Marin: posted; ">>>If the zinc was doing its job, weed and algae should not grow on it.>>" True. A zinc corrodes fast enough to slough off marine growth. A good marine grade zinc should also be stamped with the appropriate mil spec number or at least meet the spec... Howver, all stainless steel bolts are not created equal....I foget which alloy is best for underwater , but lower grade bolts are ferrous (attracted to a magnetic) and will have local crevice type corrosion which can eat away active portions of the bolt, probably even if a good zince is connected nearby....These are also called active stainless steels and are close to aluminum in galvanic activity.. Passive stainless steels like 304,316 are almost as inactive an monel.... Cheers, Rob Brueckner 1972 Hatteras Yachtfish