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
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