Protection and machinery in the transition to fast battleships

How light can a capital ship's protection become before it stops being a fast battleship?

There is no single percentage; classification emerges from armor coverage and thickness, subdivision, and the machinery density that produces speed.

Full armor coverage, including an all-or-nothing scheme, points toward a battleship, while cruiser-caliber protection points toward the older battlecruiser model. Internal subdivision and machinery layout determine how much survivability can be retained. Engine rooms placed within a dense protected subdivision impose different risks from machinery arranged against the hull. Improving horsepower per ton and per unit of volume is the key enabler that allows speed without surrendering battleship protection.