Because it's inspired by the center battery ironclad concept and in a casual search the name I came to for the first was a Royal Navy vessel. And happily, that's one of the nations I can do a flag for in Cosmoteer paint tiles.
Flags look better than just painting the back half black but are easier than trying to paint on a sail pattern or superstructure and my broadsiders tend to be too symmetric to tell which way they're pointing without some sort of direction indicator.