 The armor protection of the Gangut-class ships had to protect against two different threats as revealed during the Russo-Japanese War. Japanese high explosive shells had riddled the unarmored portions of Russian ships and had even sunk several ships with their heavy armor belts unpenetrated. The Russians decided that the entire side of the ship needed to be armored, even though this would limit the thickness of the main belt. They developed a system where the outer armor would break up or at least slow shells down and burst immediately behind the outer armor and an inboard armor bulkhead would stop the splinters and shell fragments from reaching the vitals. This system likely would have worked against the British armor-piercing shells that performed so badly at the Battle of Jutland, but would have failed against the improved shells introduced afterwards with their redesigned fuses. A related weakness was that the turrets and conning towers lacked the inboard splinter bulkhead even though they used armor thickness roughly equivalent to that of the main belt.
