IJN Battleship Design A-125
Originally the Nagato class of the IJN was only considered as a single ship class consisting of only Nagato as the Diet of Japan 1916 did not vote for a new pair of battleships only a single one which became Nagato and two Battlecruisers which later became IJN Amagi and IJN Akagi. But later that year, at the end of the summer, the then American president Thomas Woodrow Wilson announced the 1916 Naval Act or "Big Navy Act" which meant the USN had to aquire 10 new battleships and 6 new Battlecruisers (4 Colorado, 6 South Dakota and 6 Lexington class capital ships) among other vessels, the parliament of Japan had no choice but to vote for 3 new battleships which became IJN Mutsu, IJN Kaga and IJN Tosa.
This pause allowed the designers to improve the Nagato class while remaining in the same tonnage level as that ship and by June 1917 a modified design was born with 5 turrets arranged as the later Tosa/Kaga class yet on a smaller hull then those of the later class.
There must had been debates of to what design Mutsu's construction must be based on, a repeat Nagato, an improved Nagato with 5 turrets similar to the Tosa/Kaga class then under design process or as a 3rd sister of that new class. Finally the choice was set on a repeat Nagato as it seems because it would reduce costs of the overall new construction programme (1+1 Nagato and 2 Tosa VS 1 Nagato and 3 Tosa) and also that this way the Nagatos could form a heterogeneous squadron.
This and the fact there were lack of money to order the ship led to Mutsu being laid down a year later then Nagato in 1918 June despite the Yokosuka Naval Arsenal's Shipyard was free at that time. (Yamashiro was launched in 1915)
As you can see the chosen design for the 5 turret Nagato was much different from Nagato herself, the hull was more streamlined flush deck, no risen forecastle but rather a flared bow eg the deck was constantly rising in height from the centre of the ship to the point of the Nose. Improvements in Engine technology allowed the same amount of shp to be mounted in a reduced space giving room to the 5th turret and trunking the boiler uptakes into a single funnel, though the casemated 1st and 2nd deck had to be reworked and the boat stowage became somewhat cramped as a result.
The design had the following characteristics:
Dimensions: 201,35 (pp) x 213,3 (wl) x 215,8 (oa) x 32,76 x 9,0 meters
Displacement: 33.800tons (standard)
Armour: 305mm Belt inclined to 25 degrees, 70mm Deck, 76mm Slopes
Engines: 80.000shp Gihon Steam Turbines, 4 shafts
Range: 10.200km at 30km/h (5.500nm at 16knots)
Speed: 49km/h (26,5knots)
Armaments:
5x2 410mm/45 Type 3 / 3rd Year Type Cannons,
16x1 140mm/50 Type 3 / 3rd Year Type Guns,
4x1 76mm/40 Type 3 / 3rd Year Type AA Guns,
4x1 53cm Above Water Torpedo Tubes
Official drawing:
https://iiif.dl.itc.u-tokyo.ac.jp/repo/s/hiraga/document/c052c841-0ddd-4dd8-89ed-70c0ba502949#?c=0&m=0&s=0&cv=0&xywh=-659%2C0%2C8261%2C4927
Data table with comparison of Nagato:
https://iiif.dl.itc.u-tokyo.ac.jp/repo/s/hiraga/document/3c0b3daa-7ded-4adc-9a6c-b9b4cdf28673#?c=0&m=0&s=0&cv=0&xywh=1432%2C569%2C2561%2C1528
https://iiif.dl.itc.u-tokyo.ac.jp/repo/s/hiraga/document/debc356c-0ffe-468c-b6cf-2babd310b651#?c=0&m=0&s=0&cv=0&xywh=1111%2C740%2C3074%2C1833
Various calculations:
https://iiif.dl.itc.u-tokyo.ac.jp/repo/s/hiraga/document/9f786591-af6d-4959-ac62-7f4a65523558#?c=0&m=0&s=0&cv=0&xywh=-659%2C0%2C8261%2C4927
