Cruiser Projects and Designs of the IJN

Tzoli

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Here I gathered all the various cruiser designs and projects proposed for the Imperial Japanese Navy

Protected Cruisers:
Not found any so far except the cancelled sister ships of the Unebi and Tone classes

Light Cruisers:
Design C-19 (1912)
- A primary mine-layer cruiser design offered before WW1 but not ordered.
Dimensions: 159,41 (pp) x 15,24 x 5,18 meters
Displacement: 6.000tons (standard)
Engines: 28.500shp Steam Turbines, 2 shafts
Range: 18.500km (10.000nm)
Speed: 50km/h (27knots)
Armour: none
Armaments:
4x1 152mm Guns
6x1 76mm Guns
4x1 533mm Torpedo Tubes
100x Mines

Design C-31A (1915) - A WW1 era design for a fast well armed cruiser possibly the baseline design which evolved into the Tenryu and later the Kuma/Nagara/Sendai classes or a development of the proposed 1914 large scout cruiser.
Dimensions: 161,54 (pp) x 15,85 x 5,1 meters
Displacement: 7.000tons (standard)
Engines: 106.000shp Steam Turbines, 4 shafts
Range: 9.200km (5.000nm)
Speed: 65km/h (35knots)
Armour: 25mm Deck, 76mm Belt
Armaments:
9x1 140mm Guns
1x3 533mm Torpedo Tubes
48x Mines

Design C-32 (1915) - A smaller 4.100ton cruiser, very likely part of the Tenryu design process
Dimensions: 140,21 (pp) x 13,41 x 4,11 meters
Displacement: 4.100tons (standard)
Engines: 65.000shp Steam Turbines, 4 shafts
Range: 11.000km (5.900nm)
Speed: 63km/h (34knots)
Armour: 25mm Deck, 63mm Belt
Armaments:
5x1 140mm Guns
2x3 533mm Torpedo Tubes
48x Mines

7.200ton Proposal (1916) - A well armed and armoured scout cruiser proposal first envisioned in 1914 but their slower construction time resulted their postponing to 1916. In 1916 in was re-formed into a 7.200ton cruiser and 3 ships were considered in the 8-4 and 8-6 naval programmes. This designed formed the basis of the later Furutaka class because while originally it was designed with 14cm guns in mind new 20cm guns were considered as well
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)
View: https://i.imgur.com/amHU9p9.png
Dimensions: unknown
Displacement: 7.200tons (standard)
Engines: unknown
Range: 11.000km (8.000nm)
Speed: 67km/h (36knots)
Armour: 51mm Deck, 76mm Belt
Armaments:
4x2,4x1 140mm Guns
4x2 610mm Torpedo Tubes

Tenryu-Kai type (1917) - 6 modified or improved Tenryu class cruisers were considered to be built from 1917 onward as part of the 8-4 and 8-6 naval programmes. Their designer was Teiji Kawase and only very limited info I could gather was that it's displacement would be 3.500tons compared to 3.230tons of the Tenryu class which indicate more armour somewhat more armament and larger 610mm torpedo tubes with the associated increased engine power.
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)
View: https://i.imgur.com/bUGIY8e.png
View: https://i.imgur.com/odZJ3E9.png

Furutaka Class original Design (1918) - The design process eventually leading the creation of the Furutaka class Heavy Cruisers was first started with a smaller ship evolved from the previous 7.200ton design to 8.000tons and 5-6 twin 140mm guns in a pyramidal arrangement while the AA armament only consisted of 2 single 76mm guns while the torpedo armament unchanged from the 4 twin 610mm tubes. Armour wise it was much thinner as well with the armoured box protecting only the machinery by 64mm belt and 25mm deck. Eventually based on the data of the USN's Omaha class but especially on the British Hawkins class, this design evolved into the IJN's first heavy cruiser type.
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)
View: https://i.imgur.com/6rfmXWk.png
View: https://i.imgur.com/s6NRAs5.png

5.600ton Proposal (1919) - A post war fleet escort cruiser design with good armament and very high speed. Possibly a development of the 7.200ton type though not much info is available on it.
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)
View: https://i.imgur.com/WYXdTJB.png
Dimensions: unknown
Displacement: 5.600tons (standard)
Engines: 102.800shp Steam Turbines, 4 shafts
Range: unknown
Speed: 74km/h (40knots)
Armour: unknown
Armaments:
4x2 140mm Guns
6x2 533 or 610mm Torpedo Tubes

Tenryu-Kai type (1921) - A different source provides more data on the Tenryu-Kai type cruiser putting it to 1921 and with somewhat more displacement.
Dimensions: 143 (oa) x 12,4 x 4 meters
Displacement: 4.080tons (standard)
Engines: 51.000shp Steam Turbines, 3 shafts
Range: 11.000km (8.000nm)
Speed: 61km/h (33knots)
Armour: Over Machinery: 25mm Deck, 63mm Belt
Armaments:
2x2 140mm Guns
1x1 76mm DP-AA Guns
2x2 610mm Torpedo Tubes

Design C-43 (1923) - A squadron leader cruiser proposal found in the Yuzuru Hiraga archive but likely Hiraga's own design number because C-42 was Oyodo and C-44 was the Agano-Kai. Might be connected to the earlier 5.600ton design.
Source: (Hiraga number: 21231001)
Dimensions: 152,4 (pp) x 14,17 x 4,85 meters
Displacement: 5.600tons (standard) 7.000tons (Full load)
Engines: 42.750shp Steam Turbines, 3 shafts
Range: 9.200km (5.000nm)
Speed: 65km/h (35knots)
Armour: Over Machinery: 25mm Deck, 64mm Belt
Armaments:
4x2 140mm Guns
2x1 76mm DP-AA Guns
2x2 610mm Torpedo Tubes
100x Mines

5.000ton Proposal (1926) - A well armed true light cruiser on limited displacement and average speed, it's origins are a mystery but might be connected to the numerous Vickers export designs of the time.
Dimensions: unknown
Displacement: 5.000tons (standard)
Engines: 60.000shp Steam Turbines, 4 shafts
Range: unknown
Speed: 59km/h (30,5knots)
Armour: unknown
Armaments:
3x3 152mm Guns
2x2 120mm DP-AA guns
2x2 533mm Torpedo Tubes

Design 1927 or C-37 (1927) - The earliest interpretation of what became the Mogami class cruisers. Yet to re-find where I've found data of it, but I seems to remember from Eric Lacroix's Japanese Cruiser book
Dimensions: 197meters (wl)
Displacement: 8.500tons (standard) 11.200tons (Full load)
Engines: 152.000shp Steam Turbines, 4 shafts
Range: unknown
Speed: unknown
Armour: 38mm Deck, 125/143mm Belt (Magazines/Machinery)
Armaments:
5x3 155mm Guns
4x1 127mm DP-AA Guns
4x3 610mm Torpedo Tubes
2x Scout Planes

Squadron Leader Design (1928) - Essentially an early interpretation of what became the Agano and Oyodo class cruisers a decade later a cruiser adequately armed and armoured with good speed to lead destroyer and submarine squadrons as well as for doing scouting duties. 6 ships were envisioned but cancelled and replaced by 4 Takao-Kai type cruisers in 1930. Though Eric Lacroix's book states "A main armament of six 50-cal 14-cm guns, in twin mounts with a high elevation" this indicates more to the 55 calibre high angle twin turret envisioned for the Kongo Replacement designs of 1928-29.
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)
View: https://i.imgur.com/IMUgWsm.png
View: https://i.imgur.com/XzB1SOs.png
Dimensions: unknown
Displacement: 5.000tons (standard)
Engines: 42.750shp Steam Turbines, 3 shafts
Range: 9.200km (5.000nm)
Speed: 65km/h (35knots)
Armour: 51mm Deck, 76mm Belt
Armaments:
3x2 140mm/50 Type 3 or 140mm/55 Type 88 Guns
2x3 610mm Torpedo Tubes
2x Scout Planes

Tone as designed (1934) - Like the Mogami originally the Tone class heavy cruisers too were designed with 155mm guns in mind, in triple turrets all forward and likely similar modified Takao or further modified Takao style brdige, but the Tomozoru incident as well as the worsening political situation meant these ships got modified Mogami bridges and the standard 20cm guns as the other IJN heavy cruisers of the time
Source: (Eric Lacroix - Japanese Cruisers of the Pacific Wa
View: https://i.imgur.com/0N586GD.png
View: https://i.imgur.com/iGFqSMz.png

Squadron Leader Design (1937) - The immediate precursor of the Agano class cruisers but on a smaller displacement and with Oyodo style armament and better heavy AA. 5 such ships were considered to be built in the Maru 3 programme but voted down by the Diet.
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)
Dimensions: unknown
Displacement: 5.000tons (standard)
Engines: unknown
Range: 13.000km (7.000nm)
Speed: 67km/h (36knots)
Armour: 100mm Belt
Armaments:
2x3 155mm/60 Type 90 Guns
4x2 76mm/60 Type 98 DP-AA Guns
3x2 25mm/60 Type 96 AA guns
2x4 610mm Torpedo Tubes
1x Seaplane

Kuma Class Original Torpedo Cruiser Conversion proposal (1938) - As to be compatible with the new night fighting doctrine the IJN experimenting with, 3 of the 5 Kuma class cruisers were to be converted into a Torpedo Cruiser with very heavy torpedo armament to destroy or scatter entire enemy fleets. The conversion was actually done but to a modified design and for only 2 ships. The original proposal had these armaments for the cruisers:
4x2 127mm/40 Type 89 DP-AA guns, 4x2 25mm/60 Type 96 AA guns and 11x quadruple 610mm Torpedo Launchers: 5 per side and 1 one the centreline! The production bottleneck for the torpedos and their launchers as well as the 127mm DP-AA guns meant that the revised design only applied to two ships with most of their original armament remained impact with reduced torpedo launcher number.

Design C-39 (1938) - The first known preliminary design leading to the Agano class light cruisers with different engine power, longer range and slightly smaller displacement, different torpedo arrangement and with two funnels.
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)
Dimensions: 162 (pp) x 173 (wl) x 174,5 (oa) x 15,2 x 5,6 meters
Displacement: 6.585tons (standard) 7.800tons (Full load)
Engines: 112.000shp Steam Turbines, 4 shafts
Range: 14.800km (8.000nm)
Speed: 67km/h (36knots)
Armour: 20mm Deck, 55/60mm Belt (Magazines/Machinery)
Armaments:
3x2 152mm Type 43 Guns
2x2 76mm/60 Type 98 DP-AA Guns
2x3 25mm/60 Type 96 AA guns
2x4 610mm Torpedo Tubes
18x Depth Charges
2x Seaplanes

Design C-39 Modified (1938) - After the baseline C-39 design was made some modifications were made on the design mostly to the hull length and reduced engine power:
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)
Dimensions: 170 (wl) x 15,2 x 5,6 meters
Displacement: 6.450tons (standard) 7.600tons (Full load)
Engines: 99.000shp Steam Turbines, 4 shafts
Range: 14.800km (8.000nm)
Speed: 65km/h (35knots)
Armour: 20mm Deck, 55/60mm Belt (Magazines/Machinery)
Armaments:
3x2 152mm Type 43 Guns
2x2 76mm/60 Type 98 DP-AA Guns
2x3 25mm/60 Type 96 AA guns
2x4 610mm Torpedo Tubes
18x Depth Charges
2x Seaplanes

Oyodo Preliminary Design 1 (1938) - One of the design studies considered for the Submarine squadron leader type cruisers of the Oyodo class. Heavy DP-AA guns 2 forward and two on the sides, aircraft capacity for 6-8 planes and two catapults
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)
Dimensions: unknown
Displacement: 5.000tons (standard)
Engines: unknown
Range: 18.500km (10.000nm)
Speed: 67km/h (36knots)
Armour: unknown
Armaments:
4x2 127mm/40 Type 89 DP-AA Guns
6x3 25mm/60 Type 96 AA guns
6-8x Seaplanes, likely E16A Paul types

Oyodo Preliminary Design 2 (1938) - A refined version of the previous design where the catapults located at almost side by side.
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)
Dimensions: unknown
Displacement: 6.600tons (standard)
Engines: unknown
Range: 18.500km (10.000nm)
Speed: 67km/h (36knots)
Armour: unknown
Armaments:
4x2 127mm/40 Type 89 DP-AA Guns
6x3 25mm/60 Type 96 AA guns
6x Seaplanes, likely E16A Paul types

Oyodo Preliminary Design 3 (1938) - The basic design which developed further into the Oyodo class almost the same as the original just different aircraft handling facilities and shorter range but with the long 45m catapult aft.
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)
Dimensions: unknown
Displacement: 8.200tons (standard) 9.800tons (Full load)
Engines: 110.000shp Steam Turbines, 4 shafts
Range: 18.500km (10.000nm)
Speed: 65km/h (35knots)
Armour: unknown
Armaments:
2x3 155mm/60 Type 90 Guns
4x2 100mm/60 Type 98 DP-AA Guns
8x2 25mm/60 Type 96 AA guns
6x Seaplanes, likely E16A Paul types

Design C-44 Agano Kai (1941) - An improved or modified Agano with 4 main turrets and higher speed. 7 units ordered in 1941 to replace the aging 5.500ton type (Kuma, Nagara, Sendai) cruisers
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)
Dimensions: 175 (pp) x 184 (wl) x 16,4 x 5,9 meters
Displacement: 8.250tons (standard) 9.670tons (Full load)
Engines: 153.000shp Steam Turbines, 4 shafts
Range: 11.600km (6.300nm)
Speed: 70km/h (38knots)
Armour: 20mm Deck, 55/60mm Belt (Magazines/Machinery)
Armaments:
4x2 152mm Type 43 Guns
4x2 76mm/60 Type 98 DP-AA Guns
3x3 25mm/60 Type 96 AA guns
2x4 610mm Torpedo Tubes
2x Seaplanes

Armoured Cruisers:
Only found one so far, the First Class Armoured Cruiser Design from 1904/05. A modern looking 6" armed cruiser with most of its guns in twin circular turrets and often seen accompanied with the First class battleship drawing from the same year. The origins are not entirely clear but likely the result of the Battle of Tsushima. Drawings were found in the Hiraga archive as well as in Japanese books and magazines.
Source: (Battleships and Battle Cruisers of the IJN by Ishibashi Takao)
Source: (Hiraga number: 20200201)
Dimensions: 144,78 (oa) x 21,34 x 7,92 meters
Displacement: 12.000tons (standard)
Engines: unknown
Range: unknown
Speed: 41km/h (22knots)
Armour: 51mm Deck, 51mm Slopes, 152mm Belt
Armaments:
8x2,4x1 152mm Guns
6x1 120 Guns
4x1 457mm Torpedo Tubes

Heavy Cruisers:

Design C-41 (1923)
- A heavy cruiser proposal found in the Yuzuru Hiraga archive but likely Hiraga's own design number because C-41 was actually the Agano class. Might be connected to the Myoko class design process. Very heavy displacement compared to armour and armament.
Source: (Hiraga number: 21231001)
Dimensions: 225,5 (pp) x 21,64 x 5,33 meters
Displacement: 14.000tons (standard) 18.000tons (Full load)
Engines: 140.000shp Steam Turbines, 4 shafts
Range: unknown
Speed: 68km/h (36,5knots)
Armour: 25/47mm Deck, 51/102mm Belt (Magazines/Machinery)
Armaments:
4x2 200mm/50 Type 3 Guns
2x1 120mm/45 Type 10 DP-AA Guns
2x4 610mm Torpedo Tubes

Design C-42 (1923) - Another heavy cruiser proposal found in the Yuzuru Hiraga archive but likely Hiraga's own design number because C-42 was actually the Oyodo class. Might be connected to the Myoko class design process. Smaller and slightly slower than the previous proposal
Source: (Hiraga number: 21231001)
Dimensions: 210,3 (pp) x 20,11 x 5,26 meters
Displacement: 12.000tons (standard) 15.200tons (Full load)
Engines: 125.000shp Steam Turbines, 4 shafts
Range: 14.800km (8.000nm)
Speed: 67km/h (36knots)
Armour: 25/47mm Deck, 51/102mm Belt (Magazines/Machinery)
Armaments:
4x2 200mm/50 Type 3 Guns
2x1 120mm/45 Type 10 DP-AA Guns
2x4 610mm Torpedo Tubes

Myoko Preliminary Design (1924) - Possibly connected to the Myoko design process 20cm turret calculations connected to the Myoko papers shows Hiraga played with the ideas of well armoured triple and quadruple turrets on a slightly shorter hull and thinner armour
Source: (Hiraga number: 21231001)
Dimensions: 198,12 (pp) x 17,52 meters
Displacement: 10.000tons (standard)
Engines: unknown
Range: unknown
Speed: unknown
Armour: 38mm Deck, 89mm Belt
Armaments:
4x3 or 3x4 200mm/50 Type 3 Guns
2x1 120mm/45 Type 10 DP-AA Guns
2x2 13,2mm AA Guns
2x Seaplanes

Myoko Preliminary Design (1924) - One of the designs considered for the Myoko class, almost identical to the as built class but with longer range.
Source: (Hiraga number: 22160101, 201, 301)
Dimensions: unknown
Displacement: 10.000tons (standard)
Engines: 130.000shp Steam Turbines, 4 shafts
Range: 14.800km (8.000nm)
Speed: 67km/h (36knots)
Armour: unknown
Armaments:
5x2 200mm/50 Type 3 Guns
4x1 120mm/45 Type 10 DP-AA Guns
2x2 13,2mm AA Guns
2x Seaplanes

IJN Haguro upgun proposal (1933) - According to some Japanese sources and the Japanese Wiki there was a new 20,3cm cannon under development in the late 1920's early 1930's which was expected to be mounted on new cruisers and would had been comparable to the USN 8"/55 cannons. This weapon would had been a longer /55 calibre one and would had been first tested on IJN Haguro and to be mounted on the Takaos but due to budgetary issues this new weapon did not enter service.
Source: (Japanese Wikipedia)

Takao-Kai type Heavy Cruiser (1929/30) - Soon after the Takao class cruisers ordered a new follow-up design was considered but development was postponed until the end of decade. This was unlucky because the 1st London Naval Treaty was held in 1929 with its ratification in 1930 meant all new heavy cruiser construction had to be halted as the nations reached their quota (And the RN could not keep with them...) Hence the very limited info available on this design. Only some basic requirements were set before the programme cancelled which included better heavy AA likely twin instead of single 127mm/40 Type 89 DP-AA guns and improved armour likely 140mm Magazine belt and the usual 5 twin 20cm guns which might had been the new /55 calibre ones as well. We could expect that the Tower like bridge would had been modified similar to what first proposed for the Mogami class cruisers. Though knowing how the original Takaos were overweight by 1350tons from the set 10.000tons this improved version we could had expected even more!
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)

Tone-Kai type Heavy Cruiser (1938/39) - At the 1938 revised Maru 4 programme, two modified Tone type cruisers were considered to be built to form a homogeneous squadron with the other two Tones. Not much is known about this design except that it was based on the Tone class but with substantial more displacement of 13.000tons standard instead of the 11.200tons of the original Tones which might indicate better armour and engine power and command facilities.
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War), Japanese Wiki

Ibuki Class Heavy Cruiser Design C-46 "Torpedo Modification Proposal" (1942)- While the Ibuki class IJN Ibuki and Ikoma hull numbers 300 and 301 were known cruiser based on the Suzuya, there was a proposal to change these heavy cruisers into more potent torpedo platforms. Though the description is rather vague how the torpedo launchers would had been placed, it was said that the original 4 quad launchers either to be replaced or even strengthened by 5 quintuple launchers, 2 per side one one centreline in place of the Aircraft handling facilities even deleting the Catapults themselves. This would had been provided a great deal of torpedo firepower rivaling that of the Kitakami and Oi's conversions with either 15 or 23 tubes per side!
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)

AA Cruisers:

Tenryu CLAA Conversion proposal 1
- In 1936, plans were drawn up to convert the 2 Tenryu class light cruisers into Anti-Aircraft ships akin to the C class in the Royal Navy and to improve these old small cruisers fighting capabilities. The conversion envisioned the replacement of the the entire armament by:
4x2 127mm/40 Type 89 DP-AA guns, 4x3 25mm/60 Type 96 AA guns, 2x3 533mm Torpedo Tubes, 36x Depth Charges. Due to more pressing issues as well as budgetary restrictions these conversions did not followed this time.
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)

Tenryu CLAA Conversion proposal 2 - In 1938, new revised plans were drawn up to convert again the 2 Tenryu class light cruisers into Anti-Aircraft ships, this time the armament was changed to more modern weapons:
4x2 76mm/60 Type 98 DP-AA guns, 4x3 25mm/60 AA guns, 2x3 533mm Torpedo Tubes, 36x Depth Charges. Again due to budgetary restrictions these conversions did not followed and seems to be abandoned altogether.
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)

Kuma CLAA Conversion proposal - In 1938, next to the Torpedo cruiser conversion proposal, Anti-Aircraft conversion too was envisioned for all the Kuma class ships akin to the C class in the Royal Navy. The conversion envisioned the replacement of the the entire armament by:
2x2 100mm/60 Type 98 DP-AA guns, 4x3 25mm/60 Type 96 AA guns, 4x2 533mm Torpedo Tubes, 36x Depth Charges. Due to more pressing issues as well as budgetary restrictions these conversions did not followed this time.
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)

Oyodo CLAA Conversion proposal - In late 1941 early 1942 due to the ever increasing effectiveness of aircrafts it was proposed to convert the then under construction Type C (Oyodo class) cruisers into heavy AA ships with an unparalleled armament of 12x twin 10cm AA guns! The admiralty seems to be not too fond of the idea believing their carriers could protect the fleet not to mention each such cruisers would eat up as much as 3 Akizuki class Destroyer gun mounts and which was a bottleneck in AA gun construction for the IJN. This design became known as the Japanese Atlanta and most sources show the 3-2/2-3 arrangement version of the design.
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)
Dimensions: 180 (pp) x 189,7 (wl) x 192,1 (oa) x 16,6 x 5,95 meters
Displacement: 7.150tons (standard) 9.200tons (Full load)
Engines: 110.000shp Steam Turbines, 4 shafts
Range: 13.000km (7.000nm)
Speed: 64km/h (34,5knots)
Armour: 30/50mm Deck, 60/75mm Belt
Armaments:
12x2 100mm/60 Type 98 DP-AA
6x3 25mm/60 Type 96 AA guns
2x4 610mm Torpedo Tubes

IJN Suzuya AA Conversion Proposal (1942) - After the battle of Midway in which Mikuma sunk and Mogami was heavily damaged only 3 operational Mogami class cruisers remained in service. Between late 1942 and early 1943, while Mogami was underwent repair and conversion into an aviation cruiser to increase the fleet's scouting capability, plans were drawn up to convert both Suzuya and Kumano into Anti-aircraft cruisers. The plans included the replacement of all the main 20cm twin turrets to twin 12,7cm DP-AA mounts as well as increase of light AA gun armament. Though original plans included to convert both of the two fully operational cruisers to Anti-aircraft ships, most likely to wartime shortage of 12,7cm guns as well as suitable dockyard capacity, Suzuya would only got her aft guns replaced by the mentioned AA weaponry while still able to provide the same firepower as the Furutaka and Aoba classes.
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)

IJN Kumano AA Conversion Proposal (1942) - While IJN Suzuya would only got half the AA gun replacement that was proposed, IJN Kumano could had received the full conversion creating a potent AA platform rivalling even that of the USN's Atlanta class AA light cruisers. Other changes likely would be included the replacement of the main gun rangefinder with High-angle one as well as more Machine-gun director towers for controlled AA fire.
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War)

Design C-45 (1942) - Pure Anti-Aircraft cruiser designs from start envisioned as Akizuki class flotilla leaders, 4 ships were ordered one for each surface battle group and essentially enlarged Akizukis with seaplane facilities on board. Artists tend to describe them as Aganos with 10cm main guns.
Source: (Eric Lacroix - Japanese Cruisers of the Pacific War), Japanese Wiki
Dimensions: 172 (wl) x 15,8 x 5,7 meters
Displacement: 5.800tons (standard) 7.150tons (Full load)
Engines: 103.000shp Steam Turbines, 4 shafts
Range: 13.000km (7.000nm)
Speed: 63km/h (34knots)
Armour: 50mm Deck, 50mm Belt
Armaments:
4x2 100mm/60 Type 98 DP-AA
6x3 25mm/60 Type 96 AA guns
100x Depth Charges
2x Seaplanes

Others:

Design X (1915/18) Large Light Cruiser
or Light Battlecruiser - Found in the Hiraga archive, drawings of a long slim and fast cruiser with single 305mm cannons, clearly inspired by the British Courageous class. This Japanese design is more lightly armorued than her British cousins but would had been less costly as well due to the smaller size.
Source: (Hiraga number: 21710201)
Dimensions: 201,17 (wl) x 18,9 x 6,09 meters
Displacement: 11.000tons (standard)
Engines: 135.000shp Steam Turbines, 4 shafts
Range: unknown
Speed: 67km/h (36knots)
Armour: 25mm Deck, 76mm Belt
Armaments:
3x1 305mm/45 Type 37 Cannons
8x1 140mm/50 Type 3 Guns
2x1 76mm/40 Type 3 DP-AA Guns
4x1 533mm Torpedo Tubes

Design X -Revised (1915/18) Large Light Cruiser or Light Battlecruiser - Some data armament changes in one of the papers found in the Yuzuru Hiraga archive shows a revised version of the Design X with 4 turrets instead of 3 and other armament changes but weirdly on a smaller hull and same displacement. Another weird thing is that Yuzuru Hiraga compared or as it seems derived it's hull or calculations from the Kuma class light cruisers which were while ordered in 1917 finished only in 1920 so a design date might be closer to 1918?
Source: (Hiraga number: 21710501,21711401)
Dimensions: 199,64 (wl) x 18 x 6,06 meters
Displacement: 11.000tons (standard)
Engines: 135.000shp Steam Turbines, 4 shafts
Range: unknown
Speed: 67km/h (36knots)
Armour: 25mm Deck, 76mm Belt
Armaments:
4x1 305mm/45 Type 37 Cannons
7x1 140mm/50 Type 3 Guns
4x1 76mm/40 Type 3 DP-AA Guns
6x1 533mm Torpedo Tubes

Extra:

Chichibu Class Big Gun Cruiser

While the Chichibu herself was basically a hypothetical design it should be mentioned here because it's impact both on the USN and IJN!
Some brief mention was made here many years ago:
Th Chichibu class as mentioned, was a 2-4 ship strong class believed by the USN intelligence to be built from the late 1930's. The actual data on the design was limited to displacement, speed and armament: 12-15.000tons, around 30knots and 6x 12" cannons with exact description that these are based on the German Deutschland class Heavy or Big Gun Cruisers.
The erroneous information the US gathered could be connected to multiple factors:
- Japanese misinformation and secrecy regarding their armed forces
- Inadequate information gathering from the Military Intelligence side
- False information gathering to persuade the politicians for new warship constructions
Either one or all of the above meant that the USN had information that now two hostile nations (Japan and Germany) have cruisers equipped with larger than 8" guns, a potential dangerous threat to the treaty design cruisers of the time both 6" and 8" armed ones. Hypothetical wargames and extrapolations of known data resulted that the RN and USN needed something bigger than a 10-12.000ton 8" armed cruiser to fight with these new kind of vessels. On the RN side the Deutschlands influenced the 9,2" armed cruisers designs of the 1938-40 period while for the USN both these and Chichibus resulted in developing and eventually building the Alaska class large or battlecruisers which were designed primary to fight these vessels as well as enemy 8" armed cruisers.
And now we turn back to the Imperial Japanese Navy which by late 1941 early 1942 got hold of information that the USN were ordering and building 6x large heavy cruisers with possibly larger than 8" guns. This resulted in designing a large cruiser to counter these new ship type the B-65 class with 3x3 310mm weapons. But as new and more accurate information were gathered by the Japanese the design was revised with the deck armour thickened, main armament enlarged to 3 twin 356mm cannons and likely either the allocation or drop of the quadruple 610mm torpedo tubes.
This created a sort of funny (in my opinion) construction circle: False information about a Japanese cruiser class resulting in building the Alaskas by the USN against which the Japanese designed their own cruiser to counter and to which to some extent the Soviet Stalignrads were designed and constructed as well!
 
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cattalonX

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Tzoli

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I cannot copy the text from an image!
But that number sure isn't in the Archive.
 

Nick Sumner

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A really useful post, thanks for this Tzoli!
 

Tzoli

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Will include the sources as well when I re-find them.
 

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The 1943 edition of the Jay Launer's Fighting Fleets magazine indicates the possible existence of a hybrid cruiser called Tukaku class.
More information appeared in the 1944 edition of The Enemies' Fighting Ships wich described a second type called Tsushima class.
In the Warships International magazine of december 1970 a little reference of these ships appeared in the David Dickson Japan's Hybrid Battleships and Carriers article.
The same information appeared in chapter 12 of R. D. Layman and S. McLaughlin The Hybrid Warships.
Tsushima class:
displacement: 9000t
dimensions; 650ft oa x 63 ft
propulsion: 96000 shp, 34 kts
armament: 4 x 2 - 8in, 4 x 2 5.1in, 2 x 4 24in TT
aircraft; 11-13 seaplanes
complement: 1100.
 

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Tzoli

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I've seen that drawing but this was mid-late war speculation and likely errenous information on the Tone class. It did not effect warship development like the Chichibu. I've included the Chichibu because it's effect on the development of the Alaska and B-65 classes.
 

lancer21

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Kuma CLAA Conversion proposal - In 1938, next to the Torpedo cruiser conversion proposal, Anti-Aircraft conversion too was envisioned for all the Kuma class ships akin to the C class in the Royal Navy. The conversion envisioned the replacement of the the entire armament by:
2x2 100mm/60 Type 98 DP-AA guns, 4x3 25mm/60 Type 96 AA guns, 4x2 533mm Torpedo Tubes, 36x Depth Charges. Due to more pressing issues as well as budgetary restrictions these conversions did not followed this time.

Hi Tzoli,

Just a question about this Kuma CLAA proposal, was it really supposed to carry just 2x2 type 98 100mm, or it's just a simple error and it's supposed to be 4x2?

I ask because the smaller Tenryu was supposed to carry either 4x2 Type 89 127mm or Type 98 76 mm guns, and indeed the Kuma torpedo cruiser plan also had 4x2 Type 89 127mm guns, while during the war Isuzu CLAA was fitted with 3x2 Type 89 127mm guns.
 

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My little contribution:
Never finished CV Ibuki, built over the hull of the never finished light cruiser of the same name. I get the image from the book Portaaviones de todo el mundo, from Gino Galuppini. Editorial Espasa-Calpe S.A. Madrid 1984

Attachments 1648669707314.jpeg

 

Tzoli

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Kuma CLAA Conversion proposal - In 1938, next to the Torpedo cruiser conversion proposal, Anti-Aircraft conversion too was envisioned for all the Kuma class ships akin to the C class in the Royal Navy. The conversion envisioned the replacement of the the entire armament by:
2x2 100mm/60 Type 98 DP-AA guns, 4x3 25mm/60 Type 96 AA guns, 4x2 533mm Torpedo Tubes, 36x Depth Charges. Due to more pressing issues as well as budgetary restrictions these conversions did not followed this time.

Hi Tzoli,

Just a question about this Kuma CLAA proposal, was it really supposed to carry just 2x2 type 98 100mm, or it's just a simple error and it's supposed to be 4x2?

I ask because the smaller Tenryu was supposed to carry either 4x2 Type 89 127mm or Type 98 76 mm guns, and indeed the Kuma torpedo cruiser plan also had 4x2 Type 89 127mm guns, while during the war Isuzu CLAA was fitted with 3x2 Type 89 127mm guns.
Difficult to get hold info of, Eric Lacroix only states 10cm twin mounts I might had erroneously stated only 2 twins.
 

gral_rj

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Here I gathered all the various cruiser designs and projects proposed for the Imperial Japanese Navy

Protected Cruisers:
Not found any so far except the cancelled sister ships of the Unebi and Tone classes

How many planned sisters to the Unebi? One, to mirror the Naniwa class?
 

Tzoli

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Here I gathered all the various cruiser designs and projects proposed for the Imperial Japanese Navy

Protected Cruisers:
Not found any so far except the cancelled sister ships of the Unebi and Tone classes

How many planned sisters to the Unebi? One, to mirror the Naniwa class?
One more Unebi and one more Tone
 

Hood

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One more Unebi
What source do you have for a proposed sister for Unebi? Most sources state only Unebi was ordered as an alternative source of cruisers other than Britain. Her sinking on her delivery seemed to put off the IJN from ordering a replacement from France.
 

Tzoli

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One more Unebi
What source do you have for a proposed sister for Unebi? Most sources state only Unebi was ordered as an alternative source of cruisers other than Britain. Her sinking on her delivery seemed to put off the IJN from ordering a replacement from France.

From here:
Which seems to be an earlier extract of the Wikipedia article because now both the Wiki article about Unebi and Chiyoda states the compensation money was used for the construction of Chiyoda and not the allocated money for a 2nd Unebi.

I will check Conway's All the World Fighting ships as well.
Conway 1860-1905 gives very little filling info, no mention for a sister ship.
 
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Tzoli

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By the way here are the Design X cruisers aka the Japane Courageous:
ddwnkbn-194f702c-a33b-4ea5-9fc8-dc9b0f6c8bfc.png


And the revised design:
ddwq8lc-f9c29a82-b2b9-47c7-96e3-30c382873b61.png
 

G4H4CK256

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???
Chichibu?
The cruiser with a 305mm main gun cannot be sure
The following is an early concept of the super A cruiser
15,000 tons, at least eight 10-inch guns, and a speed of 36 knots
Source?
View attachment 676123
Only this picture, I'm not sure if it in the Hiraga archive
That's from Japan Center for Asian Historical Records. here's the link: https://www.jacar.archives.go.jp/das/image/C14121165700
Speaking of cruiser with large calibre guns, there was a plan in 1934 to mount 10in guns on Mogami class https://www.jacar.archives.go.jp/das/image/C14121167000
1649041344353.png
 

Tzoli

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Are you sure that these table shows proposals? It looks to me as ship construction proposals or even wargame to counter the USN.
On the 2nd page the rightest most column shows USN BB names:
West Virginia
Colorado
Massachusetts
California
Tennessee
Idaho
New Mexico
Nevada
Arizona
the second row on the 3rd page contains rest of the USN BB's in 1937:
Pennsylvania
Oklahoma
Maryland
New York
Texas
Arkansas
missing from the list:
Utah, Wyoming, Mississippi

8x Unknown Battleships
4x Fast large Battleships, Note there were no 65.000ton (aka Yamato) 34knot / 63km/h BB design of the IJN!
2x 41.600tons likely the Nagatos
The Fusos and Ises seems to be missing.
The cruiser part is also weird.
4x 18.000ton 36knot cruisers
2x 15.600ton 36knot cruisers
2-2 other cruisers? Kongos maybe?

8x 10.000ton 36knot cruisers (Myoko and Takao classes)
4x 7.100ton 36knot cruisers (Furutaka and Aoba classes)
6x 8.510ton 36knot cruisers (Mogami and Tone classes)

87??? 5.500ton 36knot cruisers (There were 14 such cruisers: 5x Kuma, 6x Nagara, 3x Sendai)
Where are the two Tenryus and the Yubari?

32x 2.000ton 36knot destroyers (There were 10x Fubuki, 10x Ayanami, 4x Akatsuki 10x Asashio (5x Kageros were laid down in 1937))
48x 1.400ton 36knot destroyers (15x Minekaze, 9x Kamikaze, 10x Mutsuki, 4x Hatsuharu, 10x 1.700ton Shiratsuyu)


I need a Japanese-English translator to decode this table!
 

G4H4CK256

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Are you sure that these table shows proposals? It looks to me as ship construction proposals or even wargame to counter the USN.
On the 2nd page the rightest most column shows USN BB names:
West Virginia
Colorado
Massachusetts
California
Tennessee
Idaho
New Mexico
Nevada
Arizona
the second row on the 3rd page contains rest of the USN BB's in 1937:
Pennsylvania
Oklahoma
Maryland
New York
Texas
Arkansas
missing from the list:
Utah, Wyoming, Mississippi

8x Unknown Battleships
4x Fast large Battleships, Note there were no 65.000ton (aka Yamato) 34knot / 63km/h BB design of the IJN!
2x 41.600tons likely the Nagatos
The Fusos and Ises seems to be missing.
The cruiser part is also weird.
4x 18.000ton 36knot cruisers
2x 15.600ton 36knot cruisers
2-2 other cruisers? Kongos maybe?

8x 10.000ton 36knot cruisers (Myoko and Takao classes)
4x 7.100ton 36knot cruisers (Furutaka and Aoba classes)
6x 8.510ton 36knot cruisers (Mogami and Tone classes)

87??? 5.500ton 36knot cruisers (There were 14 such cruisers: 5x Kuma, 6x Nagara, 3x Sendai)
Where are the two Tenryus and the Yubari?

32x 2.000ton 36knot destroyers (There were 10x Fubuki, 10x Ayanami, 4x Akatsuki 10x Asashio (5x Kageros were laid down in 1937))
48x 1.400ton 36knot destroyers (15x Minekaze, 9x Kamikaze, 10x Mutsuki, 4x Hatsuharu, 10x 1.700ton Shiratsuyu)


I need a Japanese-English translator to decode this table!
from what I understand, it looks like a table of required military forces at the end of 1940, after the collapse of naval treaty.
I made some translated version but it might be wrong to some extent

8x Battleships (戦艦) - Nagato class, Fuso class, Ise class, and Hiei + one more Kongo class?
4x Fast Battleships (高速戦艦) - 2x New fast battleships (新高戦, aka early concept of Yamato class), 2x Kongo class

4x Middle size Aircraft Carrier A (中空母 甲) - 18000t class (early concept of Shokaku class)
2x Middle size Aircraft Carrier B (中空母 乙) - Soryu, Hiryu
2x Large Aircraft Carrier (旧大空母) - Akagi, Kaga
2x Small Aircraft Carrier (小空母) - Hosho, Ryujo

8x Large Heavy Cruisers (大巡) - 10000t, Myoko class and Takao class
4x Middle size Heavy Cruisers (中巡) - 7100t, Furutaka class and Aoba class
4x Middle size Heavy Cruisers, modified (中巡改) - 8510t, Mogami class and Tone class (10in gun conversion was considered)

8x 5500t class cruisers (乙巡) as a flotilla leader + 4x 5500t class????

32x Large Destroyers (大駆) - DDs after Hatsuharu class?
48x First class Destroyers (一駆) - Fubuki class, Ayanami class, Akatsuki class + old DDs + 8 unfinished DDs?

etc.
 

Tzoli

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That 25cm armed Mogami I still find dubious as we know how lightly built Mogami to begin with. I can only think 4-5x1 25cm, not much a firepower increase over 15x 15,5cm or 10x 20,3cm
I can only take into account the old 25,4cm Type 38 guns designed in 1905 and equipped by the old armoured cruisers.
The guns data:
15,5cm/60 Type 90 -55,87kg shell with 925m/s MV -> Kinetic Energy: 23,9MJ
20,3cm/50 Type 90 no.2 - 125,85kg shell with 840m/s MV -> Kinetic Energy: 44,4MJ
25,4cm/45 Type 38 - 226,8kg shell with 825m/s MV -> Kinetic Energy: 77,18MJ

5x3x 15,5cm = 358,5MJ
5x2x 20,3cm = 444MJ
4x1x 25,4cm = 308,72MJ
5x1x 25,4cm = 385,9MJ
3x2x 25,4cm = 463,08MJ
4x2x 25,4cm = 617,44MJ

Energy wise 5x 25cm guns roughly equals to 15x 15,5cm and 6 guns equals to 10x 20,3cm and I did not take into account the fire rates of these weapons not to mention the much larger turret and barbette required for the twin 20,5cm turrets.
Only pros are the better range and better penetration capabilities of the 25,4cm gun with new AP shells.
We have no knowledge any new cruiser calibre weapon development of the Japanese in this period and if a new gun would had been designed that would take quite a few years to develop!
It sure gives interesting aspects for the Mogamis though...
 

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Did the Japanese consider building these even possible? What time frame did they
estimate on completion.
 

Tzoli

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Knowing this is the only mention of a 25cm armed Mogami I doubt any serious considerations were made.
 

Tzoli

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Part of my design number collection came from this blog entry:

Which mentions two other design numbers I do not know about: C-40 and C-43.
This C-43 was apparently not the Yuzuru Hiraga C-43 design as he too mentions that design.

C-40 was apparently to be part of the Maru 4 Programme which at first included but later left out 2x 13.000ton heavy cruisers (Improved Tone type) (according to the Japanese Wiki) so I think this would be the Tone-Kai type cruiser

C-43 was written as 計画のみ - Plan Only. Based on the design dates near it (C-42 - Oyodo of 1939 and C-44 - Agano-Kai of 1940) this design could be from around 1940 and might be the mysterious Heavy Cruiser that was planned around this time and of drawing exists from a news article showing a 4x3 20cm armed cruiser with aft torpedo tubes and catapults.
 

G4H4CK256

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Part of my design number collection came from this blog entry:

Which mentions two other design numbers I do not know about: C-40 and C-43.
This C-43 was apparently not the Yuzuru Hiraga C-43 design as he too mentions that design.

C-40 was apparently to be part of the Maru 4 Programme which at first included but later left out 2x 13.000ton heavy cruisers (Improved Tone type) (according to the Japanese Wiki) so I think this would be the Tone-Kai type cruiser

C-43 was written as 計画のみ - Plan Only. Based on the design dates near it (C-42 - Oyodo of 1939 and C-44 - Agano-Kai of 1940) this design could be from around 1940 and might be the mysterious Heavy Cruiser that was planned around this time and of drawing exists from a news article showing a 4x3 20cm armed cruiser with aft torpedo tubes and catapults.
View attachment 677886
the author says on the website that C-41~C-43 could be non-treaty cruisers or preliminary designs for Myoko class
also, I've never heard of the cruiser design with 4x3 20cm and aft torps (could you show me the drawing?)
 

Tzoli

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The first C-41-43 was from the Hiraga Archive and likely Hiraga's own designation. The later C-43 was the one I'm talking about.

That cruiser with 4x3 turrets was from this article:
Zao_01.png

Class_A_1941.jpg

and which already made into the WoWs as Zao (Originally Senjo or Unzen).

Officially we only know that 20cm cruisers were planned for the 1940 or 41 programmes. Ive not yet found any other info about this cruiser. Ive heard a rumor that Wargaming found evidence or even documents on them but I'm very skeptic about this.
 
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G4H4CK256

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The first C-41-43 was from the Hiraga Archive and likely Hiragas own designation. The later C-43 was the one I'm talking about.

That cruiser with 4x3 turrets was from this article:
Zao_01.png

Class_A_1941.jpg

and which already made into the WoWs as Zao (Originally Senjo or Unzen).

Officially we only know that 20cm cruisers were planned for the 1940 or 41 programmes. Ive not yet found any other info about this cruiser. Ive heard a rumor that Wargaming found evidence or even documents on them but I'm very skeptic about this.
ah, that one is totally fake (you can see the word "本書想定" on the top left, that means everything about this design is fabricated by the magazine)
 

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Even without the note that it is fictitious the floatplanes and catapults at the stern make me doubtful. As far as I know scouting for the fleet was an important role for the Japanese cruisers which is why they tended to have two catapults and space for at least three floatplanes amidship. Grouping all of the torpedoes at the stern also seems like it would make a vulnerable to a hit devastating the stern and crippling the steering of the ship.
 

Tzoli

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Indeed. Since it's emergence on the net like 10-15 years ago the number of issues were:
- The torpedo tubes are way too aft there is very little space for them and basically no space to reload the aftermost ones.
- USN style aft and single catapult for one floatplane (Even the Aganos carried two)
- Mainmast with crane for only boat transport. IJN used a single mast for both aircraft and boat handling, otherwise the boats lowered by davits. (Though there were a few Myoko preliminaries showing small cranes on the turrets for aircraft handling due to forward oriented mainmast)
- Rather "thick" hull eg the nose is quite wide not reminiscent of the fine lines of the IJN cruiser well known of.
- Turreted 10cm AA guns, well they sure are well protected but it would be more likely to use Oyodo style shielded mounts.
 

Tzoli

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Here I show you my drawings based on the data I was able to collect about the Myoko projects.
First the Hiraga pre-designs the C-41 and C-42:
Myoko Class Preliminary Design C-41
df5asz1-15a758a4-a598-4c2d-8a9d-2f5c59e82542.png


Originally the Myoko class heavy cruisers started their life as improved Furutaka / Aoba class cruisers with the required specifications of the Admiralty by late 1922:
- Maximum 10.000tons displacement
- 4x twin 20cm turrets, 3 forward 1 aft
- 4x single 12cm AA guns
- A torpedo armament of 4 twin 61cm Type 8 Torpedo tubes in the hull
- Protection against direct hit from 15cm and indirect hit from 20cm weaponry
- 10.000nm nautical range
- High speed of 35,5knots
- Carry 2 seaplanes

From the requirements Yuzuru Hiraga created the Myoko class heavy cruisers which were modified after laid down by Kikuo Fujimoto by the pressure of the Admiralty. As Hiraga was the lead designer of these cruisers as well as the previous Furutaka class there is a large amount of documents can be found in his archive. Among them I've found two designs which might be connected to the Myoko class development. These two designs labelled C-41 and C-42 by Hiraga (Note that these design numbers are off numbers at the time and later used for the Agano (C-41) and Oyodo (C-42) classes) and showing some early estimates of the Admiralty requirements:
- 36-36,5knots speed
- 125/140/160.000shp power plants
- 12-14.000ton standard displacement
- 4 twin 61cm torpedo tubes on the deck
- 2 single 12cm AA guns
- 4 twin 20cm main guns

For the C-41 design to reach such high speed Hiraga choose a long and sleek hull some 237,75m long on the waterline and 140-160.000shp powerplant. But the resulting long hull with long machinery spaces meant a large area to be armoured even with thinner and different plates than of the final Myoko class. The resulting large hull and long armoured area mean the design became quite heavy, 14.000tons standard which is basically twice that of the Furutakas and Aobas while the armament only slightly better with two more 20cm guns.
It is not known why the hull became soo long and heavy. The length could be attributed to early calculations for high speed but the heavier tonnage is questionable, likely the result of using more numerous but smaller boilers which might indicate four, rather then three funnels, but the data page only consist some basic data. On the other hand the weight of machinery indicates the contrary:
Myoko machinery weight: 2.689,5 / 2.730,2tons (48,3 / 47,6tons per shp)
C-41 140K shp version: 3.000tons (45,7shp per ton)
C-41 160K shp version: 2.900tons (55shp per ton)

None the less I've made this drawing based on the findings in Hiraga. I've chosen the required 3 forward 1 aft arrangement for the main turrets, a Furutaka style flying off platform and a logical placement for the 2 single 12cm AA guns on top of the hanger.

The design had the following characteristics:
Dimensions: 225,55 (pp) x 237,74 (wl) 21,64 x 5,33 meters
Displacement: 14.000tons (standard), 18.000tons (full load)
Engines: 140-160.000shp Kampon Steam Turbines, 4 shafts
Range: 18.500km at 26km/h (10.000nm at 14knots)
Speed: 68km/h (36,5knots)
Armour: 102mm over Machiney, 51mm over Magazines Belt, 47mm over Machinery, 25mm over Magazines Deck
Armaments:
4x2 20cm/50 Type 3 / 3rd Year Type Guns
2x1 12cm/45 Type 10 / 10th Year Type DP-AA Guns,
4x2 610mm Torpedo Tubes
2x Seaplanes, I've chosen Yokosuka E1Y

Original Documents:

Myoko Class Preliminary Design C-42
df5hgps-60926f85-1a07-4f42-b26a-9ff23645008e.png


Design C-42 was shown on the same paper as C-41 and looks like a smaller version of that design, saving 2.000tons by reducing hull length considerably and engine power moderately with some decrease in operational range. The design was still much longer and heavier than what became the Myoko, otherwise it has the same equipment as C-41.

To look different than a shorter C-41 I've re-arranged to turrets to 2 forward and 2 aft and changed the Furutaka style flying off platform to a regular flying off platform (the height would had been considerable if it would be put on a superfiring turret)

The design had the following characteristics:
Dimensions: 210,3 (pp) x 20,12 x 5,26 meters
Displacement: 12.000tons (standard), 15.200tons (full load)
Engines: 140-160.000shp Kampon Steam Turbines, 4 shafts
Range: 14.800km at 26km/h (8.000nm at 14knots)
Speed: 67km/h (36knots)
Armour: 102mm over Machiney, 51mm over Magazines Belt, 47mm over Machinery, 25mm over Magazines Deck
Armaments:
4x2 20cm/50 Type 3 / 3rd Year Type Guns
2x1 12cm/45 Type 10 / 10th Year Type DP-AA Guns,
4x2 610mm Torpedo Tubes
2x Seaplanes, I've chosen Nakajima C1N

Original Documents:
 

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Next are the 3 pre designs or maybe Kikuo Fujimoto's modifications insisted by the Admiralty_
Preliminary Design 1
df5jv2j-8cda9866-0a83-41f5-a82a-80c086e9bfa0.png


So far this is the earliest drawing I've found in the Hiraga archive connected to the Myoko class heavy cruisers development. Based on the description of the Myoko class development cycle from Eric LaCroix's Japanese Cruisers of the Pacific War, this design is one of the modified versions of Yuzuru Hiraga's own drawing. While the Admirality requested both good AA, heavy torpedo armament and 4 twin 20cm turrets for the new 10.000ton cruisers, Hiraga proposed and likely actually designed a cruiser with 5 twin turrets and no torpedo tubes implying that the hull mounted torpedo weaponry would be too dangerous for the ship and he either deleted them or used twin rotating tubes. When the class were still under design process, Hiraga travelled to Great Britain for study and naval diplomatic work, his assistant Kikuo Fujimoto taken over the design but he was not as strong willed or likely stubborn as Hiraga and he was forced by the Admirality to modify the designs to accept 8 fixed torpedo tubes and increased AA armament from 4 to 6 guns. This first change was likely resulted in this design you see here.
As you can see the torpedo tubes, 4 per side were located above the turbine room and under the aircraft handling facilities, the drawing does not include info on the aircraft or it's launching mechanism but later drawings show either a flying off platform or a place for the catapult. The hanger appears to be a smaller version used on the Furutakas and Aobas. Four 12cm DP-AA guns were shown with provisions for two more on the sides above the boiler rooms and the Bridge structure which was clearly based on the Furutakas, under construction at the time. The main armament shows the usual layout as found on the final Myokos and the follow up Takaos.
Armour wise the design is different, having 38mm deck instead of 35 of the final design and 102mm Machinery and 76mm Magazine Belt instead of the final homogeneous 102mm.

The design had the following characteristics:
Dimensions: Unknown, likely 192 (pp) x 201,5 (wl) 19 x 5,9 meters
Displacement: 10.000tons (standard)
Engines: 130.000shp Kampon Steam Turbines, 4 shafts
Range: 14.800km at 26km/h (8.000nm at 14knots)
Speed: 66km/h (35,5knots)
Armour: 102mm over Machiney, 76mm over Magazines Belt, 38mm Deck
Armaments:
5x2 20cm/50 Type 3 / 3rd Year Type Guns
6x1 12cm/45 Type 10 / 10th Year Type DP-AA Guns,
2x1 Machinge Guns, I've chosen 40mm/39 14th Vickers Pom-Pom / 14th Year Type HI AA guns
8x1 610mm Torpedo Tubes
2x Seaplanes, I've chosen Nakajima C1N

Original Documents:

Preliminary Design 2
df5m3hy-02a61326-ac38-4e79-a7d5-009176cec4df.png


This is another design as part of the Myoko development process. A modified version of the previous Design 1.
It shows four more hull mounted 61cm torpedo tubes, in this case forward of the front turrets and replacing the six 12cm single DP-AA guns by two twin ones on top of the hanger. A circle behind the 4th turret off centre indicates a possible location for a catapult hence I've put one there.

The design had the following characteristics:
Dimensions: Unknown, likely 192 (pp) x 201,5 (wl) 19 x 5,9 meters
Displacement: 10.000tons (standard)
Engines: 130.000shp Kampon Steam Turbines, 4 shafts
Range: 14.800km at 26km/h (8.000nm at 14knots)
Speed: 66km/h (35,5knots)
Armour: 102mm over Machiney, 76mm over Magazines Belt, 38mm Deck
Armaments:
5x2 20cm/50 Type 3 / 3rd Year Type Guns
2x2 12cm/45 Type 10 / 10th Year Type DP-AA Guns,
2x1 Machinge Guns, I've chosen 40mm/39 14th Vickers Pom-Pom / 14th Year Type HI AA guns
12x1 610mm Torpedo Tubes
2x Seaplanes, I've chosen Yokosuka E1Y

Original Documents:

Preliminary Semi-Final Design 3
df5oevu-8f34ebb3-3b51-46a0-9d6b-a878cde54e0e.png


This design is likely was the final design chosen for construction of the Myoko class cruisers.
It is essentially a modified Design 1.
But with 6 torpedo tubes per side above the turbine room, but instead of a catapult the drawing shows a flying off platform and cranes on the 4th turret to put the planes on it. The bridge superstructure still resembles that of the Furutaka though

The design had the following characteristics:
Dimensions: Unknown, likely 192 (pp) x 201,5 (wl) 19 x 5,9 meters
Displacement: 10.000tons (standard)
Engines: 130.000shp Kampon Steam Turbines, 4 shafts
Range: 14.800km at 26km/h (8.000nm at 14knots)
Speed: 66km/h (35,5knots)
Armour: 102mm over Machiney, 76mm over Magazines Belt, 38mm Deck
Armaments:
5x2 20cm/50 Type 3 / 3rd Year Type Guns
2x2 12cm/45 Type 10 / 10th Year Type DP-AA Guns,
2x1 Machinge Guns, I've chosen 40mm/39 14th Vickers Pom-Pom / 14th Year Type HI AA guns
12x1 610mm Torpedo Tubes
2x Seaplanes, I've chosen Nakajima C1N

Original Documents:
 

Tzoli

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Following are the unique triple and quad turret variants based on the data and calculations made by Hiraga:
Triple Turret Variant
df5qgtw-95dc7118-54e5-476b-ab6e-08143214021e.png


In a document I've found in the Hiraga archive shows various calculations for a 10.000ton cruiser using different hull shapes and thus sizes and what is more important the usage of triple or quadruple 20cm turrets with good barbette and turret armour! The calculations imply it was associated with the development of the Myoko class cruisers. The 1.032tons armament weight indicates either three triple or two quad turrets and two single 12cm DP-AA guns. By reducing the armoured area of the ship by using three or two turrets the belt and deck armour could be increased compared to the 5 twin turrets Myoko actually built with!
While the papers calculates 2 single 12cm guns I've put the usual 6 single guns as likely the Admiralty would had forced the designers to adopt this change.

The design had the following characteristics:
Dimensions: Unknown, likely 192 (pp) x 201,5 (wl) x 203,8 (oa) 19 x 5,9 meters
Displacement: 10.000tons (standard)
Engines: 130.000shp Kampon Steam Turbines, 4 shafts
Range: 14.800km at 26km/h (8.000nm at 14knots)
Speed: 66km/h (35,5knots)
Armour: 114mm over Machiney, 95mm over Magazines Belt, 51mm over Machiney, 38mm over Magazines Deck
Armaments:
3x3 20cm/50 Type 3 / 3rd Year Type Guns
6x1 12cm/45 Type 10 / 10th Year Type DP-AA Guns,
12x1 610mm Torpedo Tubes
2x Nakajima E4N Seaplanes

Original Documents:

Triple Turret Variant 2
df5z7bl-96f6ba98-8bab-499b-b18a-76e82abff5fd.png


This drawing was originally my first take on the triple turreted Myoko:
Because the Hiraga document hinted 12 barrels so either 4 triple or 3 quad turrets hence this 4 turreted version I show here.
I'm not sure it is possible to mount 12 barrels on 10.000tons but the usual splinter protection of the Japanese cruiser gun turrets and the shorter belt and deck armour compared to the 5 turreted original Myoko together with some other weight saving might be just enough to reach that goal.
Reduced ammo for standard loadout fewer torpedo tubes and DP-AA guns might be necessary as well.

Quad Turret Variant
df5wxku-da4d7dff-990b-42c8-b28d-30a714758e4e.png


In a document I've found in the Hiraga archive shows various calculations for a 10.000ton cruiser using different hull shapes and thus sizes and what is more important the usage of triple or quadruple 20cm turrets with good barbette and turret armour! The calculations imply it was associated with the development of the Myoko class cruisers. The 1.032tons armament weight indicates either three triple or two quad turrets and two single 12cm DP-AA guns. By reducing the armoured area of the ship by using three or two turrets the belt and deck armour could be increased compared to the 5 twin turrets Myoko actually built with!
While the papers calculates 2 single 12cm guns I've put the usual 6 single guns as likely the Admiralty would had forced the designers to adopt this change.
This is the quad turret version. While the armament was as specified by the Admiralty (8 guns) apparently Hiraga played with a quad turret arrangement which I present here. Though the two turrets seems inadequate, they each feature 4 barrels and good armour and allows the shortest belt and deck armour saving considerable weight, on the other hand sacrificing redundancy because one lucky hit could knock out half the armament of the ship.

The design had the following characteristics:
Dimensions: Unknown, likely 192 (pp) x 201,5 (wl) x 203,8 (oa) 19 x 5,9 meters
Displacement: 10.000tons (standard)
Engines: 130.000shp Kampon Steam Turbines, 4 shafts
Range: 14.800km at 26km/h (8.000nm at 14knots)
Speed: 66km/h (35,5knots)
Armour: 114mm over Machiney, 95mm over Magazines Belt, 51mm over Machiney, 38mm over Magazines Deck
Armaments:
2x4 20cm/50 Type 3 / 3rd Year Type Guns
6x1 12cm/45 Type 10 / 10th Year Type DP-AA Guns,
12x1 610mm Torpedo Tubes
2x Nakajima E4N Seaplanes

Original Documents:

Quad Turret Variant 2
df5z7bu-9595684b-2fa3-46e5-8408-08ec022bc107.png


This drawing was originally my first take on the quad turreted Myoko:
Because the Hiraga document hinted 12 barrels so either 4 triple or 3 quad turrets hence this 3 turreted version I show here.
I'm not sure it is possible to mount 12 barrels on 10.000tons but the usual splinter protection of the Japanese cruiser gun turrets and the shorter belt and deck armour compared to the 5 turreted original Myoko together with some other weight saving might be just enough to reach that goal.
Reduced ammo for standard loadout fewer torpedo tubes and DP-AA guns might be necessary as well.
 

Tzoli

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And finally the Myoko class as commissioned and the proposed upgunning of the IJN Haguro

Myoko Class Heavy Cruiser as Finished
df61bje-33e9ddc4-9af5-469c-849e-f6f7f48e7095.png


Here I show you the look of the Myoko class heavy cruisers as commissioned at first.
Originally the Myokos were ordered without any torpedo armament , 5 twin 20cm gun turrets and only 4 12cm DP-AA guns. Due to the requirements by the Admirlaity the design changed while under construction: 8 61cm deck mounted torpedo tubes were added which were later increased to 6 per side totalling to 12 tubes, and the AA armament enhanced by 2 more 12cm gun mounts. This however resulted in less living space for the crew and increased displacement, to fix these issues, a deck structure was erected around the bridge and the first pair of 12cm guns placed on them, while 2 turrets rangefinders got removed (originally all 5 20cm turrets would had their own range finders)
The difference between this and their pre-modernised state are the lack of splinter shields for the 12cm DP-AA guns.
Between 1928 and 1935, the AA guns received their protecting shields and various minor hull elements got changed, but otherwise the ships of this class looked like this.

The design had the following characteristics:
Dimensions: Unknown, likely 192 (pp) x 201,5 (wl) x 203,8 (oa) 19 x 5,9 meters
Displacement: 10.000tons (standard), 13.280tons (full load)
Engines: 130.000shp Kampon Steam Turbines, 4 shafts
Range: 14.800km at 26km/h (8.000nm at 14knots)
Speed: 66km/h (35,5knots)
Armour: 102 Belt, 35mm Deck, 32mm Slopes over Machinery
Armaments:
5x2 20cm/50 Type 3 / 3rd Year Type Guns
6x1 12cm/45 Type 10 / 10th Year Type DP-AA Guns,
12x1 610mm Torpedo Tubes
2x Nakajima E4N Seaplanes

IJN Haguro Upgun proposal
df63i3m-dbfb6145-8565-4f46-9cce-ce7a1554efae.png


Based on the description of the Japanese wikipedia about the 20cm guns:
https://ja.wikipedia.org/wiki/五十口径三年式二〇糎砲#仮称3号20cm砲
The Japanese started developing a new 20cm gun in the early 1930's, an 55 calibre gun similar to the USN's guns. One gun was manufactured in 1931 and tests showed promising results but the programme was cancelled in 1933 likely due to budgetary reasons. None the less the IJN decided to upgun the Myoko class heavy cruiser IJN Haguro as well as the 4 units of the Takao class which were finishing construction at that time. No data I could found on the performance of the gun but I've decided to use a turret similar to the triple 15,5cm ones.
Here I show you IJN Haguro in 1937 as it would looked like with the new guns.

The design had the following characteristics:
Dimensions: Unknown, likely 192 (pp) x 201,5 (wl) x 203,8 (oa) 19 x 5,9 meters
Displacement: around 10.535tons (standard), 15.390tons (full load)
Engines: 130.000shp Kampon Steam Turbines, 4 shafts
Range: 14.800km at 26km/h (8.000nm at 14knots)
Speed: 63km/h (34knots)
Armour: 102 Belt, 35mm Deck, 32mm Slopes over Machinery
Armaments:
5x2 20cm/55 Type 93 Guns
4x2 12,7cm/40 Type 89 DP-AA Guns,
2x4 13mm/76 Type 93,
2x3 610mm Torpedo Tubes
3x Seaplanes: Kawanishi E7K Alf and Nakajima E8N Dave
 
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Tzoli

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And here is the similar upgunning proposal for Takao:
df65k5y-ed705ac6-a8c1-4156-bce1-7ae38630d429.png


The design had the following characteristics:
Dimensions: 192,5 (pp) x 201,7 (wl) x 203,8 (oa) 19 x 6,11 meters
Displacement: around 10.535tons (standard), 15.390tons (full load)
Engines: 130.000shp Kampon Steam Turbines, 4 shafts
Range: 15.700km at 26km/h (8.500nm at 14knots)
Speed: 66km/h (35,5knots)
Armour: 102mm over Machiney, 127mm over Magazines Belt, 35mm over Machiney, 47mm over Magazines Deck, 32mm Slopes over Machinery
Armaments:
5x2 20cm/55 Type 93 Guns
4x1 12cm/45 Type 10 DP-AA Guns,
2x4 13mm/76 Type 93,
4x2 610mm Torpedo Tubes
3x Seaplanes: Kawanishi E7K Alf and Nakajima E8N Dave
 

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