Ōsumi Class Aircraft Carrier
Background
(In this one off AU THERE IS NO OTHER DETAILS)
Premise: Japan is less expansionist during WW1, and thus naval development programs are cut back rather to promote economic & industrial development.
In 1925, the construction on IJN Hōshō began, with the plan to construct a trio of vessels in order to initialize aircraft carrier doctrine for the Navy. The boats were designed to have staggered construction periods with the first boat, Hōshō to be built from 1925-1927, the second boat Shokaku from 1926-1928, and the third boat (unnamed) from 1927-1929. The Shokaku & the Unnamed third carrier would later be officially canceled in 1925 due to economic conditions at the time.
In June 1926, due to unknown circumstances (Historians believe the welding process went haywire) Hōshō erupted into flames whilst her construction was 64% complete. Her hull would subsequently be scrapped on site.
Due to the Washington Naval Treaty being signed in 1922, the Japanese were permitted 81,000 tons of aircraft carrier tonnage. However, under Article XX, Pt. 4: “an aircraft carrier was defined as a warship displacing more than 10,000 tons constructed exclusively for launching and landing aircraft. Carriers lighter than 10,000 tons, therefore, did not count towards the tonnage limit.”
With the Washington Naval Treaty taking effect in 1923, the major naval powers agreed to a short term (2 year) construction pause for aircraft carriers over 10,000 tons. As a result, the conversion of Amagi and Takao that had been the primary target for the IJN’s new carrier’s were put on hold for a short period to adhere to the treaty’s pause. However, in 1923 the Great Kanto Earthquake critically damaged the hull of Amagi, causing the cancellation of the conversions as the hull of Amagi was far too damaged to modify. The carrier conversion of Takao was scrapped via the Prime Minister’s orders as a single, expensive & ineffective carrier made little sense.
Design & Construction
Due to the loophole in the Washington Naval Treaty, the IJN in 1928 permitted the revival of Hōshō’s sister ships. However, the design sphere has changed since Hōshō’s initial design work was completed in 1922. The Hōshō design would form the basis of Ōsumi’s design, however involving more modernized materials.
The IJN permitted construction of a pair of light carrier vessels under the Han’i Kessen Doctrine (Construction of 2x light carriers, 2x fleet carriers, and 2x seaplane tenders). In 1930 Ōsumi would be launched, with some minor outcry from other naval powers as they believed Ōsumi to be a violation of the Washington Naval Treaty as Japan was not counting it as part of their tonnage. However, Ōsumi being under 10,000 tons meant it was exempt from the treaty limitations and excluded from Japanese tonnage allocations. The Ōsumi would finish construction work in 1933, with her commissioning on May 27th, 1933 on the 28th anniversary of the Battle of Tsushima. Her sister, IJN Atsumi, would launch in 1931 with commissioning in 1934.
Service
Ōsumi would serve as a training vessel for her first few years of service, with her first landing trials being conducted in late 1933 with the assistance of the Sempill Mission, a delegation of 30x naval aviators led by Captain William Forbes-Sempill intending to help the IJN develop its naval aviation operations. The lessons learned in Ōsumi & Atsumi would impact the fitting out of the Ryūjō and the Sōryū Class with more advanced landing equipment.
Ōsumi would form the initial basis of the First Carrier Division in 1933, with her sister joining a year later. Ryūjō and Sōryū would later join the Division after their fitting out period was completed in 1935 & 1937 respectively.
The First Carrier Division would be sent to China as a result of the Shanghai Incident taking place in 1937, with Ōsumi arriving at the mouth of the Yangtze River to support Japanese troops on the ground. Ōsumi’s fighters would shoot down an unknown number of Chinese Nationalist fighters, however Chinese records estimate 5x fighters shot down in three different incidents. The First Carrier Division would pull back away from Shanghai in early March, with Ōsumi sortieing back to Kure Naval Base for a refit of her landing systems.
Ōsumi would continue to act as an escort & training carrier until 1939 where she would deploy alongside Ryūjō as part of the 2nd Carrier Division off of Shanghai, where 3x Chinese Nationalist Martin B-10 bombers would attempt an attack run against the 2x carriers after they were spotted by reconnaissance aircraft. The bombing run would prove unsuccessful as both carriers would conduct evasive maneuvers to avoid the bombs falling. However, Ōsumi would be the only carrier of the 2nd Carrier Division for the rest of the year as Ryūjō made back for Japan for an overhaul of her stability figures, as the evasive maneuvers during the bomber attack proved that she needed an overhaul. Ōsumi’s fighters would shoot down a total of 15x Chinese fighter planes, with loss of 5x aircraft of their own (1x due to shoot down, 3x due to ditching, and 1x due to unknown reasons).
Ōsumi would participate in the Attack on Pearl Harbor in May of 1942 acting as an auxiliary carrier, providing scouting, anti-submarine, and general combat air patrols. None of her aircraft would take part in the actual battle itself, however the carrier did take part in the landing of 2x Mitsubishi A6M “Zero”s from Ryūjō that had gotten lost. Ōsumi would not take part in the Battle of Midway as she was in the Philippines at the time of battle. Her sister Atsumi however would take part in her place.
Ōsumi would later deploy out of Naval Base Truk for a few months in 1943, before being recalled to mainland Japan to train new cadets while her sister was in refit. She would spend most of 1943 and 1944 training new pilots for the navy after the severe losses in Midway. In late 1944, the IJN pulled Ōsumi and Atsumi from training to act as aircraft and fuel transport ships to Iwo Jima and Okinawa.
Fates
The IJN Ōsumi conducted transport runs to Okinawa, with her final cargo run being February 24th. She would sail back to Nagasaki the next day. She would remain in Nagasaki until the end of June when she would begin deliveries of fighters to Korea due to the foreseen Declaration of War by the USSR. Her last sortie would be on August 6th when she departed Nagasaki for Busan. She would spend 2 days in Busan before making back to Nagasaki on August 9th, however radio reports out of neighboring cities and towns near Nagasaki reported a complete destruction of the city (Due to the dropping of the Atomic Bomb), so the Captain of the Ōsumi decided to make for Tsushima until further developments were revealed. She would remain in Tsushima when surrender was declared on August 14th. Her crew sabotaged the engine and gear systems to prevent seizure. When the island was seized by the American Occupation Forces, she would be dragged out of port. American authorities wanted Ōsumi to take part in Reparation Transport of Japanese Troops from Korea, but the destruction of her engine systems prevented it. She would be dragged out of her anchoring in Tsushima by a tug, where she would then be dragged to offshore Pearl Harbor, to be sunk in tests of ASM-N-2 Bat guided bombs from a PB4Y Privateer, making Ōsumi the last Japanese carrier sunk. Her wreck is located at 19°58'26"N 158°34'00"W
On one of the cargo runs in 1945 to Iwo Jima, Atsumi would be stalked for several hours by the submarine USS Spearfish. Around midnight on January 10th, 1945 USS Spearfish would launch 2x torpedoes towards IJN Atsumi. The first torpedo would strike the dead center of the ship while the second would hit the rudder. Atsumi would explode into a massive rupture of flames 30 miles North-West of Iwo Jima, with an explosion so bright and loud those on the island were woken up by it. She would sink with all hands, and 25x stored aircraft on board. RV Petrel found her wreck location on June 5, 2018 at 24°55'55"N 140°58'15"E
Specifications of IJN Ōsumi (1933)
Type: Training Aircraft Carrier
Displacement: 9,567 (standard)
Length: 563 ft
Beam: 75ft 7in
Draught: 15ft 5in (mean)
Installed power: 8x boilers with 22,000 kW
Propulsion: 2 shafts; 2x steam turbines
Speed: 27 knots
Range: 9,245 nmi at 12 knots
Complement: 523
Armament:
4 × single 120mm GP guns
3 × single 6.5 AA guns
Aircraft carried: 18-24x
Typical 1933 Air Fleet:
Combat Oriented: (20x total)
12x Nakajima A2N Fighters
6x Mitsubishi B2M1 Torpedo Bombers
2x Yokosho C2M Reconnaissance Aircraft
Training Oriented: (18x Total)
18x Yokosho K4Y Training Aircraft