At the same time that the Fubuki-class destroyers were being mass-produced, large cruisers were being successively completed, conscious of the Washington Naval Treaty, which stipulated that vessels under 10.000 tons were not subject to restrictions. Among them was the Myōkō-class cruiser, later considered the precursor of the 10.000-ton heavy cruisers. The Myōkō-class was initially conceived as an enlarged and improved version of a planned 7.000-ton reconnaissance cruiser equipped with six 20 cm guns, a plan that never materialized. Unlike the cramped 7.000-ton reconnaissance cruiser, the Myōkō-class, designed for a 10.000-ton hull, featured a relatively reasonable design with ten 20 cm guns.
In fact, the original plan for the Myōkō-class included not only ten 20 cm guns but also numerous torpedo launchers, and even an ambitious proposal to achieve speeds of over 35 knots with an engine reaching 130.000 horsepower. However, this would have cramped the internal space with torpedo launchers and high-power engines, leading to decreased habitability and an inability to allocate sufficient space to fuel tanks, resulting in inadequate range. Concerned that such a design would compromise the cruiser's performance, the plan was ultimately abandoned. As a result, the Myōkō-class achieved a modest speed of 32 knots with 100.000 horsepower, typical for cruisers of that period. However, it also boasted a long range of over 10.000 nautical miles at 14 knots and good habitability.
As a side note, when the third ship, Ashigara, was dispatched to the United Kingdom as an invited ship for the coronation ceremony of King George VI, it received high praise: "Excellent habitability. The Imperial Japanese Navy understands that the condition of the crew greatly influences the combat capability of the ship. Truly, it is a leading civilization in Asia."
On the other hand, pleased with this recognition, Imperial Navy officials decided to focus on improving habitability and enhancing the welfare of the crew in the construction of future ships.
Despite its merits, the Myōkō-class harbored a hidden truth. It was designed from the beginning to address concerns that the United States and Britain might impose restrictions on auxiliary vessels under 10.000 tons in response to Japan's successive completion of excellent cruisers and destroyers. The design incorporated the ability to replace the 20 cm guns with 23 cm guns in the future, inspired by the plan to upgrade the Fusō-class battleships from 36 cm to 41 cm guns. Of course, upgrading the main guns would inevitably increase the displacement beyond 10.000 tons, but that was a matter for the future. In the not-too-distant future, many of Japan's heavy cruisers, including the Myōkō, would be saved by the adoption of 23 cm guns.
Memo
Myōkō-class (sister ships Nachi, Ashigara, Haguro, as well as sub-classes Takao, Atago, Maya, Chōkai)
- At the time of completion:
-- Overall length 200 m, beam 20 m
-- Standard displacement: 10.000 tons (notified value to each country. The actual figure is 12.000 tons)
-- Eight boilers, four shafts, 10.000 horsepower, 32 knots
-- Five twin 20 cm turrets
-- Four single 12 cm anti-aircraft guns
- After refit (conducted after the expiration of the disarmament treaties for each ship):
-- Overall length 200 m, beam 20 m
-- Standard displacement: 13.500 tons
-- Eight boilers, four shafts, 10.000 horsepower, 31 knots
-- Four twin 23 cm turrets
-- Six twin 12,7 cm anti-aircraft guns
-- Numerous 25 mm autocannons (varying for each ship)
At the time of completion, the Myōkō-class cruisers boasted exceptional firepower for auxiliary vessels under 10.000 tons, with the installation of 20 cm guns numbering up to ten. However, it was anticipated that other naval powers would follow suit and construct powerful 10.000-ton class vessels in response to the Myōkō-class. Therefore, the design was based on the assumption that in the future, these vessels would be refitted with 23 cm guns to ensure their ability to outgun such potential adversaries.
The 23 cm guns had a 40% increase in shell weight compared to the 20 cm guns, resulting in enhanced firepower. However, to accommodate the increased weight of the turrets and barrels, and to compensate for the added weight of anti-aircraft weaponry, one turret (the third turret) was reluctantly removed, reducing the total to four turrets with eight guns. Nevertheless, the 23 cm shells, weighing up to 170 kilograms, possessed ample penetration and destructive power against any cruiser in the world. Despite being a design from the Taishō era, the cruisers maintained first-class combat capabilities, thanks to the significant improvement in anti-aircraft capabilities.
. . .
Despite the global economic downturn, the successive enhancement of high-performance and distinctive auxiliary vessels by the Imperial Japanese Navy, such as the Fubuki-class destroyers and the Myōkō-class cruisers, greatly intrigued American and British naval and government officials. Keen on reducing military expenditures and prioritizing economic policies such as recession and unemployment measures, the US and Britain proposed disarmament conferences to curb Japan's moves in this direction. In 1930, a conference primarily aimed at limiting the number of auxiliary vessels, including cruisers, destroyers, and submarines, was convened in the United Kingdom. This would later be known as the "London Naval Treaty."