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IJN AA cruisers

ceccherini

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Japanese Wikipedia entry on Akizuki class DD stated that IJN started in 1936 to study dedicated AA vessels following the example of Dido class. They contemplated anything from a 1000 ton torpedo boat armed with a single 10 cm or 12.7 cm AA gun to a quite large cruiser equipped with 12 double 10cm/65 mounts, with serious consideration on converting part of the existing 5500 ton type light cruisers rearming them with 12,7cm/40. Akizuki was the result of this requirement, it's size and gun number being considered the best choice for optimal distribution of the fleet's AA capabilities. By 1941 a new class of AA cruisers was being considered: this cruisers, no.815-818, were not intended to offer a complementary higher end AA capability but only to serve as sort of command ship for Akizuki class DD. Has anyone any more information and pictures of this various projects?
 

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JFC Fuller

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Over the years there seems to have been considerable confusion on this topic and I very much doubt I have got it right myself but I think there are three different ship types caught up in this story:

Kai-Akizuki (V7 design): an improved Akizuki class that combined the Akizuki class' armament with Shimakaze machinery (according to some English language sources), I have seen some sources claim that the Kai-Shimakaze class were subsequently replaced in the programme by Kai-Akizuki class ships (though none of either class were completed)

Skokatu Escort cruisers no.815-818 (V18 design): Apparently intended as leaders for Akizuki groups with additional flag facilities and some seaplanes and catapult

Oyodo class AA conversion: Supposedly a proposal to convert in-construction or already programmed Oyodo class ships to an AA configuration with 10-12 x 10cm guns. No idea how serious this proposal actually was; Oyodo was completed as designed and the second ship (Niyodo) was cancelled without being laid down (though materials may have been gathered and machinery started???)
 
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Tzoli

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That one drawing is probably the first artist impression of the proposal of the AA cruiser

Oyodo's hull was capable to hold such large amount of guns as my drawing suggests:

And in Japanese Cruisers of the Pacific war by Eric Lacroix states as follows:
1569177307437.png
1569177028887.png
1569177384920.png
Super Type A cruisers: The battlecruisers or large cruisers of the B-65 class intended to counter the USN's Alaska class which in fact intended to counter partly the German Deutschlands but mostly the fake Chichibu class big gun cruisers.
Type A cruisers: Heavy cruisers of 203mm Guns
Type B cruisers: Destroyer squadron leaders of the Agano and Kai-Agano class
Type C cruisers: Submarine squadron leaders of the Oyodo class
 

JFC Fuller

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Lacroix pretty much confirms what I wrote above but for added context:

The V18 design AA cruisers were intended to be 5,800 tons standard with 4 x 2 10cm HA guns. I assume there would have been two Type 94 HA directors for fire control. Beyond that the only reason I can think for them getting to be so much larger than the Akizuki class is the addition of Agano style Torpedo (2 x 4 tubes) and catapult (1 x 26m) along with more protection and endurance. They were to have formed their own Sentai and were therefore not leaders for Akizuki DesRons.

Also confirms that the 24 x 10cm gun cruiser was a proposal for converting (during construction) Oyodo and Niyodo, made sometime around 1938-1939. IMO it would have made them much more useful ships but would have increased demand for the 10cm guns and mountings. It doesn't seem to have been an idea that was take particularly seriously though. Looking at the plans of Oyodo I think it would have been hard to convert the ship to the configuration shown above unless she had been in the very early stages of construction, her wing 10cm guns were open backed and had their own magazine space, additional magazine space could only have been found where the 15cm magazines were and in the bomb shed under the hangar. Also, that configuration shows no aviation facilities which would be unique for a Japanese cruiser design of this period.
 
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Tzoli

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Some time ago I made various layouts which I deem possible for the arrangement of the 12 twin mounts on the Oyodo hull:
 

JFC Fuller

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I admire your thoroughness, having thought about it I suspect your layout I/1 would have been the most likely configuration based on the existing internal layout of the Oyodo class and general Japanese practice - depending how far construction had advanced when the switch was considered. It may not be the most efficient arrangement from a gun direction perspective though.
 
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Foo Fighter

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Just a thought, how long could the stores of ammunition allow sustained fire in a saturation attack? Not that long imho.
 

Hood

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Yes the internal layout would dictate the turret locations, not to mention the topweight.
I don't think its necessarily true that the centreline arrangements would be symmetrical fore and aft. Given the relative blank canvas the rear portion of the ship is once the aviation facilities are removed more guns may of been intended aft that forewards. Likewise two beam mounts per side feels the likely maximum.

I have some doubts about the drawing ceccherini posted, I think this was from a (1980s?) Japanese magazine in an article on the unbuilt cruisers. Some of the other drawings in that article have been deemed to be speculative at best.
 

ceccherini

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Lacroix pretty much confirms what I wrote above but for added context:

The V18 design AA cruisers were intended to be 5,800 tons standard with 4 x 2 10cm HA guns. I assume there would have been two Type 94 HA directors for fire control. Beyond that the only reason I can think for them getting to be so much larger than the Akizuki class is the addition of Agano style Torpedo (2 x 4 tubes) and catapult (1 x 26m) along with more protection and endurance. They were to have formed their own Sentai and were therefore not leaders for Akizuki DesRons.

Also confirms that the 24 x 10cm gun cruiser was a proposal for converting (during construction) Oyodo and Niyodo, made sometime around 1938-1939. IMO it would have made them much more useful ships but would have increased demand for the 10cm guns and mountings. It doesn't seem to have been an idea that was take particularly seriously though. Looking at the plans of Oyodo I think it would have been hard to convert the ship to the configuration shown above unless she had been in the very early stages of construction, her wing 10cm guns were open backed and had their own magazine space, additional magazine space could only have been found where the 15cm magazines were and in the bomb shed under the hangar. Also, that configuration shows no aviation facilities which would be unique for a Japanese cruiser design of this period.
But Oyodo was finalized in late 1939 if I remember correctly. In late 1938 they were still undecided about building a light carrier, a cruiser-carrier or the Oyodo as actually built to satisfy the requirement. It's doubtfull they consider the convertion of a ship that was not even fully designed. If this was the case why not make a dedicated project from the ground up? Maybe they were contemplating a new project based on a hull derived by agano/oyodo, as the speculated Japanese "Atlanta" was always reported to be in the various publications that made mention of it.
 

ceccherini

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That one drawing is probably the first artist impression of the proposal of the AA cruiser

Oyodo's hull was capable to hold such large amount of guns as my drawing suggests:

And in Japanese Cruisers of the Pacific war by Eric Lacroix states as follows:
View attachment 619150
View attachment 619149
View attachment 619151
Super Type A cruisers: The battlecruisers or large cruisers of the B-65 class intended to counter the USN's Alaska class which in fact intended to counter partly the German Deutschlands but mostly the fake Chichibu class big gun cruisers.
Type A cruisers: Heavy cruisers of 203mm Guns
Type B cruisers: Destroyer squadron leaders of the Agano and Kai-Agano class
Type C cruisers: Submarine squadron leaders of the Oyodo class
Wonderful drawings, really actractive designs expecially the first and the second
 

Tzoli

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Well oyodo ordered in 1939, laid down in 1941 February, launched 1942 April and finished in 1943 February so a project in 1941 to finish her as a CLAA wasn't out of practicality as in mid 1941 the hull might sufficiently incomplete to be finished to such a modified design.
And here is her internal layout:
1569250626511.png
 

JFC Fuller

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But Oyodo was finalized in late 1939 if I remember correctly. In late 1938 they were still undecided about building a light carrier, a cruiser-carrier or the Oyodo as actually built to satisfy the requirement. It's doubtfull they consider the convertion of a ship that was not even fully designed. If this was the case why not make a dedicated project from the ground up? Maybe they were contemplating a new project based on a hull derived by agano/oyodo, as the speculated Japanese "Atlanta" was always reported to be in the various publications that made mention of it.
We can only speculate but I can imagine a scenario whereby the IJN had money for the ships approved, materials were about to be ordered and slots on slipways were identified when someone who was (rightly) unconvinced of the concept decided to see if those resources could be put to better use. Either way, it never seems to have been a very serious proposal.

Like Hood I have concerns about the image in the first post:

1) I see no reason why the fore-most and aft-most turrets are raised one deck above the flush-deck hull, if anything it would increase issues with top weight
2) The big gaps between the aft and foremost turrets and those directly behind them also make little sense - three closely located turrets like a Dido would be more logical
3) As mentioned previously, the absence of torpedo gear (especially) and aircraft facilities is also very unusual for a Japanese cruiser of this era
4) Director layouts, I see no reason for these to be all centreline, one on either beam (as in Oyodo as built) and one fore and one aft in the usual locations again seems more likely
 
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Tzoli

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For torpedoes and Aircraft:
For a dedicated AA cruiser, torpedoes would be not of much use as it's mission was fleet AA defence and not night torpedo attack on enemy fleet,s though that many 10cm guns would devastate any destroyer and would be a menace to cruiser if it could get in range.
For aircraft the facilities which the IJN adopted: The midship catapults would take up precious space for AA armament and again AA defence does not require scouting and spotting.
 

JFC Fuller

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For torpedoes and Aircraft:
For a dedicated AA cruiser, torpedoes would be not of much use as it's mission was fleet AA defence and not night torpedo attack on enemy fleet,s though that many 10cm guns would devastate any destroyer and would be a menace to cruiser if it could get in range.
For aircraft the facilities which the IJN adopted: The midship catapults would take up precious space for AA armament and again AA defence does not require scouting and spotting.
I don't buy this. The Akizukis were designed to provide AA defence to carrier groups and ended up with torpedos, the Japanese generally put them on everything. Similarly Japanese doctrine emphasised the use floatplanes for reconnaissance - even if not for the ship actually carrying them. I am less concerned by the lack of aircraft but the lack of torpedos and some of the other design features are quite suspect.
 

ceccherini

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But Oyodo was finalized in late 1939 if I remember correctly. In late 1938 they were still undecided about building a light carrier, a cruiser-carrier or the Oyodo as actually built to satisfy the requirement. It's doubtfull they consider the convertion of a ship that was not even fully designed. If this was the case why not make a dedicated project from the ground up? Maybe they were contemplating a new project based on a hull derived by agano/oyodo, as the speculated Japanese "Atlanta" was always reported to be in the various publications that made mention of it.
We can only speculate but I can imagine a scenario whereby the IJN had money for the ships approved, materials were about to be ordered and slots on slipways were identified when someone who was (rightly) unconvinced of the concept decided to see if those resources could be put to better use. Either way, it never seems to have been a very serious proposal.

Like Hood I have concerns about the image in the first post:

1) I see no reason why the fore-most and aft-most turrets are raised one deck above the flush-deck hull, if anything it would increase issues with top weight
2) The big gaps between the aft and foremost turrets and those directly behind them also make little sense - three closely located turrets like a Dido would be more logical
3) As mentioned previously, the absence of torpedo gear (especially) and aircraft facilities is also very unusual for a Japanese cruiser of this era
4) Director layouts, I see no reason for these to be all centreline, one on either beam (as in Oyodo as built) and one fore and one aft in the usual locations again seems more likely
Clearly it is a speculative drawing, still the turrets raising make sense to me: splashes interferences are reduced without the displacement penalties of an equivalent increase of the freeboard. Also distancing the turrets improve the firing arc, Worcester was similar but with much longer guns, so I didn't see a problem with it even in a shorter hull. Also Mogami had a similar disposition, so it is historically plausible. But yes, the absence of a floatplane carrying capability seems not very plausible. For torpedos: the actual Oyodo hadn't them while intended as a surface combatant, I would be surprised if they put them in an AA conversion or in an AA derivate.
 
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JFC Fuller

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Clearly it is a speculative drawing, still the turrets raising make sense to me: splashes interferences are reduced without the displacement penalties of an equivalent increase of the freeboard. Also distancing the turrets improve the firing arc, Worcester was similar but with much longer guns, so I didn't see a problem with it even in a shorter hull. Also Mogami had a similar disposition, so it is historically plausible. But yes, the absence of a floatplane carrying capability seems not very plausible. For torpedos: the actual Oyodo hadn't them while intended as a surface combatant, I would be surprised if they put them in an AA conversion or in an AA derivate.
The issue for me is the contradiction; if the design objective is to reduce the length of the armoured box then closely spaced/all superfiring turrets make sense, but they come with the downside of added top weight (CoG problems). If top weight is going to be a problem (e.g. on a ship with heavy turrets like the Worcester class) then the Mogami/Ibuki layout makes sense. The picture in the first post combines the worst of both layouts, a long armoured magazine length but with the entire armament pushed up a deck and therefore negatively impacting top weight (and increasing the amount of armour needed to cover the additional vertical surfaces). Furthermore, there never seems to have been an issue with the Oyodo or the Akizuki's having spray impact their foremost turrets thus rendering this suggested layout seemingly unnecessary.
 

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Also, JFC, I seriously doubt having the heavy AA mounts on the beam ends given the impact on handling, especially with inclination. At best single rifle mounts with open backs. At least with midships alignment the effect on the ship's handling is lessened once you lose weuight deep down when you deplete the magazines.
 

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Skokatu Escort cruisers no.815-818 (V18 design): Apparently intended as leaders for Akizuki groups with additional flag facilities and some seaplanes and catapult
Does anyone have a drawing for this one? I have never heard of it before.

Dave G
 

Tzoli

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That is the true CLAA design from start. Think of it as mixing the Akizuki with the Agano:
4x2 10cm, Catapult for seaplanes, probably torpedoes and probably optional DC. Hull most likely the same size as Agano
 

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The turrets look too spherical for the 10cm Type 98 Guns, otherwise nice models
 

Dilandu

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Well oyodo ordered in 1939, laid down in 1941 February, launched 1942 April and finished in 1943 February so a project in 1941 to finish her as a CLAA wasn't out of practicality as in mid 1941 the hull might sufficiently incomplete to be finished to such a modified design.
And here is her internal layout:
Thank you for the plan! I'm toying with the idea of AU-Oyodo refitted as a missile cruiser for adapted Funryu-2 SAM system -

1591454100758.png

- and those blueprints are very helpful.
 
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