 The Shōkakus were the first Japanese carriers to incorporate a torpedo belt system. Based on model experiments that began in 1935, it consisted of a liquid-loaded "sandwich" of compartments outboard of the torpedo bulkhead. The experiments showed that a narrow liquid-loaded compartment was necessary to distribute the force of a torpedo or mine's detonation along the torpedo bulkhead by spreading it across the full width of the bulkhead and to stop the splinters created by the detonation. Outboard of this were two compartments intended to dissipate the force of the gases of the detonation, including the watertight compartment of the double bottom. The two innermost compartments were intended to be filled with fuel oil that would be replaced by water as it was consumed. The torpedo bulkhead itself consisted of an outer Ducol plate 18 – 30 millimeters (0.71 – 1.18 in) thick that was riveted to a 12-millimeter (0.47 in) plate. The IJN expected the torpedo bulkhead to be damaged in an attack and placed a thin holding bulkhead slightly inboard to prevent any leaks from reaching the ships' vitals.
