No - nothing else is corroding. The bronze sea-cocks are all healthy - they
are painted over and the diver checks them every 2 months - whenever the 3
small prop zincs are replaced.

After reading all the very helpful responses to my initial post I am now
leaning towards the theory that the prop is somehow electrically isolated
from the engine block (and the large zinc)- as unlikely as that seems.
However, I won't know until the next haul-out when it can be checked with a
meter.

BTW the large zinc had some sea growth on the surface at the last dive
inspection - so it is not protecting anything...



"Ernest Scribbler" wrote in message
...
"claus" wrote
The (folding) prop on our boat is made of Bronze.

Are you trying to protect the prop or are you trying to protect something
else from being corroded by the prop? Bronze is way up on the cathodic end
of the galvanic scale, meaning it *should* corrode zinc fairly rapidly.
And of course it will also corrode other anodic metals like aluminum or
iron if there's no zinc handy. What other metals are involved? Is there a
problem other than the prop zincs eroding?