Thump, I'm sure you are right. I did just go out and check the battery connections again. There is no corrosion of any kind anywhere. The + on the cranking battery was.........well, not loose by any means, but not cinched down either. I cinched it down tight.

I had lunch with several guys, and all boat owners said that they believed that all new boats with a 24v trolling motor come wired this way from the factory/dealer. That is, only the Trolling Motor is connected to the 2 in-series batteries, and the cranking motor has all accessories. The reason is, that's the one that gets the alternator connected.

And it's worked nonstop until now.

I just read the Humminbird installation manual that I kept on .PDF. All it says about voltage is to A) connect only to 12v source, and B) to put a 3 amp inline fuse in the connecting line. It has that. So, it seems that a voltage spike that might cause damage would blow that fuse. This is a dead drop in voltage. And if starting the motor knocked the DF'ers offline, they would start back up manually if/when the big motor shuts down. These do not.

So, I suspect if I had checked the connection to battery while on the water, I'd have figured it out. Stinking boat layout sucks, though, and you either have to be standing outside the boat to get into the battery compartment, or you have to stand on the transom and hold on to the big motor! Still, that was laziness that I didn't check that first.