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Old 15-10-2011, 16:14   #1
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35amp to 80amp Alternator: Too Much ?

hello,
my small yanmar ysb8 is in need of a new alternator.
the current one is 35amps. a local shop has 40 and 80amp alternators to fit my engine model.
is a 80amp alternator going to take away too much power from my already small diesel?

thanks
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Old 15-10-2011, 16:22   #2
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Re: 35amp to 80amp alternator. too much?

80 amps is too big....the engine doesn't have enough horsepower
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Old 15-10-2011, 16:37   #3
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Re: 35amp to 80amp alternator. too much?

If you are driving your alternator with a diesel engine, and assuming adequate flywheel mass and a 3:1 engine drive pulley to alternator pulley ratio, a high efficiency, high current, 12V alternator will require about 1 hp per 30 amps of output power + about 1-2 hp for increased engine operating loads; when operating at its maximum rated output current.

Hence, an 80A alternator will use about 4.5 HP of your motive power, leaving only about 3.5 HP to drive the boat.
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Old 15-10-2011, 16:46   #4
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Re: 35amp to 80amp alternator. too much?

Quote:
Originally Posted by GordMay View Post
If you are driving your alternator with a diesel engine, and assuming adequate flywheel mass and a 3:1 engine drive pulley to alternator pulley ratio, a high efficiency, high current, 12V alternator will require about 1 hp per 30 amps of output power + about 1-2 hp for increased engine operating loads; when operating at its maximum rated output current.

Hence, an 80A alternator will use about 4.5 HP of your motive power, leaving only about 3.5 HP to drive the boat.
And then if you're over propped or have a dirty bottom you'll be down to next to nothing if the alt was putting out near full current. Over propping is very common and moves your HP curve..

For us to be helpful it would be good to know what you have for a battery bank as it may not even be able to take 80A....
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Old 15-10-2011, 16:53   #5
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Re: 35amp to 80amp alternator. too much?

While I generally agree with what Chief, Maine, and Gord said, there's another way to look at it.

Any small diesel is capable of handling an 80-amp alternator at anchor, and if you plan to charge your batteries with the diesel at anchor, that's probably a good bet. It would load the diesel nicely and charge the battery bank faster, etc., etc.

There are several ways to limit the output of the alternator, and thus the load on the diesel:

- a smart regulator, like the MC-612 or 614 can be programmed to de-rate the alternator to any desired percentage of it's maximum output;

- these regulators also can be fitted with a simple ON-OFF switch which cuts the alternator output in half -- that would just about equal the output of your existing alternator -- whenever desired.

Remember, too, that as batteries charge their charge acceptance rate drops, so the alternator puts out less current and the load on the engine drops.

Finally, it takes just over one shaft horsepower per ton to drive a displacement boat to hull speed.

FWIW,

Bill
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Old 15-10-2011, 17:14   #6
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Re: 35amp to 80amp alternator. too much?

WOW,
Thanks for the great information guys.
I think i will go for the 40amp option that he has for my engine.

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Old 15-10-2011, 18:08   #7
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Re: 35amp to 80amp alternator. too much?

I could tell a great Story of an engine like yours with a 100 amp Alternator. Lipstick on a pig would put it mildly
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Old 15-10-2011, 18:44   #8
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Re: 35amp to 80amp Alternator: Too Much ?

Why don't you take the original alt to a re-builder? I had ours re-built for around 80 bucks. That included bearings and 14 new diodes. Ours was a big one so it went to an over-the-road truck alternator shop. They do starters too. Look for a shop like this one with old alternators stacked in every corner ten to twenty deep and ask for a spare.

I also agree on the size issue. One thing you can do is replace your charge regulator with a Balmar charge controller. It will act as a programmable multistage battery charger and it will back off when your battery reaches full charge.
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Old 18-10-2011, 13:19   #9
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Re: 35amp to 80amp Alternator: Too Much ?

The pulley on a diesel engine can usually be loaded by max 25% of the rated engine power. An alternator has a efficiency of 50%. This means that you can get max 52 A from an 8 hp engine.

BUT on Yanmar YSB8 the alternator only run at the same speed as the engine. It have a maximum speed of 3400 rpm and its max power @ 3200 rpm. This is a very low speed on a alternator. So if you have a 80 A alternator, you have to see what you get out of it @ 3400 rpm. Probably it will be well below 52 A.

One alternative is to make a larger pulley and have the smaller alternator.
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Old 31-10-2011, 15:47   #10
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Re: 35amp to 80amp Alternator: Too Much ?

Hello,

There has been some very good engineering info and advice presented. However, just because an alternator is rated at 80 amps does not man that it will necessarily be producing that much at all times. It just means that the windings and diodes are capable of conducting that much if the load demand gets high enough. Just manage your load so that you will never exceed more than the engine is capable of delivering while steaming. That means don’t turn on too many electrical gadgets at once. Also you could put in some preferred trips as is done on large vessels, but that would get into some elaborate electrical controls. The charge controller for the batter is a good idea, since it is never a good idea to dump too many amps into a battery, as the plates can overheat and warp, resulting in internal shorts. The 80 amp alternator would give you opportunity to run more electrical equipment while at anchor or at the dock, and also give you opportunity to install and charge more batteries if you wish, as you would have the extra charging capacity.

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Old 31-10-2011, 17:29   #11
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Re: 35amp to 80amp Alternator: Too Much ?

If your batteries are huge and if you tend to discharge them lots then you may take the 80A model. Otherwise I would get the smaller one and invest the difference (and some more) into an external, quality advanced regulator.

b.
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