All or nothing (armor)
Then, armorers would attempt to design the application of barriers and deflectors which would protect vital areas of the hull, the superstructure, and its interior compartments from enemy shellfire, underwater mines, and torpedo attacks.
However, the continuing advances of larger-caliber guns, greater muzzle velocities, more accurate fire at longer ranges, and more energetic explosive fills of the shells demanded drastic improvements in armor protection.
The design was intended to ensure that battleships could (a) survive against the heaviest armor-piercing shells in use in the early part of the 20th century, (b) be able to carry powerful armament, and (c) retain useful speed and endurance.
The logic of the design was simple: if the ship were hit in vital areas (the ammunition and propellant magazines; the propulsion plant; the fire-control, command and communications sections), her survival was in jeopardy.
The propulsion plant, communications systems, weapons, ammunition stores, and command and control of the ship were located in a single area within and beneath the armored citadel.
Save for the turrets, the ammunition hoists, the conning tower and part of the steering gear, nothing in the way of armor protected the remainder of the ship.
The citadel can be visualized as an open-bottomed (closed top) rectangular armored raft with sloped sides sitting within the hull of the ship.
Although it was desirable for the citadel to be as small as possible, the space enclosed was an important source of reserve buoyancy and helped prevent the ship from foundering when other compartments had flooded.
As the caliber of guns grew and fire-control systems improved, engagement ranges increased, so that a greater number of hits would result from plunging fire against the ship's thin deck armor rather than its well-protected sides.
The most important finding of the gunnery trials on SMS Baden was that the 7-inch (18 cm) thick medium armor was completely useless against large-caliber shells.
With the signing of the Washington Treaty the Allies had an excess of old battleships, especially from the former Imperial German Navy, which were expended in gunnery and bombing trials.
The US could also afford to construct large portions of their battleships using Special Treatment Steel (STS), a ductile armor which provided both structure and splinter protection.
For example, Bismarck and Hiei were lost due to rudder damage; the relatively large and "soft" unarmored bow structures of Japanese superbattleships Yamato and Musashi proved to be their Achilles' heel as flooding there rendered them unstable and unmaneuverable long before they were actually in danger of sinking.
Competing theories exist as to how this occurred, but ultimately no practical thickness of deck armor could protect any battleship from a vertical bombing attack.
HMS Hood, built using the earlier concept of banded armor, was most likely lost when an AP shell from Bismarck passed through a thinner upper belt into her magazine,[10] Though also built using the older banded armor design, vessels of the Kriegsmarine's Bismarck class proved difficult to sink, largely because they were well-built and highly compartmentalized.
[10] Bismarck's sister ship Tirpitz suffered extensive topside damage from Royal Navy aerial strikes during Operation Tungsten but her vitals were relatively unharmed.
In particular, the Tallboy that hit amidships between the aircraft catapult and the funnel blew a very large hole into the ship's side and bottom while completely destroying the entire section of belt armor abreast of the bomb impact, which contributed to the battleship's rapid capsizing.
Hiei, built using an incremental armoring scheme, was fatally damaged by fires caused by 8-inch AP shells from USS San Francisco that penetrated secondary battery casemates protected by a medium thickness upper belt similar to that used on Bismarck.
On the second night, (14–15 November 1942) the USS South Dakota was hit at close range by 27 common, HE, and AP shells of various calibers, most of which passed through her unarmored superstructure without detonating and caused relatively little damage.
[12] No projectiles penetrated South Dakota's armor, and the ship's hull strength, buoyancy, stability, steering and propulsion were not materially affected.
[13] Though South Dakota was in no danger of sinking, she was put out of action by the damage the smaller-caliber fire wreaked upon her radars and electronic systems, which rendered her ineffective for night combat.
