The submarines of the KD6A sub-class were versions of the preceding KD5 sub-class with greater surface speed and diving depth. They displaced 1,814 tonnes (1,785 long tons) surfaced and 2,479 tonnes (2,440 long tons) submerged. The submarines were 104.7 meters (343 ft 6 in) long, had a beam of 8.2 meters (26 ft 11 in) and a draft of 4.57 meters (15 ft 0 in). The boats had a diving depth of 75 m (246 ft)[1]
For surface running, the submarines were powered by two 4,500-brake-horsepower (3,356 kW) diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a 900-horsepower (671 kW) electric motor.[2] They could reach 23 knots (43 km/h; 26 mph) on the surface and 8 knots (15 km/h; 9.2 mph) underwater. On the surface, the KD6As had a range of 14,000 nautical miles (26,000 km; 16,000 mi) at 10 knots (19 km/h; 12 mph); submerged, they had a range of 65 nmi (120 km; 75 mi) at 3 knots (5.6 km/h; 3.5 mph).[3]
On the day of her commissioning, I-73 was attached to the Kure Naval District and assigned to Submarine Division 20.[4][5] Her division was assigned to Submarine Squadron 2 in the 2nd Fleet, a component of the Combined Fleet, on 1 December 1937,[4] and then to Submarine Squadron 3 in the 2nd Fleet on 15 November 1939.[4]I-73 departed Okinawa on 27 March 1940 in company with the submarines I-68, I-69,I-70, I-74, and I-75 for a training cruise in southern Chinese waters, completing it when the six submarines arrived at Takao, Formosa, on 2 April 1940.[4][6][7][8][9][10] On 15 November 1940, Submarine Squadron 3 was reassigned to the 6th Fleet, another component of the Combined Fleet.[4] On 16 January 1941, the submarineI-72 temporarily relieved I-73 as flagship of Submarine Division 20.[5]
As Japanese military forces began to deploy for the opening Japanese offensive of the war, I-73 — with the commander of Submarine Division 20 embarked — departed Saeki Bay on the coast of Kyushu on 11 November 1941 in company with the submarines I-8, I-68, I-69, I-70, I-71, and I-72 bound for Kwajalein Atoll, which she reached on 20 November 1941.[5] Assigned to support Operation Z, I-73 got underway from Kwajalein on 23 November 1941, again with the commander of Submarine Division 20 embarked, and set course for the Hawaiian Islands.[5] While she was en route, she received the message "Climb Mount Niitaka 1208" (Japanese: Niitakayama nobore 1208) from the Combined Fleet on 2 December 1941, indicating that war with the Allies would commence on 8 December 1941 Japan time, which was on 7 December 1941 on the other side of the International Date Line in Hawaii.[5] After reaching Hawaiian waters, she conducted a reconnaissance of Kealaikahiki Channel between Kahoolawe and Lanai on 5 December 1941 and of Lahaina Roads off Maui after sunset on 6 December 1941.[5]
World War II
First war patrol
By 7 December 1941, the day of the Pearl Harbor attack, Submarine Squadron 3 was deployed south of Oahu, ordered to reconnoiter the area and attack any American ships that sortied from Pearl Harbor.[5] As part of this deployment, I-73 was stationed off the entrance to Pearl Harbor.[5] Her time off Oahu passed uneventfully, and on 17 December 1941 she departed Hawaiian waters to make for Kwajalein.[5]
Along the way, I-73 was diverted from her voyage to bombard Johnston Atoll.[5] She arrived off the atoll on 23 December 1941 and fired six 100-millimeter (3.9 in) rounds, knocking down the Civil Aeronautics Authorityhoming tower on Sand Islet, wounding one United States Marine, and later claiming a hit on a utility pole.[5] The United States Marine Corps 5-inch (127 mm) gun battery on Johnston Island returned fire, each of its guns firing ten rounds before I-73 submerged and departed the area unharmed.[5] She arrived at Kwajalein on 29 December 1941.[5]
Second war patrol
With the commander of Submarine Division 20 again embarked, I-73 got underway from Kwajalein in company with I-71 and I-72, the three submarines having orders to relieve the submarines I-18, I-22, and I-24 on a picket line in Hawaiian waters.[5]I-73 transmitted a situation report from her assigned patrol area on 15 January 1942.[5] She often is credited incorrectly with shelling Midway Atoll on 25 January 1942, but I-24 conducted that bombardment.[5]
Loss
On 27 January 1942,[5][11] the United States Navy submarine USS Gudgeon(SS-211) was on her return voyage from a war patrol off the Bungo Strait in Japanese waters and was 240 nautical miles (440 km; 280 mi) west of Midway Atoll in the Northwestern Hawaiian Islands when she received an Ultra message informing her that I-18, I-22, and I-24 were approaching her.[5] She steered to intercept them, but did not encounter them. While submerged and searching for them, however, she detected the sound of high-speed propellers off her portbow at 09:00 local time.[5] She then sighted I-73 at a range of 5,000 yards (4,600 m), identifying her as an "I-68-class submarine" with a deck gun forward of her conning tower and at least six men on her bridge, making 15 knots (28 km/h; 17 mph) on a heading of 255 degrees True.[5]Gudgeon fired three Mark 14 torpedoes at I-73 at a range of 1,800 yards (1,600 m) at 09:07 local time, then lost sight of her in heavy seas.[5] One minute and 45 seconds after firing the torpedoes, Gudgeon′s crew heard two explosions, after which I-73′s propeller noises stopped.[5]Gudgeon returned to periscope depth and saw no sign of I-73.[5] She claimed only to have damaged I-73, but Station HYPO, a U.S. Navy signals intelligence unit in Hawaii, confirmed that Gudgeon had sunk I-73.[12] Sunk at 28°24′N178°35′E / 28.400°N 178.583°E / 28.400; 178.583 (I-73), I-73 was the first warship ever sunk by a U.S. submarine.[5][11]
The Imperial Japanese Navy declared I-73 to be presumed lost with all 94 hands off Hawaii[4][5] and on 10 March 1942 administratively transferred her to the fourth reserve at Kure Japan, pending final disposition.[5] The Japanese removed her from the Navy list on 15 March 1942.[5]
Rohwer, Jürgen (2005). Chronology of the War at Sea 1939–1945: The Naval History of World War Two (Third Revised ed.). Annapolis, Maryland: Naval Institute Press. ISBN1-59114-119-2.
Empire of Japan
