It is extremely unlikely that your corrosion
has anything to do with "stray current
With an aluminum
sail drive and bonded through hulls you are creating a situation where the sail drive is the anode and it is trying to protect your through hulls. It has become the sacrificial anode.
My advise is you only bond through hulls if they show evidence of electrolytic corrosion. An isolated piece of metal in the water
is only ONE pole of a battery
. The moment you connect it with bonding to other metal in the water
that is not the same metal (same electrical
potential) you create a battery
flows from the anode to the cathode in the water and back through the bonding.
I would remove all the bonding, in particular that connects to the sail drive.
If necessary I think you can get special anodes to protect the aluminum sail drive locally, perhaps magnesium?? - check with the sail drive manufacturer.
your through hulls on each haul out
and worry about adding a zinc anode to protect them only if you see trouble.
If the aluminum on the sail drive is a GOOD alloy and is not connected to anything else it will survive without any protective anodes. Aluminum hull
boats that are TOTALLY isolated from all other metal don't even need protection. But if it has bronze or stainless prop and hardware
you may need to follow manufacturer's recommendations.
Finally, thoroughly clean the existing corrosion and repaint with a Coal Tar Epoxy
or some similar underwater paint
that will seal and electrically ISOLATE the metal from the water. Within 36 hours of the epoxy
setting, apply any anti-fouling
if you are going to use it, if you leave it too long the epoxy hardens so much that subsequent paint
will not adhere.