It is not a rule of thumb. It is an engineering fact. All the belt
drive makers have web sites and they all have pages about sizing belts.
I used the Martin sheave formulas and double checked on the Gates Belt
site. "A" and "AX" belts which are normally used on boat engines have a
maximum of about 4 HP each at the speeds and pully sizes we use. (6-7"
drive at 1500-3000 RPM and 2.5" to 3" driven at 3200-7000 RPM)

There IS a rule of thumb involved though. Counting efficiency losses
and the ever present safety factor, the rule is that an alternator
requires an average of 1 HP per 25 amps of output at 13.8V. So when you
get past about 90-100 amps a single AX belt just can't handle the power.

Joe Wood wrote:
Rule of thumb that I heard was that anything over 90 Amps should have
dual belts.

Joe Wood

Glenn Ashmore wrote:

Put another way, is a large alternator operating at 60% of capacity
more or less efficient than a smaller one operating at close to
maximum output?




--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

Getting back to your original question.....

I don't think a bigger alternator is less efficent and it might be
more efficient, but I freely admit I'm speculating. Consider:

-- The bigger alternator will have bigger windings and therefore less
I^2 R loss inside. This is a small factor, as by far the biggest heat
producer in an alternator is the voltage drop across the output diodes
and that won't be affected. Nonetheless, it should be a plus for the
bigger unit.

-- The bigger alternator may have a bigger case and should therefore
run cooler, always a good thing.

I must add that I'm not quite sure why you ask the question, Glenn --
your various posts make it clear that you have an excellent
understanding of DC systems..... What do you think?

Jim Woodward
www.mvfintry.com


Glenn Ashmore wrote in message ...
It is not a rule of thumb. It is an engineering fact. All the belt
drive makers have web sites and they all have pages about sizing belts.
I used the Martin sheave formulas and double checked on the Gates Belt
site. "A" and "AX" belts which are normally used on boat engines have a
maximum of about 4 HP each at the speeds and pully sizes we use. (6-7"
drive at 1500-3000 RPM and 2.5" to 3" driven at 3200-7000 RPM)

There IS a rule of thumb involved though. Counting efficiency losses
and the ever present safety factor, the rule is that an alternator
requires an average of 1 HP per 25 amps of output at 13.8V. So when you
get past about 90-100 amps a single AX belt just can't handle the power.

Joe Wood wrote:
Rule of thumb that I heard was that anything over 90 Amps should have
dual belts.

Joe Wood

Glenn Ashmore wrote:

Put another way, is a large alternator operating at 60% of capacity
more or less efficient than a smaller one operating at close to
maximum output?

Jim Woodward wrote:
Getting back to your original question.....

I don't think a bigger alternator is less efficent and it might be
more efficient, but I freely admit I'm speculating. Consider:

-- The bigger alternator will have bigger windings and therefore less
I^2 R loss inside. This is a small factor, as by far the biggest heat
producer in an alternator is the voltage drop across the output diodes
and that won't be affected. Nonetheless, it should be a plus for the
bigger unit.

-- The bigger alternator may have a bigger case and should therefore
run cooler, always a good thing.

I must add that I'm not quite sure why you ask the question, Glenn --
your various posts make it clear that you have an excellent
understanding of DC systems..... What do you think?

Jim Woodward
www.mvfintry.com

It is more a matter of balance. The immediate problem is that I
suddenly have a surplus of alternators. I was planning to use the big
270 amp hot rated Niehoff that I picked up off a fire engine that got
crushed by a falling wall. It needs about 10 HP at full output. It is
to be mounted off engine through a CV joint arangement to eliminate side
loads but when I add the Cat pump for the watermaker it runs the maximum
HP off the front of the engine up to about 12HP max. That is getting a
bit much even with the CV joint so when I got offered a 200 amp (cold
rated) Leece Neville fot $140 I bought it.

Now I am wondering which should be the primary and which the spare. For
all practical purposes the 750 amp bank is only going to take 175 amps
max and more like no more than 150 amps most of the time so the question
was will I be wasting enengy by swinging the heavier alternator? I
believe I agree with you that the answer is probably not. I think I
will mount the Niehoff and keep the LN in reserve.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com