Judy: I think that the square plate fastened to the hull was/is , as you say, a "zinc". But it is there to combat corrosion. I don't know but what it might have some effect on marine growth, but I believe that its primary purpose is to be a "sacrificial lamb" in the electrolytic corrosion cycle (two dissimilar metals connected by an electrolyte...the river) Heavy metals are often put into underwater (bottom) paint to inhibit marine growth because it is toxic to it. Wood ships were often sheeted with copper for that purpose. I SEE AFTER I WROTE THIS THAT BRUNO COVERED THIS SUBJECT QUITE WELL...