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I have been reading a lot of info from boaters on the topic of alternator speeds and drives and have come to the conclusion that speed is of the essence. My engine will be rumbling round at 600rpm cruising speed. I have two 70amp lucas altos with 60mm pulleys. I have a gearbox driveshaft from the crank with Two 100mm pulleys attached. I will belt drive two 50mm pulleys with these which will drive a two 150mm pulleys mounted on same shaft that will in turn belt drive the 60mm alto pulleys. This should give me an alto speed of 2500rpm at cruising and 2000rpm at idle

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13 minutes ago, Peter Duddigan said:

This should give me an alto speed of 2500rpm at cruising and 2000rpm at idle

Alternator design and specifications tends to show that maximum output is between 4000 & 6000 RPM

 

Try and get your ratio higher if you can

 

edit to add a typical alternator output graph

 

Automotive Alternator Output | AUTOMOTIVE

Edited by Alan de Enfield
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Alternators also have a maximum rpm too, above which they risk breaking up inside. The pulley ratio needs to be chosen so that at maximum engine revs you won't over speed it. More a problem for racing cars than inland waterways boats, but something to check if you are looking at unusually high ratios.

Jen

Edited by Jen-in-Wellies
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Gearbox driveshaft from the crank sounds interesting. 

 

Usually you would expect belt drive to obtain the reduction . 

 

ETA scrub that I see its an industrial engine with a gap between engine and gearbox. 

 

Sorry !

Edited by magnetman
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5 minutes ago, Jen-in-Wellies said:

Alternators also have a maximum rpm too, above which they risk breaking up inside. The pulley ratio needs to be chosen so that at maximum engine revs you won't over speed it. More a problem for racing cars than inland waterways boats, but something to check if you are looking at unusually high ratios.

Jen

 

Wise words, but I think it would have to be a very high ratio - my Alternators (as used by Barrus etc) are rated at ;

 

Cut in speed: 1200, Max speed: 15000

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2 hours ago, Peter Duddigan said:

I have been reading a lot of info from boaters on the topic of alternator speeds and drives and have come to the conclusion that speed is of the essence. My engine will be rumbling round at 600rpm cruising speed. I have two 70amp lucas altos with 60mm pulleys. I have a gearbox driveshaft from the crank with Two 100mm pulleys attached. I will belt drive two 50mm pulleys with these which will drive a two 150mm pulleys mounted on same shaft that will in turn belt drive the 60mm alto pulleys. This should give me an alto speed of 2500rpm at cruising and 2000rpm at idle

Doesn't that all come to gearing up at 15:1?  Assuming the gearbox driveshaft is 1:1.

 

If so, you will be spinning alternator at 9,000 rpm when the engine is running at 600 rpm

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1 hour ago, Mike Adams said:

If you have room it is much better to go for a larger diameter alternator rather than lots of pulleys. The larger the diameter the lower speed they operate.

something like this WWA42000-Alternator-12v-160A-For-Leece-Neville

That looks a bit like a Leece Neville ~160a (@12v nominal) large frame. 

 

I've got one of those somewhere on one of the boats. Not connected to anything it's just "somewhere on the boat".

 

Proper bit of gear. 

 

 

Edited by magnetman
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2 hours ago, Peter Duddigan said:

I have been reading a lot of info from boaters on the topic of alternator speeds and drives and have come to the conclusion that speed is of the essence. My engine will be rumbling round at 600rpm cruising speed. I have two 70amp lucas altos with 60mm pulleys. I have a gearbox driveshaft from the crank with Two 100mm pulleys attached. I will belt drive two 50mm pulleys with these which will drive a two 150mm pulleys mounted on same shaft that will in turn belt drive the 60mm alto pulleys. This should give me an alto speed of 2500rpm at cruising and 2000rpm at idle

 

13 minutes ago, Tacet said:

Doesn't that all come to gearing up at 15:1?  Assuming the gearbox driveshaft is 1:1.

 

If so, you will be spinning alternator at 9,000 rpm when the engine is running at 600 rpm

The way I read it, he is proposing to gear up by a factor of 2 (100/50) from the crankshaft to the intermediate shaft, and by a factor of 2.5 (150/60) from the intermediate shaft to the alternator, giving an overall ratio of 5:1, so 3000 rpm alternator speed at an engine speed of 600 rpm (assuming the pulley sizes quoted are the effective diameter).

But if there is room, it would be easier to fit a larger pulley (300mm for 5:1) to the crankshaft and drive the alternator directly.

Or run a belt directly off the flywheel, which will spin the alternator at something like 5000 rpm.

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2 hours ago, Peter Duddigan said:

I have been reading a lot of info from boaters on the topic of alternator speeds and drives and have come to the conclusion that speed is of the essence. My engine will be rumbling round at 600rpm cruising speed. I have two 70amp lucas altos with 60mm pulleys. I have a gearbox driveshaft from the crank with Two 100mm pulleys attached. I will belt drive two 50mm pulleys with these which will drive a two 150mm pulleys mounted on same shaft that will in turn belt drive the 60mm alto pulleys. This should give me an alto speed of 2500rpm at cruising and 2000rpm at idle

 

26 minutes ago, Tacet said:

Doesn't that all come to gearing up at 15:1?  Assuming the gearbox driveshaft is 1:1.

 

If so, you will be spinning alternator at 9,000 rpm when the engine is running at 600 rpm

 

2 minutes ago, David Mack said:

 

The way I read it, he is proposing to gear up by a factor of 2 (100/50) from the crankshaft to the intermediate shaft, and by a factor of 2.5 (150/60) from the intermediate shaft to the alternator, giving an overall ratio of 5:1, so 3000 rpm alternator speed at an engine speed of 600 rpm (assuming the pulley sizes quoted are the effective diameter).

But if there is room, it would be easier to fit a larger pulley (300mm for 5:1) to the crankshaft and drive the alternator directly.

Or run a belt directly off the flywheel, which will spin the alternator at something like 5000 rpm.

Maybe but how does it go from the 50 to the 150 pulley?

 

The way I read it was the same as you, but that the intermediate shaft is driven by the 50mm pulley and the drive is taken off the same shaft by the 150 pulley.  Thus an additional 3:1 which makes 15:1 

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5 minutes ago, Tacet said:

 

 

Maybe but how does it go from the 50 to the 150 pulley?

 

The way I read it was the same as you, but that the intermediate shaft is driven by the 50mm pulley and the drive is taken off the same shaft by the 150 pulley.  Thus an additional 3:1 which makes 15:1 

The 50mm and 150mm pulleys are on the same shaft, so rotate at the same speed.

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20 minutes ago, Tacet said:

Maybe but how does it go from the 50 to the 150 pulley?

 

The way I read it was the same as you, but that the intermediate shaft is driven by the 50mm pulley and the drive is taken off the same shaft by the 150 pulley.  Thus an additional 3:1 which makes 15:1 

 

13 minutes ago, David Mack said:

The 50mm and 150mm pulleys are on the same shaft, so rotate at the same speed.

Same RPM but surely different linear belt speed?

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36 minutes ago, Tacet said:

 

Same RPM but surely different linear belt speed?

Yes linear belt speeds will be different, because the pulleys on the intermediate shaft are different sizes. 

 

Intermediate shaft rotates at twice crankshaft speed. Alternator rotates at 2.5 times intermediate shaft speed = 5 times crankshaft speed.

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That'll give an alternator speed of 5000rpm on tickover, a top speed of around 17000. No thanks. A larger pulley on alternator would reduce this but I'm not going that route. The belt length would be silly. I'll prob up the idler to 200mm, giving an alto speed of 4000 plus at cruising and a max of 7500

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Following on from earlier suggestion of using a larger frame alternator, we have Prestolite 8MR2070 units on our boat, so have some data to hand.

 

There are trade offs in designing the drive speed (which can be lower on these units than car alternators). I would avoid running the alternator too fast, perhaps its better to optimise the output whilst avoiding high speeds where efficiency drops and noise increases. However do not run too slow as cooling is reduced and torque will increase. 

8MR2070TA_curve.jpg

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5 hours ago, Peter Duddigan said:

That'll give an alternator speed of 5000rpm on tickover, a top speed of around 17000. No thanks. A larger pulley on alternator would reduce this but I'm not going that route. The belt length would be silly. I'll prob up the idler to 200mm, giving an alto speed of 4000 plus at cruising and a max of 7500

Yes the belt is long but there were no failures in the 10 years I had the boat and charge rates were excellent flywheel is 590 on the JP. I think

@frangarused to have the same system, if he still has that will have been running 25 years.

You would need to choose your alternator well mine was from a Jaguar XJS 😷

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1 hour ago, Loddon said:

Yes the belt is long but there were no failures in the 10 years I had the boat and charge rates were excellent flywheel is 590 on the JP. I think

@frangarused to have the same system, if he still has that will have been running 25 years.

You would need to choose your alternator well mine was from a Jaguar XJS 😷

Yep still going...Alternator now changed from a A133 from a Jag to a A127 from a Discovery!!...used the same principle on a couple of other boats with large external flywheels...gives you a great charge rate at tickover and spins the fan fast enough to ensure proper cooling. Change belts every 3/4 years just because they get a bit polished and its easier to do planned maintenance than have to worry about it when it breaks.

  • Greenie 1
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