 Allocated project number A-102, the Nagato class was designed before Commander Yuzuru Hiraga was reassigned to the Navy Technical Department (NTD) responsible for ship design, although Hiraga is often credited with the design of these ships. In contrast to earlier designs, the Nagato class used the American "all or nothing" armor scheme that maximized the armor thickness protecting the core of the ship by eliminating armor elsewhere. The design had two armored decks of medium thickness rather than the single thicker deck used formerly. The ships also used a new type of underwater protection system that successfully resisted penetration by 200-kilogram (440 lb) torpedo warheads in full-scale trials. It consisted of a deep water-tight compartment adjacent to the hull, backed by a thick torpedo bulkhead that connected to the side and deck armor plates, with a deep fuel oil tank behind it.
