Could I drive a beefy alternator off my prop shaft?

[Amulet]

My last question (poxy) yielded useful advice (thanks), so here's another.

I have a Farymann engine with an alternator in the flywheel.
It delivers a miserable 10 amps provided I run the engine fast enough that the vibration loosens the keel bolts. Notional max revs 3000. No significant charge below 2500.

Could I put a pulley on the prop shaft and get a serious alternator to give me some amps? (Of course it would run backwards in astern - is that a problem?)

No criticism of the engine. PBO reviewed it and concluded it was "rather agricultural". Farming friends regard that as a compliment - they expect to park their machinery in the nettles for 9 months and then have it start first time when needed.

Last engineer that saw it said he was more used to them driving a cement mixer than a boat. No navigation lights, chart plotter and autohelm on a cement mixer (nor fridge to cool the beer) hence the need for more amps in a marine environment.

 

[VicS]

Perfectly feasible. Gearing up the speed from prop shaft speed to that required by most alternators will need a fair sized pulley on the shaft.

Can't think of any problems running in reverse will cause.

 

[halcyon]

Have you concidered the flywheel ?

When we bought our current boat it had a Sabb 8 hp with a dynostart and alternator.
The alternator was fitted to a bracket bolted to the side of the engine box, and the fanbelt ran on the flat of the flywheel, thus giving the required rpm. The battery sense wire from the alternator went to the centre tap on a big wire wound resister. The alternator output voltage was increased as required by sliding down the centre tap on the resister.

Did it work, never tried it, but the previous owner took the boat to the East coast of the USA, West Indies, Azors, Gib and back to the UK, and it charged the battery all the way.

So you do have alteratives.

Best of luck.

Brian

 

[William_H]

Yes I would suggest you look at the possibility of driving from the flywheel. I suspect that you would need a big pulley on the prop shaft which might not fit in. Alternators have the ability to be turned at high rpm which can be usefull.

However I would suggest you might try finding the diodes of the flywheel alternator and replacing them with schotky diodes These have a much lower forward volt drop. So if you are using them in a bridge diode circuit you have 2 diodes in series at any time resulting in 1.4 volts dropped using silicon diodes while schotky types will drop about .2 volts each or .4 volts total. The added 1 volt will I thinkl improve the current into your battery when charging. Of course it may be that the coils in the flywheel have resistance which limits the current charging ability but I suggest you try the schotky types first. You can buy them from electronics suppliers or even scrounge from old computer power supplies. You need schotky diodes rated at, at least 10 amps or more.

You probably don't have a charge regulator but if you do try disconnecting that. The meagre charge for short periods should not harm your batteries. (hopefully). Here I am assuming the flywheel alternator is similar to that in small o/b motors. good luck olewill

 

[Billjratt]

Going back to the propshaft question - the alternator will not cool properly when running in the wrong direction. Obviously this will not be a big problem, as I'm sure you don't go very far in reverse. Just be sure that it is turned the correct way when in forward gear. The fan behind the drive pulley is the key component, you get left and right handed ones.
Alternators "steal" about 0.9HP per 50amps generated, so you don't want too big an alt on a 10HP engine.

 

[pvb]

[ QUOTE ]
Going back to the propshaft question - the alternator will not cool properly when running in the wrong direction. ...... The fan behind the drive pulley is the key component, you get left and right handed ones.

[/ QUOTE ]Many modern alternators are in fact bi-directional, so cooling shouldn't be an issue.

 

[damo]

You may need to consider lateral stress on the shaft - will it affect the gearbox, shaft seal etc? And if the alternator isn't mounted on the engine then the belt may flap around if the vibration is out of synch.