Disappearing sacrificial zincs

[divernc]

I have been replacing my underwater sacrificial zincs(rudders and shafts) 3 times a year on a 1979 43C (in saltwater). They are usually 80% gone after 4 months. I have been in the same slip for last 16 months. My most recent set is 50-60% gone in six weeks and highly pitted. No electronics changed on boat or known changes to dock. Marina dredged about 8 weeks ago from 3' at dead low water to 6'. A few neighboring boats have come and gone. Engine raw water zincs look fine as do all underwater metals. Bonding system appears in good shape. boat. I will first check the male and female shore power connections on both ends. What are some next steps to check or ask marina to check? Thanks for all the knowledge shared against my other posts.

[mikep996]

Check this article, it may be helpful:

www.yachtsurvey.com/corro...arinas.htm

[jim rosenthal]

You might try fitting blocking diodes (galvanic isolators) on each incoming shore line's ground leg (green). Stray DC current coming through those lines may be part of the problem.
Also, you might double check your bilge- is anything in the way of wiring loose in there and leaking DC current into the bilge water? That is fairly common no matter how well the boat is maintained.

[dshuman]

Maybe check bonding and grounding with a continuity tester to see if some areas have bad cooontacts. do you have a grounding wire that runs to the shaft seal plates? I should show good continuity test with the rest of the system. Your zincs are doing their job by taking the disintegration before your bronzes do.