Questions for Andina Marie

We just completed our every-third-year haul out, and for the first time
ever, we had something very unusual on the hull. Over a large area
(approximately 1 meter across) around 2 bronze thru hulls on the
starboard side and rising up towards the waterline, the bottom paint was
gone. Not only the last coat, but all the accumulated layers going back
to the beginning of time - right back to the epoxy-coated gel coat. The
same thing was present around a different pair of thru hulls on the port
side. And finally, to a lesser extent at the rudder gudgeon bearing at
the heel of the rudder. Over the years we have owned the boat, we have
never seen this before.

It was suggested to me that this was an electrolytic effect. It seems
reasonable to me as well that it is. Would you agree?

The bronze thru hulls had no apparent dezincification. I had good
opportunity to check one of them because I needed to replace it for
unrelated reasons - it was quite sound.

We lost our prop nut zinc sometime over the winter (that is, it had
eroded to the point where it departed when the prop started spinning),
probably in February. So the boat was without anodic protection for the
month of March. Is this enough time for this effect to appear? Or has
it been underway for some time?

Finally, the bronze rudder gudgeon is connected electrically to the
engine, as were the two thru hulls on the port side. The two thru hulls
on the starboard side are not bonded. No other thru hulls on the boat
were affected, not even the others which were bonded.

All electrical outlets on the boat are GFCI protected, and the boat is
equipped with a galvanic isolator. I checked the water heater and found
nearly 30 megohms between the element and chassis ground, which seems to
be as good isolation as might be expected.

Can you help me understand what might have happened here?

Thanks in advance,

bob
s/v Eolian
Seattle