@NOMISYRRUC Interesting in that table is the Graf Zeppelin is among the largest of the carriers, only beaten/rivalled by the Essexes, Shokakus, Ark Royal and Eagle. So the Germans, with no experience in building carriers, thought it was a good idea to start with the biggest possible. That's a bit like a starting architect planning the Empire State Building as his first project.
I was surprised by how close Graff Zeppelin and Essex were in terms of waterline length and beam. However, according to Whitley in German Capital Ships of World War Two it was intended to be a much smaller ship.
The chapter on the aircraft carriers says that the design of what became Graff Zeppelin began in March 1934 one year before the denunciation of the Treaty of Versailles and 15 months before the Anglo-German Naval Agreement. The intention was to order one ship in October 1935 as part of the 1935 Programme. The basic requirements were:
- Theatre of operations - Atlantic and North Sea
- About 15,000 tonnes displacement
- Speed 33 knots (continuous)
- Armament nine 15cm or six 20.3 cm guns with a strong flak outfit
- Endurance 12,000nm
- Cruiser protection
- 60 aircraft, a third of them with folding wings
- Two catapults
- The Air Ministry had indicated a minimum flight deck length of 180m.
It also said that the design team had obtained technical details of Lexington and was using Courageous as a useful starting point. It also said that improving relations with Japan allowed them to send a team to inspect Akagi and as a result of that visit numerous alterations were made including adding a third lift and an extension of the flight deck. The sentence before that says that they believed that a larger number of smaller ships would have been better, but the needs of the aircraft complement and and minimum flight deck length pushed the displacement inexorably upwards.
The above aircraft carriers were all bigger than the 15,000 tonne ship planned in March 1934. In the case of Akagi and Lexington considerably so. Therefore, the size of the ships that they were using as a guide (in addition to the aircraft and flight deck length requirements) may have resulted in Germany's "first time" design being so large.
OTOH (if the Langley and Hosho are ignored) the first American and Japanese aircraft carriers were their capital ship conversions. These were even larger than Graff Zeppelin. Although I admit that they wouldn't have displaced that much had they been designed as aircraft carriers from the keel up. Plus 2 of the 4 British prototypes (Eagle and Furious after her 1921-25 rebuild) displaced over 20,000 tons. Therefore, the early American, British and Japanese aircraft carriers were also on an "Empire State" scale.
I should also add that the aircraft carrier quota of 135,000 ton of aircraft carriers for the British Empire and the 27,000 ton limit for individual aircraft carriers in the Washington Treaty was because the Royal Navy wanted five 27,000 ton aircraft carriers in the early 1920s but as the decade progresses they found that they could design smaller ships with the characteristics they wanted which is partially why the abortive 1931 Carrier and Ark Royal displaced 22,000 tons. Therefore, Graff Zeppelin's size could have been the result of inexperience rather than any deliberate intention to build the largest ship possible.
Having mentioned Langley and Hosho the Reichsmarine should have had a contingency plan for a quick and simple merchant ship conversion when political conditions allowed. This would have been along the lines of their OTL contingency plan to build U-boats when political conditions allowed. The plan should have been implemented as soon as possible after the Nazis came to power (January 1933). The purpose of the ship would be to get some practical experience that could be incorporated into the design of later ships and to act as a training ship for the aircrew and ground crew of the later ships.
According to Whitley (again) the Reichsmarine laid the foundations of a naval air arm in October 1928 when it obtained government approval for a few seaplanes for "experimental" purposes. The excuse for this was the fact that the Treaty of Versailles allowed the Reichsmarine to retain anti-aircraft guns. This was interpreted as also permitting aircraft to tow the necessary targets! (Whitley's exclamation mark.) In this way an organisation known as "Air Services Incorporated" was formed as a cloak for its illegal activities in the fleet. Whitley finished by saying that although eventually killed off with the advent of Herman Goering and his "everything that flies belongs to me" attitude, this service did constitute the basis of a naval air arm.
So based on the last paragraph I think October 1928 is a plausible POD for the scheme to convert a suitable merchant ship into an experimental aircraft carrier.