Old subject re-visited - shaft alternators / generators

Refueler

Refueler

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Ok ... I have a PRM box fitted to a Perkins 4-107. The PRM manual advises that it's not necessary to lock shaft and can freewheel without cause for concern.

So while sailing - I can hear the faint whir of the shaft, due to the huge 3 bladed prop under the stern. Locking it off wouldn't change my speed much anyway being a heavy Motor Sailer.

Fitting a standard car or similar alternator to engine one now would IMHO be too much load to turn well by a freewheeling shaft. Clamp on pulleys can be made up easily enough to do it.
My thoughts are based on my having a Lawn / Garden Tractor with 12V starter / alternator fitted to 13.5HP Briggs & Stratton. The alternator is tiny and charges a small battery under the seat. If I remember rightly its about 20 - 30 A/Hr.

Now the output of that alternator will not be phenomenal - but probably easily driven by a freewheeling shaft ?

Thoughts .... comments .... ?
 
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My thoughts are based on my having a Lawn / Garden Tractor with 12V starter / alternator fitted to 13.5HP Briggs & Stratton. The alternator is tiny and charges a small battery under the seat. If I remember rightly its about 20 - 30 A/Hr.

[/ QUOTE ]Are you sure it's a combined starter/alternator? Every Briggs engine I've seen uses an alternator system built into the flywheel.
 
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Thats down to the clamp on pulleys ... simple to make suitable gear ratio.
Trick then is to dis-engage when shaft is being driven at higher speed by engine.

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Electric Clutch
 
I was thinking simpler than that ... over-centre lever. Lever one way - belt is slack, lever other way belt is tight and driving.

Now the question is what speed difference is there between a freewheeling shaft sailing at 5kts and a driven shaft, boat doing 5kts ? Is it not so serious that alternator can in fact be left engaged in both situations.
 
You could set the load by manually tweeking the field current until it just doesn't quite stall the prop, and then accept whatever current you get...(disconnect the internal regulator first)
However, I have a feeling in my water you won't be happy with the results as it won't be going very fast. (and remember it has to generate enough to feed it's field before it gives you anything).
 
The size of alternator I'm talking about is tiny and I don't think the load would be enough to worry about.

Another way of course is to have meshing gears ? or friction discs.

The idea is to get 12v at lowish ampage to top up batterys while gear is used and engine stopped. To basically balance light gear use on board. Let's assume the alternator is 20A ... I'd be surprised actually if its anywhere near that - must get tge book oiut on the tractor ! ... but draw on it would be ? 5 - 10A for nav gear / lights etc. ? - I have no fridge, fancy gear to power up !
 
OK the point about field generation is valid - so what about a Generator instead of an alternator. But then of course we get into regulated voltage ...

So question is - has anyone fitted a shaft generator or alternator ?
 
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Let's assume the alternator is 20A ... I'd be surprised actually if its anywhere near that - must get tge book oiut on the tractor !

[/ QUOTE ]You can see the output details of the various Briggs & Stratton alternators here.
 
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Now the question is what speed difference is there between a freewheeling shaft sailing at 5kts and a driven shaft, boat doing 5kts ?

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Pigeons meet cat:

PDF file

Enjoy /forums/images/graemlins/grin.gif

Andy
 
What about using an old wind generator. These are made to run at low speed without the need for a field current.
 
There must be enough power available. Your prop must be bigger than that used in towed generators. Thus it should be down to gearing. What speed do the towed generators turn at?
 
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Now the question is what speed difference is there between a freewheeling shaft sailing at 5kts and a driven shaft, boat doing 5kts ?

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Driven boat at 5 kt probably means engine speed around 1800 to 2000 rpm which means shaft speed 900 to 1000 rpm assuming 2:1 reduction box. Freewheeling shaft driven by boat speed through water is likely to be less than 100 rpm (mine does this too when sailing at speed and I have roughly clocked its speed) and insufficient power to drive an alternator or any charging device with significant output. Tis often said that a locked up prop will cause less drag than a slow spinning prop.
 
according to one hydrodynamicis
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The measured drag of fixed and free wheeling propellers will be greater or lesser as compared to the opposite, depending on hull shape, strut configurations prop design and position of prop relative to hull. In other words everybody is right, depending on what their personal experience was. Therefore you cannot predict or calculate what the drag is going to be with any certainty. Only empirical data from a tow tank will tell accurately.

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I've always held that a stopped prop presents the least drag but I stand corrected. I also went and bought a feathering prop to get around the problem /forums/images/graemlins/smile.gif
 
A thought on drive ratios. For an automotive type of alternator, you will probably need at least 10:1, probably more, to spin it at say 2000 rpm to get any sort of output. For an alternator pully of 2.5", which is small, you're looking at a shaft pulley diameter of over 2 ft. You then forget to disengage, start the engine and the alternator whizzes round at 20,000 + rpm. You definitely need a tolerant generator.
 
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