Well, I finally got back to the boat and determined that in fact the alternator
is the cause of the problem. It's a Hitachi fitted as standard to the Yanmar
GM1. It's the 30 Amp model and seems to work perfectly, charging
at 14.3 V. With the engine
stopped and everything turned off except the battery selector switch it draws 3.5 Amps thru the alternator
. Disconnect one of the wires to the alternator--- zero draw. The battery selector switch had also been defective, never really switched off, but I fixed that. Now my problem will be to figure out which diode is the bad one.