Many thanks to everyone for their help. I have learned a lot throughout the course of this discussion, and I think I am finally getting to the bottom of it.
Mo, it's funny
that you mentioned the Napa 9520 belt. I think the root of the problem was a worn-out 9520 belt. Before now, I had never taken the belt off, because it looked like new from the outside (probably because it was mis-aligned and shaving itself down to fresh rubber constantly).
(note the teeth are completely missing from one section. Belt is also significantly skinnier than the fresh 9520 I replaced it with):
My best guess at what happened is this: previous owners were having belt issues, and the old 9520 was worn and stretched to the point that the original bracket couldn't hold it tight enough to recharge the batteries
. So instead of replacing the belt, they added another bracket to force it tight. But the addition of the bracket caused mis-alignment and chewed up the belt to the point that it wasn't making proper contact with the alternator pulley, and with all of the 'pivot points' and poor leverage you have with two brackets, it would never stay taut for long, and it finally got to the point where it just didn't work
I have removed the aftermarket bracket, and am now just using the original bracket and a new Napa 9520 belt (only because there were extras on the boat
, and it saved me a trip to the auto parts
store). Mo is right, the 9520 is a little on the long side and nearly maxes out the original bracket, but even with just a moderate amount of tension, I was able to produce 50+ amps yesterday without any squeaking or slipping. For now, I'm claiming victory. Thanks again for everyone's help. Now I can continue north to Beaufort
SC with confidence.