This thread caught my attention because of the mention of copper bottom paint
I have a 2005 Hunter
41 and over the years I noticed that whenever I had a fresh bottom paint
job (70% copper, hard paint
not ablative) I would start burning prop and shaft zincs up every 1-2 months or about three times as fast as before the bottom paint
job was done. Yes, my boat
has a galvanic isolator
and I had tests done to determine if there was any stray current
leakage from my boat or in the surrounding water
. Nothing was found.
I lived with this until the 3rd bottom paint job when I finally figured out what was going on. I have a solid lead fin keel
that had areas where the primer was missing and/or in poor condition. The keel
is bonded and when coated with nice fresh conductive copper paint it created a good battery
that burned up the zincs much faster than 'normal'. I even switched to aluminum
zincs which slowed the burn rate down but at one every 2-3 months that was still too fast. The problem would resolve after about 12-16 months when most of the copper paint on the un-primered areas of the keel would burn off and the zinc burn rate would be close to 4-5 months which I consider acceptable.
In an effort to fix the problem, I had the keel completely stripped and re-primed with 3 coats of epoxy
primer and then bottom paint. The zinc burning problem improved afterwards however, it took more than one attempt to get a decent but not perfect primer bond to the keel. The first time around, there were several large areas on the keel that had bottom paint separation which I assume was due to either poor primer bond with the bottom paint and/or poor primer bond with the keel that allowed conductivity through to the paint which in turn lifted the bottom paint.
After the 2nd strip and re-prime of the keel using heat lamps to ensure a good primer bond with the keel the issue was substantially improved however, on a subsequent diver inspection
, there were still several areas of 25-50 cent sized blisters
(only on the keel) where the bottom paint had lifted. It would seem that getting a perfect primer bond with the entire surface of the keel is quite difficult.
I'm fairly certain that if the keel bonding were removed that the problem would be completely solved
but I don't feel comfortable doing that.