Originally Posted by Insequent
Good info above.
But back to the OP, 's question. How do you know if you have too much zinc? What are the symptoms? What are the downsides?
The first is the voltage read from the Silver/silver chloride cell.
Too high from too much zinc can bubble paint off of metal parts, props, shafts, struts, etc., any immersed painted metals.
May cause direct damage to metals like aluminum.
It may cause a ring of paint bloom/bubble or disappearance around immersed through hulls.
On wooden boats it can cause actual damage to the wood surrounding through hulls or any other immersed metal parts or bolts/screw to secure them.
There are some good books that cover the subject.
--Metal Corrosion in boats: Nigel Warren
--The Boat Owners Guide to Corrosion: Everett Collier
I think Boatowners Mechanical and ELectrical Manual also: Nigel Calder also has a bit.
There are many other sources.