 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 . 
