Mogami-class cruiser

The Mogami-class (最上型) were a class of four warships built initially for the Imperial Japanese Navy (IJN) as light cruisers in the early 1930s under the weight and armament restrictions of the London Naval Treaty. After Japan refused to comply any longer with that agreement, all four ships were rearmed with larger caliber main armament and were reclassified as heavy cruisers. All four fought in World War II, and were sunk.

The Mogamis have been seen by naval architects as a design failure. The IJN's Naval staff insisted that each new class be superior to anything else in its category, yet designers strove to stay in compliance with treaty regulations. As a result, the initial construction of these ships was overly light; within their first few years of service, all four had to be reconstructed to remain seaworthy. They were also unstable seaboats due to excessive topweight and their welded seams cracked under the stress of firing their own main guns.

Design
For the 1931 Fleet Replenishment Program, believing themselves understrength in cruisers, the IJN chose to build to the maximum allowed by the Washington Naval Treaty. This resulted in the choice of 155 mm guns in five triple turrets (a first for Japan) in the Mogamis, also capable of 55° elevation, making the Mogamis one of the very few classes of cruiser to have a dual purpose (DP) main battery; this was coupled with very heavy anti-aircraft protection, as well as the standard reloadable, turreted torpedo tubes, also unique to the IJN.

To save weight and improve transverse stability, the class was given a more compact and lower superstructure, electric welding was used, as was aluminium in the superstructure. Aiming to meet the weight limits compelled them to fit only ten boilers (compared to twelve in the previous Takao and Myoko classes), trunked into a single funnel stack (which also saved tophamper). The new impulse geared turbines added 22,000 shp over Atago, increasing the top speed by 1.5 knots (2.8 km/h). Protection, however, was not stinted on; the class proved able to take substantial punishment.

The declared weight was 8,500 tons, though the true design weight was 9,500 and at trials they would displace 11,169 tons.

The designers, however, had overreached; excessive topweight led to instability, and gunnery trials revealed cracking hull welds. Hull bulges were retrofitted to Mogami and Mikuma, and added to Kumano and Suzuya, increasing beam to 20.5 m (67 ft) and displacement to 11,200 tons, cutting speed by 2 kt (3.7 km/h).

Following Japan's withdrawal from the Second London Naval Treaty, plans were made to modernize and expand the entire fleet. Beginning in 1939, the class was brought in for substantial reconstruction, replacing the triple 155 mm turrets with twin 203 mm (8-inch) guns, turning over the 155 mm turrets for the battleship Yamato. Indeed, the designers had designed the class in mind so that the 6-inch guns could be switched with 8-inch batteries, in effect making them heavy cruisers and skirting the London Naval Treaty, though the Japanese had withdrawn from the conference and were not signatories to the Second London Naval Treaty of 1936.

Torpedo bulges were also added; in all, displacement rose to over 13,000 tons, and speed dropped to 34.5 kt (63.8 km/h).

The United States Navy's Brooklyn-class cruisers were designed specifically to counter the Mogami-class, and as a result had a very similar armament to the pre-refit Mogamis, in a nearly identical layout, though the US-pattern 6"/47 weapon was semi-automatic, with a higher rate of fire and the three weapons in each turret mounted in a single sleeve. Japan's choice of the 155mm gun caliber is curious, as Japan already had a 6" (152mm) weapon in service, of nearly equal performance. In spite of the resulting multiplicity of similar gun calibers, Japan resented the 5-5-3 treaty ratios, and had vowed to build to the very limit allowed by the 1922 Washington and 1930 London Naval Treaties. As the French had already used a 155mm main battery in the three Duguay-Trouin-class cruisers (1922–1926), this became the largest gun caliber allowed for light cruisers under the 1930 London Naval Treaty.

War service
All four ships participated in the Japanese invasion of the Dutch East Indies. The Mogami and Mikuma were present at the Battle of Sunda Strait and contributed to the sinkings of HMAS Perth and USS Houston.

In June 1942, all four took part in the Battle of Midway, where Mogami and Mikuma collided trying to avoid a submarine attack; Mikuma was finished off on 6 June 1942 by aircraft from USS Enterprise (CV-6) and USS Hornet (CV-8). The heavily damaged Mogami limped home and spent ten months in yard, during which her afterparts were completely rebuilt, and "X" and "Y" turrets were replaced by a flight deck (with the intention to operate 11 aircraft).

In October 1944, the survivors were reunited at the Battle of Leyte Gulf. Mogami, heavily damaged by a collision with Nachi, cruiser gunfire, and aerial attack was scuttled by Akebono, while Kumano stumbled into Manila harbor on one boiler, to be put out of her misery by Halsey's aviators on 25 November 1944; the US escort carrier planes mauled Suzuya at Leyte, and she was scuttled by Okinami on 25 October.

Criticism
This class is seen by naval architects as trying to fit a quart into a pint pot. The IJN's Naval staff insisted that each new class be superior to anything else in its category, this placed an enormous burden on Japanese naval constructors and the difficulties with these ships have to be seen in this light.

The initial construction was extremely light in order to comply with the naval treaties and had to be remedied. When the Royal Navy's Director of Naval Construction (DNC) was told about these ships, by British Naval Intelligence quoting the public displacement figure, he replied that the capabilities quoted could not be achieved on this displacement and that "they must be building their ships out of cardboard or lying".

Though the placement of Turret #3 improved its firing arc, and though the class had the stability problems fixed (the preceding Takao-class cruisers were considered too top-heavy), the Mogamis are generally not considered an improvement over the Takaos. Nonetheless, the follow-up Tone-class retained many aspects of the Mogami-class design. However, the Tones were intended for a different purpose with all of their main armament forward, so their stern could accommodate extra floatplanes.