Hans, Yes; normal. As Rudy said, only the part immersed in raw water will be part of the active protection circuit. Because less material is immersed in water inside the heat exchanger, what is immersed will deteriorate faster (salt water, fastest deterioration, brackish water slower, fresh water slowest). So very much "normal." But if that's the way you use your boat, accept it as a good thing. Loosing pencil zincs is MUCH BETTER than loosing copper from the heat exchanger body, or cupro-nickel from the core pack!!! Jim Peg and Jim Healy, living aboard Sanctuary http://gilwellbear.wordpress.com <http://gilwellbear.wordpress.com/> Monk 36 Hull #132 MMSI #367042570 AGLCA #3767 MTOA #3436