mm on the upper armoured deck. These additions increased the weight of the ship's armour to , 32.6 percent of her displacement. In early 1941, in preparation for war, Mutsus barbette armour was reinforced with armour plates above the main deck and plates below it. Aircraft Mutsu had an additional boom added to the mainmast in 1926 to handle the Yokosuka E1Y floatplane recently assigned to the ship. In 1933 a catapult was fitted between the mainmast and Turret No. 3, and a collapsible crane was installed in a port-side sponson the following year; the ship was equipped to operate two or three seaplanes, although no hangar was provided. The ship was operating Nakajima E4N2 biplanes until they were replaced by Nakajima E8N2 biplanes in 1938. A more powerful catapult was installed in November 1938 to handle heavier aircraft like the single Kawanishi E7K, added in 1939–40. Mitsubishi F1M biplanes replaced the E8Ns on 11 February 1943. Fire control and sensors The ship was fitted with a rangefinder in the forward superstructure. Additional and anti-aircraft rangefinders were also fitted, although the date is unknown. The rangefinders in No. 2 and 3 turrets were replaced by 10-metre units in 1932–33. Mutsu was initially fitted with a Type 13 fire-control system derived from Vickers equipment received during World War I, but this was replaced by an improved Type 14 system around 1925. It controlled the main and secondary guns; no provision was made for anti-aircraft fire until the Type 31 fire-control director was introduced in 1932. A modified Type 14 fire-control system was tested aboard her sister ship in 1935 and later approved for service as the Type 94. A new anti-aircraft director, also called the Type 94, used to control the 127 mm AA guns, was introduced in 1937, although when Mutsu received hers is unknown. The 25 mm AA guns were controlled by a Type 95 director that was also introduced in 1937. Construction and service Mutsu, named for Mutsu Province, was laid down at the Yokosuka Naval Arsenal on 1 June 1918 and launched on 31 May 1920. Funding for the ship had partly come from donations from schoolchildren. While Mutsu was still fitting out, the American government called a conference in Washington, D.C. late in 1921 to forestall the expensive naval arms race that was developing between the United States, the United Kingdom and the Empire of Japan.