In 1861 John Elder from Glasgow brought out plans for a circular ship. It would, he claimed, have maximum stability and minimum draught, making it an ideal gun platform.
The Russian Navy, in the shape of an Admiral Popov, liked the idea and built two circular ironclads, the Popovkas or Cyclads, to patrol the Black Sea.
A variation on the design was the imperial yacht Livadia, which was more of an ellipse than a circle. Built by John Elder for Czar Alexander II she wasn’t delivered until after he was assassinated.