diy prop shaft alternator

[Ric]

I have a 9.5m sloop with two blade prop which cruises at 4-7knots. If possible I'd like to build an on the cheap prop shaft alternator. I know ideally one should have three blade prop and normally a bigger boat - but I'm not looking to produce huge amounts of amps. I'd be happy if I could get 2 amps as long as the building does not cost much. My thinking has not gone much further than getting a car windscreen motor from the scrappy, and turning it from the prop via bike chain. I'd put a relay in the output to disconnect it when the engine is switched on. Then feed the output into the regulator of my Rutland wind generator. Has anybody tried anything similar? Alternatively I would consider a ready built one if there was such a thing available for a boat of my size and it was not too astronomically priced (yeah dream on) and it was relatively easy to fit (ie no clutches etc as on the complicated ones). Any help appreciated!

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[Guest]

A couple of my customers have done exactly what you are proposing. The main thing you need to watch is a) RPM of the shaft & b) power required to drive it.

Too few revs and you simply won't see an output - you can overcome this (to a degree) with using different size pulleys (bear in mind that alternators need about 500 RPM to begin acheiving output.

Too large an alternator and it will simply stall the shaft.

With the above in mind, you can get v. small alternators that are fitted to larger outboards or small automotive types - perhaps a breakers yard is worth a bet.

Running the output through the Rutland regulator isn't really a good idea - firstly because they can only handle a small amount of current; Secondly, most alternators have their own built in regulator anyway. Note that you will need a positive supply to the regulator (normally through the D+ connection) to fire it up - this should be switched as it will pull power continuously (and while at anchor, will flatten the batteries).

Hope this helps!

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[Ric]

Thanks - do you know of any plans etc? Getting the gearing correct is obviously going to be the biggest stab in the dark. Also, does anybody make a self-install kit with all the necessary bits?

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[Guest]

Ric,

Not that I know of... (is it something as a company I should be looking at?)

To get the average RPM of the shaft, you can rent a device that will display shaft RPM (no idea what its called - I'm an electrical engineer not a mechanic!). Once you have this, you can begin to work out theoretical alternator RPM.

If you take the diameter of the shaft (or pulley fitted around it) and divide it by the alternator pulley diameter, you will get a multiplier to allow you to work out actual alternator RPM. For example:

4" driving pulley, 2.5" alternator pulley = 1.6. So shaft RPM is say, 300. 300 x 1.6 = 480 RPM for the alternator.

A chain may not be the best device to use (since the prop shaft are is usually pretty damp), I would use a standard alternator belt (only problem is you'll have to pull back the shaft to change the belt (not that it will need it often)).

The only other issue I can think of is one of possible side loading of the prop shaft bearings - this is worthwhile checking as an over tightened belt could wreck things. Pat Manley at PBO is pretty useful for that sort of question.



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[Ric]

For sure a cheap and simple propshaft alternator (without clutches etc) for small boats would have a market!

My reasoning for going for chain is that it would not put such a load on the shaft bearings as it would not depend on tightness for grip. Also, it would be less sensitive to small misalignement of the alternator and propshaft. The shaft area in my boat is actually very dry and warm as it is next to the engine and exhaust (small boat advantage!) so dampness there is not a problem.

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[Guest]

Ric - Good point about the chain. A number of people have asked for prop shaft driven units in the past... it would take quite a lot of research and engineering on our part (that unfortunately costs!). I looked at doing a 10A shaft unit (using a Honda alternator) that came out at about the £350 mark. Would you say that this would be acceptable?

James

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[Ric]

That's definitely a lot of amps for the money if you compare that to a wind generator or solar panel, so yes I'm sure people would buy that. But my gut feeling is it would take a fairly big boat and a three blade prop to power something that powerful. There are also already a few fairly complex units around for boats of above 38ft roughly, but I can't find anything for smaller boats in my research. I would be simply amazed if my small two blade prop on a 32ft boat produced enough torque to produce 10A. If it did, I'd send a cheque tomorrow!

I would guess a more realistic target for a small boat would be 2-5A. I'd be happy with that - it would complement my wind generator and panels nicely. Bear in mind that most modern cruising yachts have relatively little room around the propshaft between the rear cabin bedboards and the hull, so it might not be easy to fit a large pulley/cog to spin up a large alternator fast enough - another reason why I think it best to aim small and under the products already out there. Also, with a small unit you could dispense with the complexity of a disengagement clutch as it would probably not cause the main engine any problems to spin it continuously (though a relay to take off the current load while motoring would be sensible). I'd definitely be interested in a kit that supplied the right size cogs and ones that can be easily fitted to the propshaft, a suitable alternator, a couple of relays and circuit diagram etc, as that would save me a lot of time sourcing the bits and trial and erroring it all. I'm sure I could build the mounting myself.

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[Talbot]

I would very strongly recommend that you use a toothed rubber belt rather than chain. When brand new there is little to seperate the friction losses, howevera chain in a salty environment is soon going to start to bind up, and the friction losses will increase dramatically. even if you try to keep it supple, you will fail due to the salt environment. They only really work well on a bike if you trickle a very light oil on them all the time (scott oiler) and I am sure that you will not want to do that inside your boat.

However there is another solution:

If I was doing this I would go about it in a very different way. I would canabalise an old motorbike for the magneto coils, and then set the magnets part of the set onto the shaft and mount the magneto on the boat. You would then have a good system that would not actually be physically secured together, thus eliminating one source of friction loss. The clearances are small, but this is not an impossible project. The speed of shaft turning would not produce lots of power, but may well be enough to power your nav lights (especially if you convert to LED /forums/images/icons/smile.gif)

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[Ric]

I was thinking exactly along these lines myself, though was not sure that the shaft would be turning fast enough to produce much current. But I think this solution is very neat - it solves lots of problems at once as it is easy to install, would take up very little room around the propshaft (important in a modern yacht) and has very few moving parts. If somebody manufactured something like that which could produce (say) 2-3A I'm sure there would be a market for it (compare the costs of 2-3A with solar panels and also consider they only work 8 hours per day!)

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[Guest]

Agreed - if you could fix the magnets to the shaft and then have a stator that split into two halves it would work well.. Shaft alignment would have to be spot on to prevent the stator rattling itself to bits.

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[Ric]

If you were to manufacture one (hint!) you could mount entire unit on bearings on the shaft and just use a torque arm fixed to anything nearby (even just resting on the hull) to prevent the casing from turning. This would solve the alignment problems. Mounting would then be a matter of disconnecting the shaft and sliding the whole unit over it - though this would be easier for amateur installation than getting the alignement correct on the other suggested mounting. I'm sure a unit like that, suitable for small boats and easy to install, would sell quite nicely.

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[Guest]

hmmm - one to think about thats for sure....

Boatshow coming up - good place to canvass opinion..

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[richardandtracy]

If you're prepared to make the alternator from scratch, can I suggest a book for you to have a look at. It's 'The Brakedrum Windmill Plans' by Hugh Piggot. This details a 400W alternator for a windmill at about 400 rpm, but the formulae are given for you to alter voltages & outputs for different sizes & revs. The book also gives a basic regulator/ rectifier design for home build - so no need to use the Rutland one.

As for how much power can you take out, I suggest the maximum would be about 10% of the engine power required for a given boat speed (check revs & engine power chart). This may even be a little pessimistic.

Final thing, try to avoid belts etc. They are at best 95% efficient, so if you can make the alternator from scratch with magnets on the prop shaft, you can gain that extra 5% for no penalty.

Regards

Richard.


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[LORDNELSON]

This has been interesting as I have often thought it a shame that there is some almost free energy there which is being wasted. As a matter of interest have you compared your idea with attaching a second alternator to your engine. It would of course only charge when the engine was running but 20 to 30 amps for a short time may be as good as a much lower current produced when sailing. Have you tried measuring the torque available at the shaft when sailing? That would enable you, on an average basis, to calculate how much energy is available for generation (less mechanical transmission losses)before committing yourself.

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[Avocet]

I agree that measuring the torque available when sailing is the place to start. I was wondering if you could attach a collar with a torque arm round the shaft and hang some weights on the arm. Tighten the collar gradually with (elastic bands or tape or something (???) until there is just enough friction to keep the weights suspended without resting on the hull or stopping the shaft. I'm guessing that there won't be very much even at (say) 5 knots. I've never tried but I bet I could stop Avocet's prop with my hand at that speed (might try with something else first though!) An alternative to using weights might be to use a fisherman's spring balance on the end of the torque arm.

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