I don't discuss my member
Join Date: Feb 2003
Location: Under a boat, in a marina, in the San Francisco Bay
Thanked 42 Times in 39 Posts
Rep Power: 14
A zinc will only protect what it is in electrical contact with. If there is a direct electrical path from your prop shaft to your rigging, a fish zinc will provide protection. If not, you're wasting your time and money.
Since you know there are corrosion issues in your slip (or aboard your boat, which is much more likely) your best bet is to have a diver putting eyes on your running gear periodically.