Zincs

From Pearson Currents - Bill Shaw Answers the Mail:
Question:
Currently (pun intended) we are having some electrolysis problems. Our zincs are being eaten away every 2-3 months and I’m noticing deterioration of our through-hulls. Any ideas about the diagnosis of our problems?  

Answer:
It sounds like your zincs are only doing their job. You may be next to another boat that does not have a negative ground, which is literally turning your boat into a battery. I suggest you check out the boats around you and see if that is a possibility. The second thing you might look into is to see if there is a factory or other production facility that might be discharging into your waters. If this does not solve the problem for you, you will need to have an experienced electrician come on board, shut everything down and with the proper detection equipment, check to see if your boat has any current flowing. The electrician should be able to both identify the problem and its source.

As a precaution, don’t swim around your boat until you find out if the water you are sitting in is, in fact, not the cause of the problem.

Source: Pearson Currents Volume 6, #3 2000