Central citadel

The bulk of a dreadnought’s armor was concentrated around the “armored citadel”. This was a box, with four armored walls and an armored roof, around the most important parts of the ship. The sides of the citadel were the “armored belt” of the ship, which started on the hull just in front of the forward turret and ran to just behind the aft turret. The ends of the citadel were two armored bulkheads, fore and aft, which stretched between the ends of the armor belt. The “roof” of the citadel was an armored deck. Within the citadel were the boilers, engines, and the magazines for the main armament. A hit to any of these systems could cripple or destroy the ship. The “floor” of the box was the bottom of the ship’s hull, and was unarmored.[65] The earliest dreadnoughts were intended to take part in a pitched battle against other battleships at ranges of up to 10,000 yd (9,100 m). In such an encounter, shells would fly on a relatively flat trajectory, and a shell would have to hit at or just about the waterline to damage the vitals of the ship. For this reason, the early dreadnoughts’ armor was concentrated in a thick belt around the waterline; this was 11 inches (280 mm) thick in Dreadnought. Behind this belt were arranged the ship’s coal bunkers, to further protect the engineering spaces.[66] In an engagement of this sort, there was also a lesser threat of indirect damage to the vital parts of the ship. A shell which struck above the belt armor and exploded could send fragments flying in all directions. These fragments were dangerous, but could be stopped by much thinner armor than what would be necessary to stop an unexploded armor-piercing shell. To protect the innards of the ship from fragments of shells which detonated on the superstructure, much thinner steel armor was applied to the decks of the ship.[66] While the thickest protection was reserved for the central citadel in all battleships, some navies also extended a thinner armored belt and armored deck to cover the ends of the ship, or extended a thinner armored belt up the outside of the hull. This “tapered” armor was used by the major European navies—the United Kingdom, Germany and France. This arrangement gave some armor to a larger part of the ship; for the very first dreadnoughts, when high-explosive shellfire was still considered a significant threat, this was useful. However, it tended to result in the main belt being very short, only protecting a thin strip above the waterline; some navies found that when their dreadnoughts were heavily laden, the armored belt was entirely submerged.[67] The alternative was an “all or nothing” protection scheme, developed by the U.S. Navy. The armor belt was tall and thick, but no side protection at all was provided to the ends of the ship or the upper decks. The armored deck was also thickened. The “all-or-nothing” system provided more effective protection against the very-long-range engagements of dreadnought fleets and was adopted outside the U.S. Navy after World War I.[68] During the evolution of the dreadnought, armor schemes changed to reflect the greater risk of plunging shells from long-range gunfire, and the increasing threat from armor-piercing bombs dropped by aircraft. Later designs carried a greater thickness of steel on the armored deck;[69] Yamato carried a 16-inch (410 mm) main belt, but a deck 9-inch (230 mm) thick.[70]