On the commercial boat on which I was the Engineer, all the zincs were cast with tabs which were welded on. Replacing, they were cut off, ground, and new ones welded on. There was a
lot of zinc, around 250 lbs of it on a 106 ft, 160 gross-tons hull. You really want to consult a steel boat builder to determine how much zinc you'll need. You might also get useful information from some of the zinc manufacturers, Sea-Shield, Reliance, Canada Metals, others. Also check out the electronic protection systems at
Electro-Guard, corrosion and cathodic protection specialists for boats, yachts and small ships. . These units get more and more attractive as the price of metals, including zinc, goes up.