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post #1 of 7 Old 06-07-2012 Thread Starter
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Choosing a replacement alternator - restrictions?

The recent thread on alternator brands/sources has raised a question for me. My Yanmar 3GMD has its original 35 Amp alternator, and the front of the motor/front of the alternator is pretty close to a bulkhead. The idea of adding more charging power is appealing, as I continue to add electrical devices to my Marshall, so my questions relate to the key decision points in choosing an upgrade. Is it primarily a price issue, ie: the higher amperage the higher the price, or are there more specific issues to be considered like getting too much capacity for the battery banks on hand, or the higher amp alternators having larger dimensions that will need clearance? My mooring is inland of Portsmouth (NH) harbor, in the Piscataqua river which is a very fast estuary. I can count on running the engine for 30 minutes at beginning and end of each sail, charging two 90 amp/hour deep cycle bateries. Load on the engine is a minor concern, considering that I have plenty of diesel power for my 22' boat. I would be interested in moving up from 35 Amps to a more modern 60-90 amp unit, although I have not had problems with the current charging capacity so far. Any hints/suggestions?

Mike sullivan
1985 Marshall 22 catboat
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post #2 of 7 Old 06-07-2012
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Re: Choosing a replacement alternator - restrictions?

Assuming that your batteries are flooded type they will not accept more than 25% of their capacity if in bulk charging mode - between 50% and 80% state of charge. That works out to 45 amps. After they reach 80% they will accept a lot less than that. While a 90 amp alternator would work it wouldn't be a good investment. I would look for a 55 amp Hitachi to replace the original 35 amp unit if you feel that it is needed.

The last 20% of charge current will take many hours regardless of how large the alternator is.

Brian
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post #3 of 7 Old 06-07-2012
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Re: Choosing a replacement alternator - restrictions?

mb, generally a higher capacity alternator is longer, form front to back. And since the pulley is up front and has to line up the same way, they usually make the alternators extend further BACK as the capacity gets larger. So the position of the mounting leg(s) and pulley stays the same, and you need another inch or two aft of where the existing alternator is.

You'd want an alternator from the same series, so it bolts up the same way. With the same pulley diameter so it runs the same way. And generally if you stick to the same series, they are built to operate in the same rpm range so you don't have to worry about matching that, although you certainly CAN see if there's a better match, i.e. a different pulley size that will give you optimum charging at a lower engine speed.

You've got 180AH of batteries, let's say you take good care of them and don't cycle them below 50%. So all you need to replace is 90AH of power, and since the batteries will only accept 1/4C (1/4 of their 180AH capacity) during charging...you probably can't pump more than 45A of power into them, even with an external regular, unless they are deeply discharged.

So a common 50 or 55A alternator would be a sensible step up, maybe even a 60A alternator, since they all cut back their power as they operate and heat up, and a 60A alternator might actually cut back to 45-50A maximum power well before a half hour was up.

You might also want to consider an external regulator while you are doing this. Any alternator shop can also modify your alternator (bring out 1 wire and add one switch) so you have the option of external regulator, with the internal as a 'spare'. An external regulator is better for deep cycle batteries, it charges them faster and with voltage levels that are optimized for longer battery life. Even if you just modify the new alternator now, and add the external regulator later.
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post #4 of 7 Old 06-08-2012
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Re: Choosing a replacement alternator - restrictions?

Hmm ... so measure your alternator carefully and call the folks you intend to order from. Ask for measurements before placing the order. I've found some companies (mostly smaller ones) are more than willing to assist you in this way.

Niemiec Marine out of New Bedford, MA is where I buy my Yanmar parts. I stumbled across them via a web search when I first bought my boat and needed to work on my Yanmar. It must be a pretty small parts shop -- I always get the same guy on the phone. He gives me good advice and is willing to go the extra mile for me like you need. BTW, they have a lot more stuff than they put on their web site. If you explain your situation they might even have a good make and model of alternator to suggest. Their prices aren't always the cheapest I can find but they aren't that far from it and I am happy to pay a little extra for great service.

Tom

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S/V Tranquility Base
1984 Islander 30 Bahama
Pasadena, MD
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Re: Choosing a replacement alternator - restrictions?

Depending how many AH you use on a sail there may not be a real advantage to a larger alternator.

Do you have a good battery monitor? The best value is this one: http://www.jamestowndistributors.com...attery+Monitor

It will tell you the actual state of your batteries and show you how many AH you are using at any time as well as the cumulative amount. There is no other easy way to find the actual state of charge.

Brian
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Re: Choosing a replacement alternator - restrictions?

I buy mine in the auto wreckers, for around $35. No complaints.

Brent Swain, Boat designer, Builder, and author of "Origami Metal Boatbuilding"
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Re: Choosing a replacement alternator - restrictions?

Quote:
Originally Posted by dacap06 View Post
Hmm ... so measure your alternator carefully and call the folks you intend to order from. Ask for measurements before placing the order. I've found some companies (mostly smaller ones) are more than willing to assist you in this way.

Niemiec Marine out of New Bedford, MA is where I buy my Yanmar parts. I stumbled across them via a web search when I first bought my boat and needed to work on my Yanmar. It must be a pretty small parts shop -- I always get the same guy on the phone. He gives me good advice and is willing to go the extra mile for me like you need. BTW, they have a lot more stuff than they put on their web site. If you explain your situation they might even have a good make and model of alternator to suggest. Their prices aren't always the cheapest I can find but they aren't that far from it and I am happy to pay a little extra for great service.

Tom
-------------
Tom, I bought my current Marshall 22 in Mattapoisette (next harbor to New Bedford), and kept it there for most of the first summer on the seller's mooring. The prior owner used Niemec exclusively, and I went to their store for a few parts and spares to get ready for the sail to NH (it's a longer way, in a 22' boat ). It is a great spot, and appears to have a large inventory in a very good sized orderly space, in a heavily commercial marine center. I think the guy on the counter is just one of a bigger group that services the commercial operators, but in any case, they are highly respected in the area, and very helpful in person.

To all, thanks for the inputs on charging capacities, and size questions to resolve if i move to a bigger alternator. i installed a manual control for the batteries in my old Dufour 35 (forget the brand) that really allowed me to accelerate the charging without overdoing it. That was on a boat cruising with wife and three sons, lots of goodies, etc., and room for more guages, etc. in an orderly installation. Now, I have little space for control panels but also many fewer electric draws (so far). So far, I have no problems with my little 35 Amp set, but 35 Amp just seems so puny that I like the idea of upgrading if it is just another $100 or so. The newer versions of the Yanmar 3 cylinder's moved to a 55 Amp as stock years ago, and that is likely the right upgrarde given the info about max charging rates for my battery bank.

Thanks for thte inputs

Mike sullivan
1985 Marshall 22 catboat
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