The size and gun limits of an 18-inch battleship

How large would a battleship need to become for 18-inch guns, and what makes 20-inch weapons difficult?

Probably more than 50,000 tons for an 18-inch ship; 20-inch guns magnify barrel, handling, and structural problems sharply.

A ship built to carry and resist 18-inch weapons would probably displace more than 50,000 tons, perhaps around 52,000 to 53,000. A Hood-derived design might carry six such guns in three twin turrets, preserving room for the enlarged barbettes and machinery. Six guns reduce the options for half-salvo ranging, although two ships operating together could share enough fall-of-shot information to compensate. Moving to 20-inch guns makes the barrel itself extraordinarily long and difficult to handle. The square-cube relationship between dimensions and mass turns each calibre increase into a much larger design burden than the nominal inch suggests.