Aircraft powered airboat discussion.
sounds like you might have a battery with a cell going bad. I had that same problem on a truck with two batteries and found out finally that one of the batteries was going bad. It would crank but slow, when it started then the alternator wouldn't charge for about a minute an then kick in, I changed the alternator but didn't help. Finally took the battery out and put a battery tester to it and when I threw a load on it it went all the way over in the read. Replaced it , problem solved.