MikeP: "Re the main engs - I have gone from two zincs in the HE, none in the RW pump; to one zinc in the output side of the HE, none in the RW pump; to none anywhere in the engine; to back to one in the output side of the HE; to (currently) none in the engine at all..."
The theory is simple (but not necessarily conclusive): different metals in engine cooling systems would typically corrode via galvanic corrosion. But since the most active metal would seem to be the heavy, massive heat exchanger...with a great deal of mass compared with the less active heat exchanger....and copper [piping....maybe it could "corrode" for 100 years without effect??
We also discussed that at least the 8V71TI's Mike and I have self drain the raw water after shutdown...I checked mine repeatedly and never figured out how THAT could be with the raw water pump at one end and the shower head at the other HIGHER than the heat exchanger. But whenever I checked zincs, no raw water was present....and I could hear no syphoning....The zincs sit in air 99% of tghe time!!!!....the zincs appear to be immersed in water only when the engine is running!!! That means maybe 100 or 200 annual hours of "protection"....I can't remember about the 6V53's and 3-53's I owned.
It IS tough to decide where, if at all, to use pencil zincs.....no matter where you use them, pieces end up where you don't want them: intercoolers, heat exchangers, shower heads......But surveyors look for them, so maybe frequent replacement if they are used is the way to go. I ended up using one on the output side of the DD heat exchanger.
When I replaced my DD heat exchanger gaskets I also cleaned the input side of the heat exchangers, I found a few zincs pieces, fewer impeller pieces, and some shell pieces....looked like mussel remnants...but the cooling had been ok.
I did have a 12KW Onan running warmer than it should at 35 years of age: Disassembly of the heat exchanger revealed cast iron ends which had "grown" into the copper alloy tubing between...several inches of growth!!!!...they were maybe 3/4 blocked!!!....that growth was a crusty, porous, iron oxide or some type, I think, and I easily chipped it away with a small hammer and screwdriver...then routed out each tube...but the tubes were clean inside, just the ends were blocked with the growth....Looked like nobody had cleaned that heat exchanger in 35 years. Most of the growth was on the input end of that HE...only maybe 1/3 as much growth at the other end...Do not understand that, either. I found a former pencil zinc there and replaced it. I don't know the effect, if any, such pencil zincs might have such "growth"....It's possible they retard such growth.
I'm 90% sure Bruce Morrison the Hatt mtc guy likes pencil zincs on DD and replaced them for the new owner of my boat. Tradition maybe??
I always worried about the shower heads getting partially clogged.....Is there any reasonable way to check those and/or clear those of debris???
There was no indication temperature-wise of any sort of problem. I pulled it and found it wasn't noticeably clogged at all but I rodded it and soaked it in vinegar anyway. Not sure why the main eng HEs seem to get clogged much faster...
