6th Fleet (Imperial Japanese Navy)
Despite these advantages, IJN submarines achieved remarkably little during World War II primarily having in hindsight suffered from the antiquated strategy of the Imperial Japanese Navy General Staff, who viewed submarines as little more than scouts whose main role was to locate and shadow enemy naval task forces in preparation for a decisive surface conflict. Maybe the IJN General Staff was slow to change as the submarines were generally slow to dive, easy to track with radar and sonar, difficult to maneuver underwater, and less sturdy than their German U-boat counterparts.
Japanese submarine I-55 (1925)
The four Kaidai Type 3a vessels were the first mass-produced Japanese fleet submarines. Based largely on the indigenous Kaidai Type II ("I-52") a strengthened double hull, their design was also influenced by the largest of the German submarines in Japanese hands, the SM U-125. The hull had almost the same outer dimensions as the "I-52", but the increased thickness of the inner hull permitted a diving depth of 60 meters. Internal volume was slightly increased by making the hull slightly trapezoidal in cross-section, at the expense of 300 tons of additional displacement. External differences included an anti-submarine net cutter on the bow, as well as an O-ring for towing purposes.