Answer for Rob

Yes the AC grounding probably has something to do with the speed of
disappearance.
As the anode gets worn away often the fastenings themselves are weakened
and the anode spins off the shaft before it is all gone.
If you have access to scuba gear there is no reason you can't fit a new
shaft anode while the boat is in the water.


Rob wrote:

I have a Catalina 30 with an inboard diesel that I launched in May of
this yr- this weekend while swimming off the back of the boat I took a
swim below with a mask to check out the bottom. I noticed the zinc I
just put on the brass prop shaft 3 months ago was missing. I put it
on myself and remember how tight I made the screws- is it possible for
it to either fall off from vibration or deteriorate that fast? The
rudder zinc is still there. I keep my boat at a private dock in a
creek- the closest boat to mine is about 100 yards away. I have shore
power- is the AC ground possibly doing something to speed up the
galvanic process? Any ideas on how to replace the zinc without
shelling out for a short haul?

Thanks-
Rob