Sorry, Capt Camel, I missed your OP. Don't know how I did because I have been sailing this desk 12 hours daily since 11 March with nothing to do but surf the net & haunt CF. The things one does for cruising kitty transfusions.
anyhow back to OP. So you got the zincs out. How many were there? On the Yanmar
3GM30 there are three - one at the rear and two on the left side looking from the front. One of these is directly underneath the secondary
fuel filter which makes it a real bastard to remove. Why oh why do
engine makers do that sort of thing.
FYI I check my zincs every six months. They should be replaced if they are over one third
lost. Note down in your engine
maintenance book (everyone should have one of these babies to keep a tag on
oil use and your preventative
maintenance schedule) the state of decay and the date of
inspection of each zinc so that if they suddenly start to decay quicker you will know right away and then can search for the electrolytic reason.