To save weight, 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 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 19.2 m (63 ft) and displacement to 11,200 tons, cutting speed by 2 kt (3.7 km/h).
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. Torpedo bulges were also added; in all, displacement rose to over 13000 tons, and speed dropped to 34.5 kt (63.8 km/h).
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; they mauled Suzuya the same day, and she was scuttled by Okinami on 27 November.
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 Mogami's are generally not considered an improvement over the Takao's Nonetheless, the follow-up Tone-class retained many aspects of the Mogami-class design. However, the Tone's were intended for a different purpose with all of their main armament forward, so their stern could accommodate extra floatplanes.
|Mogami||Kure Navy Yard||October 1931||March 1934||July 1935|
|Mikuma||Mitsubishi, Nagasaki||December 1931||May 1934||August 1935|
|Suzuya||Yokosuka Navy Yard||December 1933||November 1934||October 1937|
|Kumano||Kawasaki, Kobe||April 1934||October 1936||October 1937|