Hayabusa Class Seaplane Tender (Part 1, 1934-1940/41):
As the 1930 London Naval Treaty had put limits to the number of ships Koko no Kaigun could have built(no carriers were allowed and no free tonnage was available for cruisers unless older units were decommissioned), naval designers had to start thinking outside of the box in order to circumvent the obstacle. As the aicraft carriers loophole, which considered units below 10.000T in displacement not as such and was exploited by Japan by building Ryujo, was closed by the new treaty the only units that not befall under limitations were auxiliaries. As Koko no Kaigun already had minelayers armed with 140mm guns under construction and the first of a three-unit (then reduced to two) 12.000T submarine tenders already in service and ignored by treaty restrictions designed focused on merging together both features in the design of the last missing type of ship still not in service: The seaplane tender.
Requirement were quite bold: the ships were to be well armed to stand their ground against enemy destroyers and had to carry a large complement of seaplanes in order to give the fleet a large scouting capability as to make up for the missing aircraft-carrier fleet. Soon the design grew even larger becoming a full fledged treaty-skirting unit under costant pressure from both the Naval Staff and Government. By designing an indigenous triple turret for the 140mm (5.5-inch) guns designers gave the ship enough firepower to match most light cruisers of that era. Three turrets for a total of nine guns were to be mounted on the forecastle, augmented by four twin 127mm (5-inch) mounts for dual anti-ship and anti-air duties fitted in shielded positions at midship. Four 40mm machine gun made up for light AA armament. The entire stern area was instead dedicated to floatplane operation, with a total capacity of 18 aircraft, 11 to be stored on deck and 7 inside the hangar, operated with three large cranes and two catapults. A70mm belt protecting engines and magazines was also planned, connecting with the forwardmost barbettees with diagonal bulkheads 30mm thick. Deck was to be 35mm thick, with gun turrets protected by 50mm plates on front and 30mm on roof and sides, barbettes themselves beign 50mm. Underwater protection was given by 25mm ulkheads. Thus designed the ship was at that point a seaplane tender in anything but name, with the term "seaplane cruiser" more fitting to describe it. Something that became even more striking after two quadruple banks of 610mm torpedoes, plus reloads, were added to the design.
To hold the planned armament and have space for engines and seaplanes the ship would have measured 205,3m (673feet, 7 inches) at the waterline and 209m (685feet, 8 iches) overall, waterline beam was 19,3m (63 feet, 4 inches), and draft 6,2m (20 feet, 4 inches), for a displacement of 10.543tons standard and 12.342 at full load. Eight oil-firing boiler generating 102.837shp powering four steam turbines driving as many propellers would have pushed the ships up to 34knots, while bunkerage allowed for a range of 8.000 nautical-miles at the speed of 14knots. Complement was planned between at 724 officers and men.
Design phase was completed by fall 1931 and the ships were approved for construction, with six ships ordered. The lead ship was named Hayabusa and laid down in December 1931 at Kousaten Navy Yard. The second ship, Kawasemi, started construction at the same yard a few days later. The third unit, Toki, started construction at Hoshiguma Navy Yard in January 1932.
Hayabusa was launched in 1933 and commissioned in 1934, recieving a reduced complement of 12 seaplanes: 3 Nakajima E2N and 9 Nakajima E4N
Kawasemi and Toki were both launched during the year 1934 and commissioned in 1935 and 1936 respectively. They were fitted with four quadruple 13mm machine guns in place of the 40mm ones fitted on Hayabusa. more so, they recieved the full planned complement of 18 seaplanes: 5 Kawanishi E7K and 13 Nakajima E8N. Eventually Hayabusa will recieve the same airwing too.
By 1935 two more ships, Ahodori and Fukuro, had been laid down at Yamatogawa Shipyards, but the direction taken by the Government and the Naval Staff after the attempted coup against Morimoto had made treaty-skirting ships useless for a nation that, like Japan, has started to denounce Naval Treaties. Under the second Rearmament Bill approved in 1936 Ahodori and Fukuro were reordered as Light Aircraft Carriers, conversion works starting in 1937 after the design of the ships was accordingly modified. The sixth unit, still unnamed, was canceled in favour of starting construction of the first Koko no Kaigun Fleet Aircraft Carrier, the Umineko.
As for the three Hayabusas already in service, in 1941 it was decided that their armament, high speed and excellent seaplane capacity would have suited well in escorting their half-sibling CVL and the now under construction Fleet Carriers. Thus the ships recieved an upgrade of the light anti-aircaft armament, with the 13 and 40mm machine guns removed and replaced by eight triple 25mm mouts, for a total of 24 barrels. Two machine gun directors were fitted, one on each side of the funnel, the searchlight directord landed their shilds and a torpedo director was fitted behind the bridge. Airwing was decreased from 18 to 15 planes because the new machines were larger, and made up by 4 Mitsubishi F1M2, 10 Aichi E13A1 and a single Nakajima E8N.
At the start of the war Hayabusa, Kawasemi and Toki formed Sentai 9, and escorted CVLs Ahodori and fukuro and the Fleet Carrier Umineko in their attack against Midway Island....
Ships in class: (laid down-launched-commissioned - fate)
Hayabusa 1931-1933-1934 - ?
Kawasemi 1931-1934-1935 - ?
Toki 1932-1934-1936 - ?
Ahodori 1934 - reordered as a Light Aircraft Carrier in 1937
Fukuro 1935 - reordered as a Light Aircraft Carrier in 1937
unnamed - canceled befor beign laid down in 1937