Originally Posted by a64pilot
Look up optimum alternator RPM and you may be surprised.
2 to 1 is probably good for a 7000 RPM redline car.
Itís not so much that they canít make power at less RPM, itís the cooling
they get from spinning the fan faster that your after.
Actually, it's the centrifuga
l forces they're trying to avoid in a high redline car. Above 14,000 rpm the alternator parts
want to flee. The latin for flee is fuga, hence fugal.
But in a small 9hp diesel
for a sailboat, what's wanted is to recharge the starter battery
and, when the craft it recharges is taken out for a few nights, can it recharge the battery
enough to power the anchor
light all night along with some incidentals in the starlit evening.
From what I've seen, alternators are burned up trying to charge batteries
with low voltage, batteries
that are run down too far, or batteries that are going bad. Alternators can generate more current
at low voltage. The heat that you need to cool off in an alternator is I^2*R.
I have a way to obtain more power from a small alternator at idle. Raise your voice if you'd rather recharge your battery with the engine burping at idle instead of with the engine throttled up, yelling down the quiet night, and vibrating the hull