Jim Healy I was reading your response to the zinc issue.... I have a question:  My twin Lehman engine trawler originally had one zinc in the raw water heat exchanger,  one in the oil cooler, and one in the trans fluid cooler on each engine.   The boat has a large transom zinc and 2 egg zincs per shaft.  There is also a zinc in the raw water heat exchanger in each of my two generators.. I also have a galvanic isolator.  In the past, I constantly had the channels in the oil coolers plug with a white hard deposit, so I took the zincs out of the oil cooler (cooler is grounded to the engine block) and put in a plug.  The oil coolers were lowest height on the engine and I am guessing they were constantly filled with sea water.  The raw water exchanger and trans cooler are higher up on the engine and maybe drain sea water better and did not have the same problem... My question is, how close to the metal being protected does the zinc need to be?  I have seen for years zinc guppies connected to the boat bonding system via a metal conductive cord sold.  So if the zinc needs to be immediately adjacent to the metal it protects, the guppy would be useless.  If the zinc has to be "close", in my case, the egg zincs would maybe protect the shafts and props...  The transom zinc is connected to the bonding system but is many feet from the coolers and engines....  what would it protect?   I have also heard boats can get "over zinc'd"... not sure how that fits into the story? Jim Gano