Aoba - class cruiser The were a class of two Japanese heavy cruiser s which saw service during World War II . Design . The " Furutaka " class of heavy cruisers ( also called “ A class ” cruisers in the " I . J . N . " ) was originally planned to include " Aoba " and her sister ship " Kinugasa " . The two ships were laid down in early 1924 with that intent . In 1924 the IJN Naval General Staff pressured the acting head of the Basic Design Section , Fujimoto Kikuo , to use the newly designed twin gun turrets on the " Furutaka " class ship s . Fujimoto agreed to modify the " Aoba " and " Kinugasa " , but it was already too late to retrofit " Furutaka " and " Kako " , which had been laid down in late 1922 . The two ships were subsequently renamed as a separate class . The modifications were significant . The 6×1 20 cm semi - turrets , 1 through 6 , were replaced by 3 twin turrets , two fore and one aft . The main superstructure was also modified to accommodate the different fire control requirements of the twin turrets . Other design modifications included using / 45 - cal HA guns rather than the guns . The airplane takeoff platform , part of which mounted atop the number 4 semi - turret , was replaced with a catapult fitted just fore of the number 3 twin turret . The superstructure aft of the funnels was extensively modified due to the new catapult . The catapults weren ' t ready before the ships were commissioned . " Kinugasa " had an compressed - air unit mounted in March 1928 , while " Aoba " had a gunpowder - propelled model mounted in March 1929 . " Aoba " and " Kinugasa " were otherwise built on the same hull as " Furutaka " and " Kako " , with identical machinery and armor . As built the " Aoba " was more than 900 tons heavier than its design weight , with negative effects on the performance of the ships similar to that of the " Furutaka - class " overweight condition . Modernization . In 1930 , the Naval General Staff , concerned by the limitations on the size of their navy by the London Naval Treaty won approval for an extensive modernization program of the “ A class ” cruisers . To offset the numerical superiority the U . S . Navy enjoyed , the planned upgrades included the latest weapons , protection , fire control systems , and communication equipment . In the spring of 1930 the two ships had their manually - operated 12 cm anti - aircraft guns replaced with improved electro - hydraulically operated units . " Kinugasa " was refitted with a gunpowder - propelled catapult a year later . Extensive modernization of the ships began in late 1938 , lasting until they were recommissioned two years later , as follows : The main battery was replaced by / 50 - cal Mark II guns . Although turrets and gun mounts were unchanged , the powder and shell hoists were modified for the larger shells and to prevent flarebacks in the turrets from traveling to the magazines . Light anti - aircraft defenses were improved by 4 twin sets of 25 mm machine guns and 2 twin 13 mm machine guns . The 6 pairs ( 3 per side ) of fixed torpedo tubes mounted on the middle deck were replaced with 2 quadruple mounts using the powerful Type 93 torpedo , located on the upper deck , one on each side of the catapult . The bridge structure was rebuilt almost identically to the " Furutaka - class " modifications to accommodate the latest rangefinders and fire control equipment for the main battery , antiaircraft and torpedoes . Of the twelve original boilers , two smaller ones were mixed - fuel capable . They were replaced by with oil - fired units . Their coal bunkers were replaced with fuel - oil tanks . The above modifications added 576 tons to the ships . To prevent the draft from increasing even more , and to improve stability , bulges were added , simultaneously enhancing antitorpedo protection . As a result , the ships ' beam was increased to .