Like, where did my prop (and zinc) go?
Some checking found a bad ground cable between the engine and the battery negative. With the alternator doing it's thing, the current couldn't get back to the alternator through the ground lead. So, it went into the keel through the bonding system, across sea water to the propeller, up the shaft, then to the engine and alternator ground.
The drivesaver won't keep cables from going bad, but it will save my propeller in the future. Maybe the battery negative shouldn't be connected to the bonding system?? I'm a mechanical guy, not electrical.