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Old 24-09-2010, 13:35   #1
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Alternator Size Too Large for Saab 2H ?

Hello Folks,
My surveyor noted my alternator (100 amp) was too large for my saab 2h deisel, and that it would "side load engine causing seal to leak" and cause "excessive belt wear". What does side loading mean, and what seals do you think he means?? Thanks, Dingoman
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Old 24-09-2010, 14:12   #2
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I am not a mechanic nor an engineer but, simply put, that big alternator puts too much pull on the crankshaft. This pull is sideways and pulls towards the alternator. This can hurt the seal (the front seal on the crankshaft) as no seal likes the shaft to be pulled to one side and sooner or later the seal will leak. I have heard that it can hurt the crankshaft too. That alternator can use about 4 hp.
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Old 24-09-2010, 16:11   #3
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Ok, Thanks for the reply. I'll check with the manufacturers recommendations for proper size. Thanks, Dingoman
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Old 24-09-2010, 19:02   #4
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I remember the sabb in my friend's boat and it did not look like something that can be hurt with a 100 amps alternator. But in fact, at that time they did not use alts bigger than 45-55 amps, so maybe.

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Old 27-09-2010, 11:42   #5
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I emailed the former owner and he assured me the system was very thoughtfully laid out, so I am going to keep her the way she is, as he assures me it is what works best on her..........Thanks for the reply Dingoman
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Old 27-09-2010, 12:01   #6
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I agree with your surveyor, a 100 amp alternator on a 2hp is in my opinion way oversized for a boats motor. I'm assuming this is your boats propulsion motor. Seems awful small for a 31' boat in the first place. Side loading issues are a real possibility on many very small diesels. Not sure about SAAB but seals, crank, shaft bearings are all subject to side loads.
Also a 100 amp alternator is at the limit of a single belt drive. You are darn near at the hp required to turn the alternator under full load. About 1.5 hp
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Formula for calculating alternator power requirements...

((AMPS LOAD x VOLT CHARGE) / 745.7) +15%

The amperage multiplied by the voltage equals watts, and 745.7 represents the wattage equal to one horsepower.
ormula for calculating alternator power requirements...


((AMPS LOAD x VOLT CHARGE) / 745.7) +15%

The amperage multiplied by the voltage equals watts, and 745.7 represents the wattage equal to one horsepower.
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Old 27-09-2010, 13:01   #7
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I believe the Saab 2H is a 10 horsepower diesel, not a 2HP diesel!

Still, a 100A alternator can put quite a load on that engine....close to 2HP at full rated output.

Happily, there are alternatives.

1. Ignore the problem and believe the PO's statement. May be OK, may not. Time will tell.

2. Do only shallow discharge to your house batteries before charging with the diesel. That way, the alternator won't put out 100A for very long, if ever.

3. Use an external regulator which allows alternator de-rating, e.g., a Balmar MaxCharge MC-612. You can set the maximum alternator output to any percentage you like.

I would favor the last option, since not only can you easily de-rate the alternator (which will certainly prolong its life), but there's a switch option which allows you to cut output power from the alternator by 50% anytime you like....like when you're trying to claw off a lee shore with that 10HP diesel :-)

Bill
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Old 27-09-2010, 13:37   #8
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dingoman, setting the proper belt tension on the alternator may cause or prevent more damage than simply "sizing" the alternator correctly.

Too much tension, even with a smaller alternator, will cause too much side load and damage bearings, seals, water pump, whatever is being pulled sideways. Too little tension and the blet slips and glazes.

A belt tension guage is a good idea. If not, you need to measure belt length and deflection, you'll find the procedure documented in many places. "It feels about right" usually is wrong, unless you have calibrated fingers.<G>
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Old 27-09-2010, 13:40   #9
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It's Sabb, not Saab (made in Norway, not Sweden), and the 2H puts out 18 horsepower. It's a two-cylinder, not one, and displaces a whopping 1140 cc . A very robust piece of real marine equipment weighing more than 400 pounds without stern gear. A real lugger. Methinks it might be able to handle a biggish alternator.
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Old 27-09-2010, 13:46   #10
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Not to hijack this thread......but I have asked this in another thread before, and could not obtain a definative answer.
How can I determine how many amps my alternator is capable of delivering? I believe that I have a 90 amp alternator, but I can not tell how many amps it is delivering. Also, how do you tell if it has an internal regulator??
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Old 27-09-2010, 13:58   #11
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Hello, Dockhead is correct, it is a Sabb and I'm looking at the insruction book, and under the electrical equipment page, it says alternator 12V 35A(500W) S.E.V..............? So I guess what they would have recommended was a 35 Amp alternator??????? But maybe this is just for another set-up or application, say one that only uses one battery etc........?
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Old 27-09-2010, 14:08   #12
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Sounds like it will. Just make sure the mechanical parts will stand the belt tension and the loads when running.

To the person asking about power metering issue - just run yours on a half empty battery and measure with a clamp meter.

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Old 27-09-2010, 16:55   #13
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Damn, I knew I should have looked up the model of engine first before I spoke. I thought 2hp was a bit odd. Shame on me. Anywho, 100 amp alternators are still a bit brutish in my opinion for that size motor. The only real answer would be from SABB itself concerning side loading issue. Some engines warranties can be affected.
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Old 27-09-2010, 17:24   #14
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Another pertinant query: How big is your battery bank? Unless it is fairly large, or is comprised of good AGM batteries, it will not accept the full 100 amps, and hence the loads on the belt/bearings/etc will not be so large. Even if the bank is large, the peak bulk charging rate doesn't last all that long.

And determining if there is adequate power is empirically easy: let the batteries get well discharged, start motoring along and see if your performance is satisfactory.

Incidentally, putting a switch in the alternator field lead is a good idea in low powered engines. If you ever find yourself in a situation demanding that all the engine's power be delivered to the prop, switching the alternator off temporarily by removing the field excitation is safe and quick.

Cheers,
Jim
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Old 27-09-2010, 17:45   #15
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"How can I determine how many amps my alternator is capable of delivering?"
Somewhere on that alternator it is stamped with identifying numbers. You find the numbers and look up the specs for it. That will also tell you if it is internally regulated. Or you can trace the wires and look for an external regulator somewhere nearby, it is a good idea to know what each piece in the charging system is.
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