From: La Salle, Colorado
A note on pre-London warship building, John: the key limitation for building carriers and light forces was money, given to IJN, not tonnage.
So, on the second thought, I'm against adding new capital ships in that period, except for the two extra battleships. Instead:
1)Rearm Aoba and Furutaka with 9x155mm cannons, instead of 6x203mm (Aoba class already underwent a reconstruction with turret changes IRL, so not that much extra expense) in early 30s. I admit, there is no small element of hindsight about the later prevalence of close range night combat here, but the supposed rationalization is the purpose of these ships either as scouts or elements of the newly-forming night fighting force.
Under the London treaty this, combined with extra allowed tonnage, will allow to build Mogamis as normal CAs, without any big rebuilds later (correct me if I'm counting wrong), in fact, it will allow to avoid excessive experimentation with the CA projects, sticking to the development of Takao class.
Take most 5500 tons CLs out of the fleet post-London, rebuild the oldest ones as auxliaries, trainders and fast minelayers, conserve some for later war emergency rebuild as cruisers with heavy torpedo or AA armament (I'll post the details I have in mind later). Meanwhile, use remaining CL tonnage post improved London to build a pair Oyodo-like CL, starting with 1933, with increased seaplane capabilities that initially will be supposed to serve as the new generation of scout cruisers, and eventually will be used for running with the carrier fleet. No need to sacrifice combat potential of CAs for that role. Alt_nav had the right idea here, by using a single design with small deviations for all heavy cruisers build in 30s, and extra tonnage limits will even allow to build all four post-London cruisers with 203mm armament from the beginning.
But I'm getting way ahead of myself...
2)Build one bigger Ryujo. In reality Japanese did not use up all the allowed carrier tonnage at that time. Just make project more rational. Official 12.5k of displacement you propose for it will be sufficient. With no rebuild it will not cost more money in the end.
I again propose to go through the fleet class by class, top-down. It is really hard to track all the proposals here.
So returning to post-treaties battleships. I take it, you still want to build two Yamatos and two B-65 cruisers for 6 25-27 knots BBs and 6 BCs, as in RA? That probably will be more expensive than reality, considering the need to develop armament for two new warship classes and with all expansions to shipyards and factories needed to build Yamatos, a mere two-ship series is hard to justify economically. I propose building 4 30-knots BBs with new 9x410/50 guns instead, as replacements for Fuso/Ise classes (which won't be sent to the scrapyards due to imminent war), as a compromise between the gun club and the carrier faction, slightly stronger than IRL, hepled by concerns about rearming and servicing Yamato-class ships on forward island bases during a projected extended campaign.
I will try to breakdown your comments and my responses:
1a. Rearm Aoba, Kako, Kinugasa, and Furutaka with 3 triple 6" guns instead of 3x2 8" guns. Little-to-no cost and I like building DECENT CL so this is good.
1b. With savings above as well as the 70% London Treaty we build all of the Mogami's (Mogami, Mikuma, Suyuza, and Kumano) as more developed/improved Takao's. No new hulls, more powerful ships, no reconstruction later (saving money): EXCELLENT!
1c. Convert old CLs to other purposes and begin construction of a new class of CL--Scout to escort the CVs and provide AA protection. Think I already proposed this somewhat. Build 2 in 1932, 2 more in 3rd Circle, and 6 in 4th Circle (replacing the Aagno's). No Yubari, no 3 Training Cruisers, and no Tone's.
On the CA front the Japanese could, based on what we are NOT building, add a set of final CAs with one pair in 3rd Circle and another pair in 4th Circle.
2. Build a bigger Ryujo instead of 2 CVL proposed in my earlier Post. GOOD. We could build Soryu and THEN use the 2 CVs budgeting in 2nd Circle for a pair of Hiryu Class. How about that?
3. Battleships: You misread my comments Posted above. NO YAMATOs! Instead we should build a pair of 3x3 16" BBs (3rd Circle) as designed to replace Fuso and build the two fast CB in 4th Circle OR a matching pair of BBs to go with the 3rd Circle design. Japanese could have either 2 BB and 2 BC OR 4 BB. I can go either way on this for it doesn't really matter.
How about that? Don't think we are too far apart and I really like your CA--CL ideas!