Gun count and speed draw on the same hull capacity. Adding a fourth triple 15-inch turret while keeping about 28 knots would require more length, beam, and internal volume. Extra boiler capacity would enlarge the machinery spaces as well. The heavier armament would increase the propulsion requirement and displacement of the whole design.
Notable Point · 2:43:02-2:43:25
A twelve-gun fast battleship needs a larger hull
Keeping twelve 15-inch guns and about 28 knots would force a capital ship to grow longer and wider so it could carry both the extra armament and the boilers needed for speed.