A ship cannot be made functionally proof against an 18-inch projectile across useful battle ranges. Designers would have to choose which lesser threats and ranges the armour should handle. Displacement that cannot buy practical immunity could instead go toward speed. The ship would then manage range and engagement geometry rather than attempt impossible all-round protection.
Notable Point · 1:41:34-1:42:12
When 18-inch guns make speed more valuable
Once armour cannot practically defeat an 18-inch shell, designers may gain more by managing range and speed than by adding protection.