry about cruising needs).

I've been considering implementing one, mainly because water-driven (e.g. towed) generators are meant to be super effective, especially when compared to wind generators on a downwind cruise..

Article about it here:
http://www.sailnet.com/forums/miscellaneous/22098-propshaft-alternator.html

Huge thread here:
http://forum.ssca.org/phpBB3/viewtopic.php?f=4&t=8415&hilit=shaft+generator
 
The first Rustler 44 launched had one fitted if I remember rightly and the owner occasionally comes onto this forum. If you're really interested I'll email him and ask him to contact you. Or you could talk to Adrian at Rustler.
 
here's a real live one

Idpnd is right that every installation is different so its a bit difficult to come up with a solution to suit everyone.

I often hear stories about them but rarely from people that have them... it would be good if someone else with one would says something.

If you have a hydraulic gb that is allowed side load then this is good.
If you have a slow reving 2200 rpm diesel then this is also good
room for around a 10 inch pulley on the propshaft is also good
then you might need some size and weight... my boat is 22tons and 50ft.
and a big prop... mine is 3 blade 19inch

i played round alot to get mine work right and if i had time i could make it even better.

The downsides....
it doesnt work under 5-5.5kts but this could be fixed.
It uses 2.5a before it generates anything as it is an alternator not a magnet motor.
It doesnt work at anchor

the upsides....
it generates while sailing above 5.5kts and provides all the power for everything.
it generates an additional 70a over the engine alternator if you leave it on when the engine is going.

here's the info on my propshaft alternator....
The gearbox is hydraulic so spins all the time while sailing, you cannot put it in reverse and stop it.
the prop is 19" 3 blade and the boat 22 tons 50 feet.
I use an alternator not a magnet motor. a magnet motor would be better at lower sailing speeds as it requires no initial amps. The alt needs around 2.5 amps initially for the field coils, so at borderline sailing speeds it might use amps instead of making amps... this is the disadvantage of the alternator... but at higher speeds it generates more amps.... 24v 20a at 6.5kts, 45a at 8kts.
to work properly it needs... a clean hull, clean prop, no cutless bearing play and the right ratio which changes for each vessel... mine is around 5:1.
what works on one vessel, it seems, might not work on another.

John
 
Nice to hear your experience, which I guess explains why they are not popular with the majority of boats - which don't have hydraulic gearboxes, don't have huge fixed props and don't have space to fit a pulley of the size you use - which was the points I was making in the first place.
 
I tried one on my deep keeled 10m sloop in the 70's. A standard car alternator connected to the propshaft via bicycle gears. Prop was about 12", 3 blade. As I recall the shaft spun at 300rpm when freewheeling but this dropped to about 100rpm as soon as any load was put on it. There was a lot of torque at 100rpm. At 10:1 gearing (1000rpm on the alternator) I was only getting about 5A. Higher gearing was too much for the bike bits and they collapsed. It would have been very hard to get the required gearing using belts and pulleys. I gave up and mounted that alternator on the motor so got pretty quick charging via that extra alternator. Unless you can get a low speed alternator (which won't be cheap) gearing is going to be a problem. If you are spending a lot of money why not buy a duogen, etc, and save the propshaft and gearbox bearings.
 
You need a certain set up.
If you don’t have a fixed prop and a gearbox to take a side load then a propshaft alternator is maybe not for you.
If you have the right setup then propshaft alternators do not cause extra drag that can be measured by the people sailing the boat.
 
Billjratt said:
Some boats have the prop size/ underwater configuration to generate considerable power under the right conditions - I heard of a boat that inadvertently "bump started" running down a wave in gear.
.
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Happened to me - surfing at just over 11knots- I slipped & kicked the gear lever into gear & next thing I knew was the engine was ticking over
 
Daydream believer said:
Happened to me - surfing at just over 11knots- I slipped & kicked the gear lever into gear & next thing I knew was the engine was ticking over
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Call me sceptical but there are some things that I have to see with my own eyes ...... and water pressure on a propellor, through the gearing, turning over a high-compression diesel engine without someone operating the decompressor is one of those things. :)

I'm not saying your story is not true ..... just that there may be another factor at play.

Richard
 
Daydream believer said:
Happened to me - surfing at just over 11knots- I slipped & kicked the gear lever into gear & next thing I knew was the engine was ticking over
Click to expand...

When I was deciding on props, Mr Brunton advised me against his product as he reckoned they could start my engines at speeds of over 15 knots. Incidentally their Autoprops freewheel backwards.
 
RichardS said:
Call me sceptical but there are some things that I have to see with my own eyes ...... and water pressure on a propellor, through the gearing, turning over a high-compression diesel engine without someone operating the decompressor is one of those things. :)

I'm not saying your story is not true ..... just that there may be another factor at play.

Richard
Click to expand...

Stories in the past of old aircooled lister being bump started on fast boats surfing (like English Rose), but by lifting the decompressor and dropping it when the engine got up to speed. The Listers in our old club launches were relatively easy to start by hand using decompressors. Get it swinging, drop one cylinder and when that fired drop the other. Then quickly put your fingers in your ears and enjoy the massage!
 
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