Yagumo Class Battleship (part 1, 1918-1930):
By the 1910s despite beign built up to a 35-unit mainline strength (2 BCs, 2 ACRs, 14 PCs and 18, soon to be 20, DDs) Koko no Kaigun suffered from a premature obsolescence of her large combatants as a consequense of the all-big-gun
concept initiated by HMS Dreadnought. Thus as early as of 1910 a first request for new naval construction was put forward to the Diet. The original request, calling for six new capital ships was ultimately cut down to two but eventually was approved by 1911.
Designs for Koko's first Dreadnought Battleship had already started in late 1907, ultimately producing the battlecruisers Aomegami and Kuromegami, but went on even after 1910: at first many concept were taken into account, rangin from a super-Kawachi design with superfiring turrets, to a modified Kongou Class with Armor upgraded to battleship standards at the expense of speed, and to a simple carbon-copy of the Fuso Class Battleship. Ultimately, the Kawachi design was discarded because it was too dated, and the Kongou and Fuso ones because Toumachi Naval Arsenal would have needed to expand its slipways to build vessels of that size.
By early 1913 designers started to focus thier efforts on a smaller version of the Fuso Class. A shorter hull housing only five, instead of six, 356mm turrets with comparable armour protection and and higher top speed. The two battleships were funded in July 1914, but the sudden outbreak of WWI and the repairs needed on aomegami, who hit a German mine off Tsingtao in late October 1914, delayed the start of their construction. The lead ship, to be called Yagumo, was laid down in January 1915 at Toumachi Naval Arsenal.
The plan called for a 188,6m long ship (14 less than a Fuso) with a beam of 27,2m (1,5m less), a draft of 8,9m and an assumed displacement of 24.450t at standard load. the ten 356mm/45 guns would have been argumented by sixteen casemate 152mm/50 guns, six 76mm/40 A-guns and four 533mm torpedo tubes. Armor values were to be the same as the IJN fuso. Assumed top speed was expected to be at least 23,5 knots.
The original design lasted a bit more than a year, the Battle of Jutland, the loss of Kuromegami against the Thiarians, and the modifications already implemented by the IJN to the third and fourth Fuso class units, that became the Ise class, led Koko designers to heavily revise the plans for the Yagumo class despite the lead ship was already under construction. When launched, in 1917, and ultmately commissioned in december 1918 Battleship Yagumo bore little resemblance to the ship that was originally ordered four years before.
While overall dimensions had remained the same, as it was for the 10 356mm guns making up for the main armament, mostly everything else had ben modified:
The planned sixteen casemate 152mm/50 guns were instead replaced by sixteen 140mm/50 ones, main belt was made of an uniform thickness of 305mm and an upper belt of 102mm protecting the casemat decks was added. An internal torpedo bulkhead 25mm thick was added and deck armor increased from 51 to 85mm. Engine compartment was completely revised for even higher speeds, Kampon boilers fueled by 68% of oil and 32% of coal powered Brown-Curtis steam turbines generating 63.581 shp that propelled the ship on a 25knots top speed during her sea trails. Range totaled 8.800 nautical miles at 14knots. Yagumo displaced 25.255t at standard load and 29.596t full, her draft had increased to 9,1m over her planned 8,9. torpedo tubes were deleted during construction in order to balance the increase in weight caused by the extra armor and the new engines.
Upon commission Yagumo immediately replaced Aomegami as Koko no Kaigun's Flagship.
The second ship of the class, Inaba, Was laid down at Toumachi Naval Arsenal in 1917, as soon as Yagumo was launched. Construction lasted until her launch in 1920, and she was commissioned in early 1921. Compared to her sister Yagumo, Inaba had the deck armor thickness further increased to 100mm, and she displaced 25.483t standard and 29.415t at full load, Her appearance was also different to address issues emrged during Yagumo's service. Larger flag platforms were added on the fore and aft supestructures, and a donkey boiler room added between the funnels. As smoke interference issues had been experienced on Yagumo, the foremast platforms and funnels were modified accordingly: a cap was added on the forward funnel, the searchlight platforms were extended forward and both the 110cm searchlight and thee 60cm signaling searchlight relocated farther from the funnel. Enclosed lookout stations were added and the foremast was shortened, while the mainmast heightened. The spotting top was expanded. Both ships had a crew of 1.072 officers and men.
Inaba replaced Yagumo for Flagship roles during 1921, then both Battleships attended celebrations for the newly signed Kokoan-Japanese Alliance as part of the Bilateral agreements of 1921 and easily survived the Washington Naval Treaty. In september 1923 they departed for Japan to provide support and relief after the great Kanto earthquake had devastated Tokyo and Yokohama.
In late 1925 Inaba was docked at Hoshiguma Navy Yard for an upgrad of her fire control system. During the works, all the 6-meter rangefinders on the main guns were replaced with 8-m ones, and the maximum elevation increased from 25 to 33°. The aft bridge recieved a 2,8m rangefinder and a fire control platform was added on the tripod foremast, requiring the relocation of the 90 and 60cm searchlights. Finally the casemate guns recieved canvas bags.
Yagumo was supposed to recieve a similar modernization, but all the shipyards were full with new Destroyers construction, The newer Battlecruiser Amagi had just entered service in 1926 and after the Geneva conference in 1927 Koko was allowed to scrap the Goryo class battlecruisers to build newer Battleships. Yagumo's refit was thus postponed to 1930/31: therfore it was also planned to carry on a larger and much more comprehensive refit on her. But the ratification of the London Naval Treaty in October 1930 forced Koko no Kaigun to a change of plans.
The history of the Yagumos will be continued someday. For now I want to complete the entire fleet up to the Washington Naval Treaty or shortly after, this means another DD class, a bunch of CLs and something else.