F-19 Class Minelayer:
he 1930 London Naval Treaty had put limits to the number of ships Koko no Kaigun could have built. In it's effort to circumvent it Koko no Kaigun issued the requirement for a class of large minelayers to be built to cruiser standards like the Hayabusa class seaplane tender. Despite beign scaled back from the original plan, as opposed to the Hayabusa, which grew even more ambitious with each revision, the ship that emerged fromt he drawing board was a 5.449ton standard (6.686t at full load) unit, 155,27m long, 15,9m abeam and with a draft of 6,1m. Like on the Hayabusa the forecastle housed the triple 140mm gun turrets, this time two mounts instead of three, for a total of six guns. Three single 120mm guns, two fitted abeam and one at the stern, made up for the dual purpose battery. Anti-aircaft duties were performed by sixteen 13mm machine guns in four quadruple mounts.
Side armor was thinner, at 30mm, giving effective protection only against Destroyer guns. It was split in two sections one protecting the engines and another one shifted lower into the hull to protect the magaines of the 140mm guns. Below the belt, a 10mm bulkhead acted as the anti-torpedo system. A single deck 20mm thick ran from the forward guns sto the turbine rooms, the gun themselves protected by 50mm face plates, 30mm side and roof and 50mm on the barbettes. Engine-wise, the first draft called for a 30+knot ship but the plan was subsequently revised: three oil-firing boiler generating 33.123shp powered two geared turbines that in turn drived two propellers, enough to push the ship up to 28knots. endurance was 8.000 nautical miles at 14knots.
As for the minelaying gear, no half measures were taken: the planned ships could carry 700 Mk.6-Mod.1 naval mines, or 850 Type88 ones. Up to 44 depth charges could also be carried without reducing the mine complement. Finally, a catapult was fitted at midship, with enough space to carry two Kawanishi E7K floatplanes for reconaissance duties. Three ships were ultimately ordered F-19, F-22 and F-24, the first starting construction at Toumachi Naval Arsenal in 1932.
By the time F-19 was launched, in 1935, the Navy had already started questioning of the effective usefulness of such hybrid ships, even more so as the official policy was now to ignore naval treaties, a thing that made naval construction like the F-19 completely pointless. The two sisters, F-22 and F-24, had already been redesigned as smaller units exclusevly dedicated to minelaying missiones. As for F-19, construction slowed to a near halt while the higher-ups discussed her fate. Instead of the Hayabusa Seaplane Tenders, which could easily act as scout cruisers for the Aircraft Carriers or be converted into CVLs themselves, F-19 lacked both speed and dimensions to be useful for such roles.
Ultimately, it was decided to just finish F-19 as designed, with the only change being the quad 13mm machine guns replaced by triple 25mm mounts, totaling 12 barrels. F-19 was finally commissioned in early 1938.
After the start of the Pacific War, F-19 sailed with F-22 and F-24 to the Aleutians, before beign detatched to reach Singapore in february 1942, were she was based until late 1943. She then returned to the Aleutians undertake a more active role, exploiting her higher speeds and ordinance capacity to lay minefields against the advancing US forces. While not employed in this manner, she acted as a convoy escort. In this guise she took part in the Battle of the Komandorski Islands on 27 March 1944, were she had the unwelcome privilege to be the first target of the US Battleship Nevada before it switched fire on Yagumo. Luckily, the closest shots were a straddle and F-19 managed to raise steam and escape (something that Yagumo could not...).
Returned to Koko, F-19 was refitted with a newer hydrophone, a full radar suite and a new airwing made up by Nakajima E8N. Anti-aircraft suitee was augmented by the addition of 8 triple and 20 single 25mm machine guns, bringing the total count to 56 barrels. Depth charge inventory was increased and a green camouflage pattern painted. She continued to operatee as a minelayer or a convoy escort as requirements dictated for most of 1945.
By mid 1945 F-19 had become one of the first non major-combatant ships in Koko no kaigun to be fitted with Thiarian designed (and Kokoan license-built) fire-control radars, qith an R4CA for the DP-gun director and an R5CD for her main guns. Integration between Japanese and Thiarian systems was deemed satisfactory, with F-19 proving it herself when she managed to shot down three Avengers and two Hellcats during and American raid on Attu, while F-30, wich had yet to recieve the systems, failed to score a single hit on the attacking planes and was sunk during the engagement.
At the start of Koko uprisings F-19 was still stationed in the Aleutians. Except of one of her E8N wich was shot down, she fell almost undamaged in US hands when the armistice was signed, in March 1946. F-19 then joined the Kokoan co-belligerent fleet and was painted in the standard USN Measure-22 style camouflage. A kokoan flag and the ship name were also painted on each side.
The ship was temporarily interned to Midway, then steamed to Okinawa by mid-June 1946 to support the later stages of the operations, the Kokoan flag on her side was painted over for this occasion as to not make her a clear, reconizable, target for Japanese Kamikazes.
At the end of the war F-19 operated as a repatriation ship, she was then renamed Seisei, entering service with the reorganized Koko Kaijou in May 1947. After two-and-a-half years of service, the ship was decommissioned and laid up. Most of her weapons and the aircraft facilities were removed, the mainmast was also cut down. Seisei was slated to be scrapped when the korean war borke out on June 25 1950. In order to allow for larger ship to support the USN and JMSDF* off Korea and retain an acceptable degree of patrol force to operate as Soviet deterrence the ship was recalled to active duty.
Recommissioning works were as fast as possible, a twin and a quadruple 40mm Bofors were fitted, augmented by two twin 20mm Oerlikons, the aft deckhouse was explanded and turned into a landing spot for a single Sikorsky H-5 helicopter, two depth charge racks and six throwers were fitted at the stern. A new crane was installed for boat handling and some electronics were replaced with USN ones, creating an unusual mix of Kokoan, Thiarian and USN systems, the latter of which was not so well integrated as the other two, leading to a complex shakedown in order to achieve satisfactory performances. Despite that, conflict between her eterogeneous and far-fetched radar suite kept hampering herfor the remainder of her service life.
In August 1953, a little less than a month after the end of the Korean War and after three years of service based at Teshigawa, Tenji island, Seisei was decommissioned for a second time. After a brief period spent in the mothball fleet she was scrapped by the end of 1954.
Ships in class: (laid down-launched-commissioned - fate)
F-19 1932-1935-1938 - Renamed Seisei 1947, Decommisisoned 1949, Recommissioned 1950, Decommissioned 1953
F-22 /-/-/ - reordered as F-22 design, 1934
F-24 /-/-/ - reordered as F-22 design, 1934
(*) no, it's not an error, in this timeline the JMSDF is established much earlier.