The I-1 was a Japanese submarine cruiser that entered service with the Imperial Japanese Navy in March 1926. The length of the ship at the time of launching was 98 m, width 9.1 m, and the underwater displacement - 2,800 tons. The maximum speed of the I-1 on the surface was up to 18 knots. The main armament was six 533 mm torpedo tubes with 14 spare torpedoes, and the secondary armament was two 140 mm guns. I-1 could descend to a maximum depth of 80 meters.
The I-1 was an ocean-going submarine cruiser - the first of four units of the same type as the I-1. Its strong artillery armament - for submarines - predestined it to fight on the surface with enemy merchant ships and enemy light surface ships. Similar design assumptions were also used on other submarines launched in the 1920s and 1930s, including the French Surcouf. Activities during World War II showed that such a concept of using submarines, in view of the rapid development of aviation, turned out to be completely wrong. The I-1 began its combat career in the Pacific War by firing multiple shots at American positions in Hawaii in December 1941. In March 1942, the Dutch transport ship Siantar sank. In August 1942 it was converted into a unit to support the special operations and covert landing of the Japanese marines. Until January 1943, it supported Japanese land-sea operations in the Guadalcanal area. The I-1 was lost by the Japanese fleet on January 29, 1943.
The I-6 was a Japanese submarine whose keel was laid in 1932, launched in March 1934, and commissioned in the Imperial Japanese Navy in 1935. The length of the ship at the time of launching was 98.5 m, width 9 m, and underwater displacement - 3,060 tons. The maximum speed of the I-6 on the surface was up to 20 knots. The main armament was six 533 mm torpedo launchers, and the secondary armament was a single 127 mm gun and one 13.2 mm anti-aircraft machine gun. The ship also took one Yokosuka E6Y1 seaplane on board.
The I-6 was an ocean-going submarine of the Junsen II sub-type, based on the experience and many solutions used on the I-5. Compared to its predecessor, the I-6 differed significantly in the underwater displacement, it was slightly larger, it also had other diesel engines with a much higher maximum power, which had a positive impact on the surface speed of the ship. The immersion depth has also improved (from 75 to 80 meters). The secondary armament was also slightly different. When the war in the Pacific broke out, the I-6 participated in the actions of the Japanese fleet against the base at Pearl Harbor on December 7, 1941. Shortly thereafter, in January 1942, I-6 severely damaged the USS Saratoga aircraft carrier, and in April of the same year it sank two commercial vessels RMS Clan Ross and RMS Bahadar. In the summer of 1942, I-6 was directed to the Aleutów region, but at the beginning of 1943 it was transferred to the South Pacific. The ship was lost as a result of an accident or enemy action on June 30 or early July 1944.