 The <unk> 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 . 
