I worked on a 120m old passenger ship for a while. During dry-dock each year we'd put 8 or so 2kg (ish) zinc anodes around the hull, a welded bracket around each one.

The trouble was that they're pretty expensive, and zinc has a great resale value - so we had to do extra watches around the lower decks with rigged firehoses to try and stop local divers stealing them who knew we'd just come out of drydock. (This was in the Philippines). We lost several that way.