By TZoli

Part I: The British Ancestry

The design history of the Japanese Nagato class battleships, the world’s first and then fastest 16″/410mm gun armed battleships is long and only partially documented. The commonly held statement that the Nagato class is the first indeginiously designed Japanese Battleship or the first Battleship entirely designed by Yuzuru Hiraga is inaccurate. Before the first known Japanese design (A-102) which could be connected to the Nagato class’ development in 1915/16 was born, the famous British ship building firm Vickers offered four export proposals to the Imperial Japanese Navy in 1913, all armed with 16″ guns: Designs 646,646A,647 and 647A. Designs 646 and 647 sported 16″/40 while the “A” sub variants were equipped with 16″/44. Norman Friedman briefly mentions these offers with some info on dimensions and armament: 4 twin 16″ turrets, 16x 6″ secondary guns, 8 submerged torpedo tubes and other guns as on the Kongo (eg. 16x 3″ guns). Other noteworthy feature of these proposals is the relatively high speed, outmatching any battleship of the time and matching the first generation battlecruisers in this regard.
It is very likely that these proposals heavily influenced the creation of the Nagato class battleships, both in size, speed and armament.

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Vickers Design 646

Norman Friedman mentions and provides some data for this proposal in his book: The British Battleship 1906-1946. As a basis for the drawing I’ve chosen a modified and lengthened Queen Elizabeth class battleship with some alterations on the bridge and mainmasts as well as of course 16″ armed turrets.

The design had the following characteristics:
Dimensions: 184,4 (pp) x 28,65 x 8,7 meters
Displacement: 29.000tons (standard)
Armour: 305mm Belt
Engines: Unknown, likely 68.000shp
Speed: Unknown, likely 44-46km/h (24-25knots)
Armaments:
4×2 406mm/40 (16″/40) Vickers Mark A Cannons,
16×1 152mm/50 (6″/50) Vickers Mark S Guns,
16×1 76mm/40 (3″/40) Vickers Mark N Guns,
8×1 533mm Underwater Torpedo Tubes

Vickers Design 646A

Design 646A was a slight modification of the baseline design 646 using 406mm/44 cannons (16″/44) rather the shorter /40 calibre one. This difference in main weaponry associated together with a somewhat different turret carrying them resulted in a net increase of displacement by 800tons to 29.800tons. In order to differentiate the looks of this longer barrelled Design 646 I’ve repositioned the mainmast, moving it just in front of the 3rd turret as well as changing the main crane’s facing and modelled the turrets after the one used on HMS Hood (Mark II). Previously I’ve used a somewhat enlarged turret of the Queen Elizabeth class (Mark I) turrets and arranged the 3″ Guns placement differently putting more on the forecastle deck.

The design had the following characteristics:
Dimensions: 184,4 (pp) x 28,65 x 8,7 meters
Displacement: 29.800tons (standard)
Armour: 305mm Belt
Engines: Unknown, likely 69.000shp
Speed: Unknown, likely 44-46km/h (24-25knots)
Armaments:
4×2 406mm/44 (16″/44) Vickers Mark A Cannons,
16×1 152mm/50 (6″/50) Vickers Mark S Guns,
16×1 76mm/40 (3″/40) Vickers Mark N Guns,
8×1 533mm Underwater Torpedo Tubes

Vickers Design 647

After the 646/646A designs that Vickers offered for Japan another pair was offered, this time with an added length of 4,5 meters (and a net increase of 500tons) likely to better achieve higher speeds, otherwise it has the same armament and armour as 646.

The design had the following characteristics:
Dimensions: 188,98 (pp) x 28,65 x 8,7 meters
Displacement: 29.500tons (standard)
Armour: 305mm Belt
Engines: 68.5000shp
Speed: 48km/h (26knots)
Armaments:
4×2 406mm/40 (16″/40) Vickers Mark A Cannons,
16×1 152mm/50 (6″/50) Vickers Mark S Guns,
16×1 76mm/40 (3″/40) Vickers Mark N Guns,
8×1 533mm Underwater Torpedo Tubes

Vickers Design 647A

Like the previously mentioned Design 646A was a slightly modified version of 646, the same applies to design 647A, a modification of the baseline 647 variant.
The changes are: longer calibre /44 cannons rather /40 ones, extra 1.250tons of displacement and 1.500shp more power to offset this extra tonnage.

The design had the following characteristics:
Dimensions: 188,98 (pp) x 28,65 x 8,7 meters
Displacement: 30.750tons (standard)
Armour: 305mm Belt
Engines: 70.0000shp
Speed: 48km/h (26knots)
Armaments:
4×2 406mm/44 (16″/44) Vickers Mark A Cannons,
16×1 152mm/50 (6″/50) Vickers Mark S Guns,
16×1 76mm/40 (3″/40) Vickers Mark N Guns,
8×1 533mm Underwater Torpedo Tubes

---

Part II: The Japanese Designs

As mentioned earlier the design history of the Nagato class is quite difficult to research, mostly because of missing material and the language barrier. While the language barrier could be overcome by translators and naval enthusiasts willing to translate Japanese texts, missing material is a major obstacle. It is known that the design number of the battleship class preceding the Nagatos, the Ise class was A-92, while Nagato’s design number was A-114, which means we have a missing set of 21 battleship designs not accepted for construction. Research in the Hiraga archive resulted in two design which had such numbers: A-110 and A-112 both of which had drawings.

Design A-102

First mention of the design A-102 was from an online source which describes this as the first known Nagato preliminary and which was similar to the “British Design”.
Knowing that Vickers offered designs to the IJN in 1913 the Ise class started construction in 1915 I’ve created this drawing showing elements of both:
152mm secondary guns and similar arrangement as well, the first deck level and the aft supersturcture reminiscent to that of the Queen Elizabeth Class, hull shape and tripod bridge structure following that of the Ise class and finally of course the armament of 4 twin 41cm turrets that of the Nagato class.

The design had the following characteristics:
Dimensions: Unknown
Displacement: Unknown
Armour: Unknown, likely 305mm Belt 25mm Deck and 51-76mm Slopes
Engines: Unknown, likely 70-75.000shp
Speed: Unknown, likely 45-48km/h (24-26knots)
Armaments:
4×2 410mm/45 3rd Year Type Cannons,
16×1 152mm/50 43rd Year Type Guns,
4×1 76mm/40 3rd Year Type DP-AA Guns,
8×1 533mm Underwater Torpedo Tubes

Design A-110
The first Nagato preliminary design which I’ve found drawings of (at first partial then a complete one) in the Yuzuru Hiraga archive was design A-110. This preliminary proposal seems to be the first one designed by Yuzuru Hiraga showing his characterisitic pagoda style mainmast albeit only 6 and closely placed legs without a central elevator shaft. Japanese sources describe the bridge structure as very similar that of the Ise type that is a large circular base with likely similar or round bridge on top of it. At least one pair of 76mm AA guns were placed on the aft superstructure at a rather high level and good firing arcs, though close to the blast area of the 3rd turret when firing extreme forward. The tripod aft mast style meant that the boat crane either located on both of it’s legs, on either side of the funnels or as I’ve depicted, on the hexapod mast forward. The Armour layout shows that the design was still pre-Jutland as evident of the thin 28mm deck armour, almost full hull length belt and deck armour, lacking only at the extreme ends ie the nose and stern but includes the steering room.

Original Hiraga documents:

平賀譲デジタルアーカイブ · 軍艦長門防禦装置後部改正案 · 東京大学学術資産等アーカイブズ共用サーバ

平賀譲デジタルアーカイブ · 一般艤装 軍艦長門後部改正案 二枚ノ内其一 · 東京大学学術資産等アーカイブズ共用サーバ

平賀譲デジタルアーカイブ · 一般艤装 軍艦長門後部改正案 二枚ノ内其二 · 東京大学学術資産等アーカイブズ共用サーバ

平賀譲デジタルアーカイブ · 軍艦長門大体計画改正案 · 東京大学学術資産等アーカイブズ共用サーバ

The design had the following characteristics:
Dimensions: 201,17 (pp) x 28,95 x 9,14m
Displacement: 32.500tons (standard)
Armour: 305mm Belt 28mm Deck and 70mm Slopes
Engines: 60.000shp, 4-shafts
Speed: 45km/h (24knots)
Armaments:
4×2 410mm/45 3rd Year Type Cannons,
20×1 140mm/50 3rd Year Type Guns,
4×1 76mm/40 3rd Year Type DP-AA Guns,
4×1 533mm Underwater and 4×1 Above Water Torpedo Tubes

Yuzuru Hiraga’s Preliminary-Designs

The next four designs associated with the Nagato class design historyare labelled as A,B,C and D and they show what possible changes are required after examining the After Action Reports of the Battle of Jutland 1916 May 31.
Preliminary-Design A
The first version, Design A the maximum speed was increased to 27knots over the A-110’s 24knots but in order to keep costs down the and to be able to reuse most of the hull design, engine power was boosted by 33% from 60.000shp to 80.000shp and the sloped armour thickness reduced somewhat from 70mm to 51mm over machinery spaces. The savings in armour weight resulted in only a marginal 250ton increase in displacement despite the larger engine power. This first attempt shows the importance of high speed in a battle both chasing down or escaping from the enemy.

Original Hiraga documents:
Armour scheme:

平賀譲デジタルアーカイブ · 軍艦長門改正案比較 A. 327500TONS 27KNOTS 80,000S.H..P C. 33,800TONS 26.8KNOTS 80,000S.H..P · 東京大学学術資産等アーカイブズ共用サーバ

Data tables:

平賀譲デジタルアーカイブ · Nagato改正案 大正5.8.8 · 東京大学学術資産等アーカイブズ共用サーバ

The design had the following characteristics:
Dimensions: Unknown
Displacement: 32.750tons (standard)
Armour: 305mm Belt, 28mm Deck and 51mm at Machinery, 70mm at Magazines Slopes
Engines: 80.000shp, 4-shafts
Speed: 50km/h (27knots)
Armaments:
4×2 410mm/45 3rd Year Type Cannons,
20×1 140mm/50 3rd Year Type Guns,
4×1 76mm/40 3rd Year Type DP-AA Guns,
4×1 533mm Underwater and 4×1 Above Water Torpedo Tubes

Preliminary-Design B

While Pre-Design A increased maximum speed of the A-110 study with reducing sloped armour thickness and likely overall armoured area to gain tonnage for larger engines, Pre-Desing B increased main deck armour from 28mm to 70mm while the displacement remained the same compared to A-110. This was likely achieved by reducing the armoured area of the ship notably abandoning the classical fully armoured hull and implementing the USN style all or nothing or at least partial all or nothing armour scheme where only the vital or vital plus some extra lengths were armoured and the rest of the ship ie the ends (everything forward of the 1st and aft of the 4th turrets) were unarmoured. Note that these armour thickness values were very close to the final Nagato design as built: 70mm Deck and 76mm Slopes.

Original Hiraga documents:
Armour scheme:

平賀譲デジタルアーカイブ · B. 32,500TONS 25KNOTS 60,000S.H..P D. 33,300TONS 26.9KNOTS 80,000S.H..P · 東京大学学術資産等アーカイブズ共用サーバ

平賀譲デジタルアーカイブ · B, C.及D 案 FORWARD SECTION AFT SECTION · 東京大学学術資産等アーカイブズ共用サーバ

Data tables:

平賀譲デジタルアーカイブ · Nagato改正案 大正5.8.8 · 東京大学学術資産等アーカイブズ共用サーバ

 

The design had the following characteristics:
Dimensions: Unknown
Displacement: 32.500tons (standard)
Armour: 305mm Belt, 70mm Deck and 70mm Slopes
Engines: 60.000shp, 4-shafts
Speed: 46km/h (24knots)
Armaments:
4×2 410mm/45 3rd Year Type Cannons,
20×1 140mm/50 3rd Year Type Guns,
4×1 76mm/40 3rd Year Type DP-AA Guns,
4×1 533mm Underwater and 4×1 Above Water Torpedo Tubes

Preliminary-Design C

Pre-Design Variant C uses both aspects of the previous two proposals: increased engine power for higher top speed (Pre-Design A) and increased deck armour thickness to 70mm (Pre-Design B) to create a post-Jutland fast battleship. Of course these changes over the base A-110 design resulted in a whopping 1.300tons of extra displacement and slightly more than 1.000tons over the “A” proposal. As you can see on the original drawing the armour belt and deck now don’t entirely cover the hull, and the ends have a rather light armour protection only, mostly against destroyer caliber shells and near misses to minimize hull displacement growth. It is more then likely that this Pre-Design C variant was chosen for further development as overall it got very close to the Nagato as finished though there were still some differences: Partial All-or-Nothing armour scheme, above water torpedo launchers separated by a casemate, small base for the hexapod pagoda mast, and single pole mainmast aft.

Original Hiraga documents:
Armour scheme:

平賀譲デジタルアーカイブ · B. 32,500TONS 25KNOTS 60,000S.H..P D. 33,300TONS 26.9KNOTS 80,000S.H..P · 東京大学学術資産等アーカイブズ共用サーバ

平賀譲デジタルアーカイブ · B, C.及D 案 FORWARD SECTION AFT SECTION · 東京大学学術資産等アーカイブズ共用サーバ

Data tables:

平賀譲デジタルアーカイブ · Nagato改正案 大正5.8.8 · 東京大学学術資産等アーカイブズ共用サーバ

Drawing:

平賀譲デジタルアーカイブ · 軍艦長門改正案(C) · 東京大学学術資産等アーカイブズ共用サーバ

The design had the following characteristics:
Dimensions: 201,17 (pp) x 28,95 x 9,14m
Displacement: 33.800tons (standard)
Armour: 305mm Belt, 70mm Deck and 70mm Slopes
Engines: 80.000shp, 4-shafts
Speed: 50km/h (26,8-27knots)
Armaments:
4×2 410mm/45 3rd Year Type Cannons,
20×1 140mm/50 3rd Year Type Guns,
4×1 76mm/40 3rd Year Type DP-AA Guns,
4×1 533mm Underwater and 4×1 Above Water Torpedo Tubes

Pre-Design D

The last of the Pre-designs, variant D is somewhat of a modified A proposal. The engine power of 80.000shp and thus a speed of 50km/ or 27knots remained but main deck armour thickness at 51mm is halfway between the 28mm of A-110 and Pre-Design A and the 70mm of Pre-Designs B. This resulted in a slight reduction of displacement by 200tons over version C. This mere 200ton saving over the better protected variant C seemed not worth the reduction in the defensive capabilities. None the less it shows a steady evolution of the Nagato class and how various proposals tried to solve the problems arisen after the Battle of Jutland.

Original Hiraga documents:
Armour scheme:

平賀譲デジタルアーカイブ · B. 32,500TONS 25KNOTS 60,000S.H..P D. 33,300TONS 26.9KNOTS 80,000S.H..P · 東京大学学術資産等アーカイブズ共用サーバ 

平賀譲デジタルアーカイブ · B, C.及D 案 FORWARD SECTION AFT SECTION · 東京大学学術資産等アーカイブズ共用サーバ

Data tables:

平賀譲デジタルアーカイブ · Nagato改正案 大正5.8.8 · 東京大学学術資産等アーカイブズ共用サーバ

The design had the following characteristics:
Dimensions: 201,17 (pp) x 28,95 x 9,14m
Displacement: 33.300tons (standard)
Armour: 305mm Belt, 51mm Deck and 70mm Slopes
Engines: 80.000shp, 4-shafts
Speed: 50km/h (26,8-27knots)
Armaments:
4×2 410mm/45 3rd Year Type Cannons,
20×1 140mm/50 3rd Year Type Guns,
4×1 76mm/40 3rd Year Type DP-AA Guns,
4×1 533mm Underwater and 4×1 Above Water Torpedo Tubes

Design A-112

The next step in the development leading to the Nagato class battleships was Design A-112, an improved and modified Pre-Design C.
The changes included:
– complete all or nothing armour scheme, the protected area only extends towards the ends of the 1st and 4 main turrets
– the saved armour weight went into thickening the slopes from 70mm to 76mm
– adding a casemate deck behind the 2nd turret

The changes introduced on this design remained for the rest of the development of the class. Notably the Hiraga style close legged hexapod pagoda mast; the casemated deck which improved fighting capabilities (due to the the forwardmost casemates on design C expected to be very wet); and the all or nothing armour scheme.

Original Hiraga documents:

平賀譲デジタルアーカイブ · 軍艦長門改正図 · 東京大学学術資産等アーカイブズ共用サーバ

The design had the following characteristics:
Dimensions: 201,17 (pp) x 28,95 x 9,14m
Displacement: 33.300tons (standard)
Armour: 305mm Belt, 51mm Deck and 76mm Slopes
Engines: 80.000shp, 4-shafts
Speed: 49km/h (26,5knots)
Armaments:
4×2 410mm/45 3rd Year Type Cannons,
20×1 140mm/50 3rd Year Type Guns,
4×1 76mm/40 3rd Year Type DP-AA Guns,
4×1 533mm Underwater and 4×1 Above Water Torpedo Tubes

Design A-113

This is another drawing I’ve found in the Hiraga archive connected to the Nagato class development but no design number associated nor any other description apart from a hull stress diagram and side view. The hull sides ie the freeboard height is somewhat higher then on the previous versions indicationg more thought went into seaworthiness and to reduce water spray on the deck and it is actually very close to the final hull. The aft superstructure deck just behind the 3rd turret also shows a close resemblance to the final version. The aft turrets were moved somewhat more forward reducing the armoured area, while there seems to be armour plates at the base of the forward funnel and the angle of the pagoda masts legs indicates higher platform height which might be the reason of saving weight by reducing the armour length.

Original Hiraga documents:

平賀譲デジタルアーカイブ · B.S.”NAGATO” CURVES OF BENDING AND SHEARING STRESSES. · 東京大学学術資産等アーカイブズ共用サーバ

The design had the following characteristics:
Dimensions: 201,17 (pp) x 28,95 x 9,14m
Displacement: 33.300tons (standard)
Armour: 305mm Belt, 70mm Deck and 76mm Slopes
Engines: 80.000shp, 4-shafts
Speed: 49km/h (26,5knots)
Armaments:
4×2 410mm/45 3rd Year Type Cannons,
20×1 140mm/50 3rd Year Type Guns,
4×1 76mm/40 3rd Year Type DP-AA Guns,
4×1 533mm Underwater and 4×1 Above Water Torpedo Tubes

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Part III: Final Design, Proposals, Refits and Modernizations

IJN Nagato As laid Down

IJN Nagato was laid down on the 28th Aug. 1917 with the original Hiraga hexapod pagoda mast bridge and closely placed mastlegs.
While the previous proposal which I’ve labelled A-113, still had many common elements of the final Nagato some changes still occured on the design:
The hexapod pagoda mast of shorter height (as used on the Pre Designs) was retained, the casemate or forecastle deck was lengthened, and the aft superstructure elements around the tripod mast were simplified.
Japanese sources describe that the heptapod (6 legs and one larger central shaft) pagoda mast was adopted later for the Nagato when it was still under construction hence the use of hexapod here.

The design had the following characteristics:
Dimensions: 201,17 (pp) x 213,36 (wl) x 215,8 (oa) x 29 x 9,10m
Displacement: 33.300tons (standard), 38.500tons (full load)
Armour: 305mm Belt, 70mm Deck and 76mm Slopes
Engines: 80.000shp, 4-shafts
Range: 10.200km at 30km/h (5.500nm at 16knots)
Speed: 49km/h (26,5knots)
Armaments:
4×2 410mm/45 3rd Year Type Cannons,
20×1 140mm/50 3rd Year Type Guns,
4×1 76mm/40 3rd Year Type DP-AA Guns,
4×1 533mm Underwater and 4×1 Above Water Torpedo Tubes

Hidetaro Kaneda’s Type A Bridge

Shortly after the ship was laid down in October 1917, Captain Hidetaro Kaneda proposed a plan to modify the bridge superstructure of the ship. This new firecontrol foremast included 9 legs in total, 1 larger central shaft including an elevator and 8 smaller symmetrically placed legs around it in a widely spaced manner basically an enneapod pagoda mast. The wide spacing allowed stability for the large platforms he envisioned for this proposlas and which includes:
– AA Platform for 8x 76mm AA guns
– Secondary gun and searchlight platform though the drawing seems to show light calibre guns either short barrelled 76mm, 57 or 47mm guns
– Searchlight platform for 8 searchlights
– Main 10m Rangefinder platform
– Firing command platform
– Director command platform

Hiraga did not like this kind of pagoda mast because he feared excessive vibrations at the rangefinder level as well as top weight issues from the larger number of guns at higher level. But the Admiralty liked the idea and somewhat later Kaneda proposed a different “B” type bridge with an oval shaped conning tower, sloped heavy armour and removed AA gun platform.

Original Hiraga documents:

平賀譲デジタルアーカイブ · 〔櫓檣改正案検討資料〕 · 東京大学学術資産等アーカイブズ共用サーバ

The design had the following characteristics:
Dimensions: 201,17 (pp) x 213,36 (wl) x 215,8 (oa) x 29 x 9,10m
Displacement: 33.300tons (standard), 38.500tons (full load)
Armour: 305mm Belt, 70mm Deck and 76mm Slopes
Engines: 80.000shp, 4-shafts
Range: 10.200km at 30km/h (5.500nm at 16knots)
Speed: 49km/h (26,5knots)
Armaments:
4×2 410mm/45 3rd Year Type Cannons,
20×1 140mm/50 3rd Year Type Guns,
8×1 76mm/40 3rd Year Type DP-AA Guns,
8×1 76mm/25 41st Year Type Guns or 57mm/40 or 47mm/50 Hotchkiss Guns.
4×1 533mm Underwater and 4×1 Above Water Torpedo Tubes

Hidetaro Kaneda’s Type B Bridge

The second modified bridge variant Captain Hidetaro Kaneda proposed in late 1917. This variant still used a 8 legged or octopod pagoda mast though there was a centreline shaft for elevator but that only went from the bridge or conning tower level upwards. The uniqueness of this bridge is the oval shaped conning tower with inclined armour plates for added protection. Yuzuru Hiraga again wasn’t really fond of this bridge either but the Admiralty liked the idea so Hiraga had to redesign his own pagoda mast with 6 closely spaced legs to the finallyy adapted heptapod (6 legs and 1 central shaft) with wide legs as the final bridge structure for Nagato.

Original Hiraga documents:

平賀譲デジタルアーカイブ · 〔櫓檣改正案検討資料〕 · 東京大学学術資産等アーカイブズ共用サーバ

The design had the following characteristics:
Dimensions: 201,17 (pp) x 213,36 (wl) x 215,8 (oa) x 29 x 9,10m
Displacement: 33.300tons (standard), 38.500tons (full load)
Armour: 305mm Belt, 70mm Deck and 76mm Slopes
Engines: 80.000shp, 4-shafts
Range: 10.200km at 30km/h (5.500nm at 16knots)
Speed: 49km/h (26,5knots)
Armaments:
4×2 410mm/45 3rd Year Type Cannons,
20×1 140mm/50 3rd Year Type Guns,
4×1 76mm/40 3rd Year Type DP-AA Guns,
4×1 533mm Underwater and 4×1 Above Water Torpedo Tubes

IJN Nagato as Finished

25th November 1920 saw the commissioning of the world’s then strongest, fastest and possibly tallest battleship entering commission into the Imperial Japanese Navy: The IJN Nagato.
The first battleship to be launched and commissioned with 41cm/45 main armament, 49km/h (26,5knots) maximum speed and both a tall pagoda mast – it’s tallest point at 41m from the waterline – and a 58,5m tall mainmast aft.
The tall pagoda mast allowed the installation of a large 10m rangefinder at a stunning 32 meters high location for excellent target acquisition and accurate firing solutions, while the even taller mainmast allowed long range radio communications. The main armament continued to be top of the line util the end of the decade when HMS Nelson commissioned into the Royal Navy but her speed in the battleship category was not surpassed until 1939 by the French battleship Richelieu!!!

The 1933-36 Modernizations

While the Washington Naval Treaty forbid the construction of new capital ships except for the United Kingdom which lacked 16″ armed battleships hence allowed the construction of the two Nelsons, Japan was expected to hold only Nagato at first and not finish Mutsu but due to diplomatic manoeuvring and requests, by the end of the 1920’s Japan too held two 16″ / 41cm armed battleships. The other two navies to command 16″ armed vessels were of course the RN with Nelson and Rodney and the USN with Colorado, Maryland and West Virginia. Interestingly, sometimes these ships were called the great seven as these were the most powerful warships of their time.
The ratification of the Washington Naval Treaty allowed the signatory nations to take a break from expensive capital ship constructions and to look elsewhere to improve their fleets. This was about to change at the abortive 1927 First Geneva and later the signed 1930 First London Naval Treaty which at first allowed constructions of capital ships up to 35.000tons displacement and a maximum 16″ armament but eventually the treaty extended the battleship construction “holiday” till 1936/37 which forbid construction of new capital ships but allowed the modernisation of existing ones.
This option was extensively used by most of the major powers navies:
The RN started modernising it’s extensive fleet including the 5 Queen Elizabeth and 2 Nelson class battleships as well as the 2 Renown and 1 Admiral class Battlecruisers,
The Italian Navy it’s 2 Conte di Cavour and 2 Andrea Doria class battleships,
The USN was absent from this modernisation fever,
The French Navy due to budgetary problems only partially modernized her 3 Courbet and 3 Bretagne class Battleships but instead started desigining and building the 2 Donkerque class Battlecruisers,
And finally the Imperial Japanese Navy’s 2 Fuso, 2 Ise and 2 Nagato class battleships as well as 3 out of 4 of the Kongo class Battlecruisers.

Nagato started her modernisation in August, 1933 at the Kure Naval Arsenal and lasted till January 1936, while Mutsu entered her modernisation phase in September 1934 at the Yokosuka Naval Arsenal and lasted till September 1936. During this time both the stern and nose sections got lengthened with a total of 9,14meters, the Gihon Steam Turbines replaced by Kampon-Parsons produced ones and the 21 Boilers got reduced to just 10 but overall engine power increased from 80.000 to 82.000shp, the funnels too got replaced by a single larger one. Both the forward pagoda superstructure and the aft tripod mainmast heavily reconstructed with many added platforms, the AA armament too got strengthened by 4 twin 12,7cm Type 89 DP-AA guns and 10 twin 25mm Type 96 Machine Guns. In place of the aft trapezoid shaped superstructure deck, aircraft facilities and a folding catapult was installed with a capacity to carry 3 seaplanes. Protection wise too the ships got upgraded with 127mm extra deck plating one deck below the main 70mm one.

The design had the following characteristics:
Dimensions: 201,17 (pp) x 221,07 (wl) x 224,94 (oa) x 34,6 x 9,50m
Displacement: 39.120tons (standard), 42.850tons (full load)
Armour: 305mm Belt, 127mm Deck and 76mm Slopes
Engines: 82.000shp, 4-shafts
Range: 15.800km at 30km/h (8.560nm at 16knots)
Speed: 50km/h (26,75knots)
Armaments:
4×2 410mm/45 3rd Year Type Cannons,
18×1 140mm/50 3rd Year Type Guns,
4×2 127mm/40 Type 89 DP-AA Guns,
10×2 25mm/60 Type 96 AA Guns
3x Seaplanes, 2x Kawanishi E7K Alf, 1x Nakajima E8N Dave

IJN Nagato as Modernized

Late War AA Refit

The worsening war situation and the ever increasing aerial threat from the USN and USAAF as well as war experience showed that warships of the IJN were very much deficient in protecting themselves from such threats. As such every warship in the fleet gradually received more and more AA mostly in the form of 25mm machine guns but in some cases 12,7cm DP-AA guns as well to increase their effectiveness in fighting enemy aircraft. This was the case with the Nagato as well, which got her AA refit in mid-late 1944 in the form of one twin 12,7cm Type 89 DP-AA guns on both sides, a large number of 25mm triple and single AA guns around the hull especially on the stern, on the 2nd and 3rd turret tops and on a specially created AA platform connecting the bridge and the funnel increasing their number to 119 barrels. The ship also received radar sets of the usual types: Type 13s on the mainmast aft, Type 21 on top of the pagoda mast and Type 22s on the Main Rangefinder deck. These additions required removal of at least 6 casemated guns partly due to topweight and partly due to battle damage.
English language sources state that during this refit the funnel and aft tripod mast was cut in height to improve the firing arcs of the AA guns but Japanese sources say this was done later in early 1945 and not late 1944 hence my drawing show them in their intact form.

The design had the following characteristics:
Dimensions: 201,17 (pp) x 221,07 (wl) x 224,94 (oa) x 34,6 x 9,50m
Displacement: 39.120tons (standard, probably)
Armour: 305mm Belt, 127mm Deck and 76mm Slopes
Engines: 82.000shp, 4-shafts
Range: 15.800km at 30km/h (8.560nm at 16knots)
Speed: 46km/h (25knots)
Armaments:
4×2 410mm/45 3rd Year Type Cannons,
12×1 140mm/50 3rd Year Type Guns,
6×2 127mm/40 Type 89 DP-AA Guns,
25×3,10×2,24×1 25mm/60 Type 96 AA Guns
3x Seaplanes, Mitsubishi F1M Pete
Sensors:
Type 13 Air-Search Radars (2 sets)
Type 21 Air/Surface-Search Radars (1 set)
Type 22 Surface-Search Radars (2 sets)

Proposed Aircraft Carrier Conversion

Between 1942 June 2nd and 6th a major naval battle happened in the middle of the Pacific Ocean near some uninteresting atoll called Midway, between the United States Navy and the Imperial Japanese Navy. During this battle the IJN lost many excellent and skilled pilots as well as 4 fleet carriers: Akagi, Kaga, Hiryu and Soryu. The catastrophic loss in this battle meant that the Imperial Japanese Navy has got a shortage of these new, long striking range capital ships: the aircraft carriers. To remedy this carrier construction and development received high priority in the Navy but at the same time alternative means were looked at to fill up the gap casued by the loss of the 4 fleet carriers mentioned earlier. Emergency plans called for the conversion of heavy cruisers and battleships/battlecruisers still available to the navy greater numbers. It was estimated by the Naval General Staff and the Naval Technical Department that the heavy cruiser conversions would require 9 months and yield an aircraft capacity of 30 planes; 18 months would be needed for the capital ships to get a capacity to carry 54 planes and the following flight deck size:
– 4-4 Myoko and Takao class cruisers would have had a 200 x 23,5m flight deck,
– 3 Mogami and 2 Tone class cruisers would have had a 195 x 23,5m flight deck,
– 4 Kongo and 2 Nagato class capital ships would have had a 220 x 34m flight deck,
– and the 2 Fuso and 2 Ise class battleships a 210 x 34m one.

The Furutaka and Kako classes were considered too small for this role and hence not included while somewhat later the cruiser conversions were discarded as they were required for fleet service by the conclusion of the Naval Ministry. The Yamato class too was not included as they were too new, too important and too powerful for such a conversion though in theend the 3rd ship, Shinano was sselected for a full conversion. The Kongo class was also deemed to require too much reconstruction work and their speed stillmade them very useful as heavy escorts for the carriers.
The Nagato class was also discarded as their 41cm cannons were still quite powerful for surface action. That left the 2 Fusos and 2 Ises for conversion of which the Ises took priority due to battle Hyuga’s turret explosion. To speed up the conversion a modified plan was created which meant only a partial flight deck was to be installed first and full conversion would be taken at a later time so Ise was modified in Kure and Hyuga in Sasebo. Fuso and Yamashiro would be modified similarly after the two ships completed their reconstruction with the expected start of the work in 1943 June, Fuso in Kure and Yamashiro in Yokosuka.
As a side note this carrier crisis in the IJN led to the conversion of the battleship Shinano, the heavy cruiser Ibuki into aircraft carriers and the construction order for the Unryu class simplified Hiryu type carriers.

Here I show you my artis rendition of Nagato had she been converted into a fleet carrier. I’ve originally chosen a Junyo type bridge but decided to modify Shinano’s bridge instead to fit on this hull. As for the aircraft storage, I could put 52 aircraft in her Hanger (40 + 4 spares of each type) though it would be quite crowded, but if a Shinano type deck parking was taken into account the 54 aircraft capacity could easily be reached.

The 3 variants shows the 3 possible deck materials: Wooden deck, Full metal deck, Shinano type linoleum deck.

The design had the following characteristics:
Dimensions: 201,17 (pp) x 221,07 (wl) x 224,94 (oa) x 34m (fd) 45,6m (max) x 9,50m
Displacement: Unknown
Armour: 305mm Belt, 127mm Deck and 76mm Slopes
Engines: 82.000shp, 4-shafts
Range: 15.800km at 30km/h (8.560nm at 16knots)
Speed: 50km/h (26,75knots)
Armaments:
6×2 127mm/40 Type 89 DP-AA Guns,
24×3,7×1 25mm/60 Type 96 AA Guns
Sensors:
Type 13 Air-Search Radars (2 sets)
Type 21 Air/Surface-Search Radars (2 sets)
Type 22 Surface-Search Radars (2 sets)
Single Hanger and na Aircraft capacity of 45-60 planes depending on storage (Hanger only or Hanger+Flight Deck)
22+4 Mitsubishi A6M Zero fighters
8+4 Nakajima B6N Jill Torpedo Bombers
10+4 Yokosuka D4Y Judy Dive Bombers