I. Origins of the scout cruiser design
The dynamic development of the Japanese cruiser fleet began after the signing of the Washington treaty. The signatories agreed to put on hold the construction of new battleships over the period of ten years and to scrap some of the warship construction projects already underway.
Some ships were dismantled on slips, while others, whose hulls had already been completed, were used as test vessels. The test results were to lay the foundations for the future warship designs. Although the treaty of February 6, 1920 put a hold on the design and construction of new battleships, it shifted the momentum of the American – Japanese arms race to the smaller vessels such as cruisers and destroyers. It also opened up the competition in the newest class of warships – the aircraft carriers. By ratifying the Treaty on February 6, 1920, Japanese government generated a 355 million yen budget surplus: the resources that were originally allotted to the construction of battleships and line cruisers could now be used to build new vessels under the “8+6” and “8+8” Fleet Completion Programs. Among the shipbuilding programs that had been given a green light were the construction of two scout cruisers (in the 7,500 ton class) – Kako and Furutaka, three first class destroyers, six submarines, one auxiliary ship, four transports and two aircraft carriers – Akagi and Amagi.
On March 10, 1922 Japanese Minister of the Navy, Tomosaburo Kato, announced the Naval Arms Limitation Program (Kaigun Gunbi Seigen Keikaku), which called for the commissioning of 6 large scout cruisers (Dai Junyokan): two 7,500 ton class vessels and four 10,000 ton ships. The program was approved at the 46th session of the Japanese parliament as the “New Shipbuilding Replenishment Plan in accordance with the Washington Treaty” (Taisho 12 Nendo Washington Joyaku Niyoru Kantei Seizo Shin Hojű Keikaku). It might be worthwhile at this point to take a look at some cruiser designs that were proposed between February 1922 and summer of 1923 when the final budget decisions were made. The 1922 budget (Gunkan Seizo Hi) covered the construction of two cruisers (Furutaka and Kako) in the 7,100 T class. Under the 1923 budget (Hojo Kantei Seizo Hi) two additional 7,100 T cruisers were authorized (Aoba and Kinugasa) in addition to four new Nachi type cruisers displacing 10,000 T.
Having secured the financial resources to build the vessels, the Imperial Navy proceeded with preparing the design requirements of the new ships. Constructor Captain Yozuru Hiraga and his assistant Constructor Lt. Cmdr. Kikuo Fujimoto were tasked with drawing the specifications of Aoba and Furutaka cruisers. In August 1921 Hiraga proposed the adoption of the plans for the experimental cruiser Yobari for the design and construction of the new 7,500 ton scout cruisers. The ships’ armament was to include six new 20 cm (8”) L/50 guns with maximum elevation of 40 degrees mounted in single turrets. The prototype of the gun had just been successfully tested at the Kure arsenal. The guns’ broadside weight of fire of 6x115.2 kg packed almost twice the punch of the U.S. Navy Omaha class cruisers. The guns would be arranged in a pyramid centerline layout forward and aft, where turrets No. 2 and 5 would be placed one level above the others. Anti-aircraft artillery was to include 4 8 cm L/40 guns placed amidships, near the smoke stacks. The units were also to be equipped with 61 cm torpedo tubes (in place of the standard 51 cm launchers) arranged in 6 twin batteries on the main deck. The armor protection of the deck and hull was to be made of NVNC steel plates varying in thickness from 35 to 75 mm. Anti-torpedo protection was based on the arrangement used on battleship Nagato. The cruisers were to be powered by four sets of Mitsubishi-Parsons turbines consisting of a high pressure turbine, low pressure turbine and a cruise turbine housed in four engine rooms separated by bulkheads. The total power output was expected to be 95,000 HP, which would translate into the top speed of 35 knots. The steam was delivered from 12 Kanpon “RO GÔ” boilers. Ten boilers were of oil-firing types, while two burned used mixed fuel types (oil or coal). The boilers were placed in seven boiler rooms. Boiler rooms No. 2, 3, 4 and 5 housed two boilers each, while boiler rooms No. 6 and 7 each contained a single boiler. Two mixed fuel boilers were to be installed in a separate boiler room No. 1 under the ship’s bridge. Three exhaust ducts directed the fumes into the smoke stacks. Two forward ducts were merged and fed the exhaust fumes into a single stack. The ships would carry a supply of 1,400 tons of oil and 450 tons of coal, which would give them an endurance of 7,000 nm at 14 knots. The preliminary design was approved by the Navy General Staff (Gunreibu) and the Ministry of the Navy in August 1921.