Alternate Midway

Combinedfleet.com also states that Junyo (and her sister Hiyo) also never made better than 23 knot during the entire war. The ship was also still shaking down when it was used at Midway, having been commissioned on 5/3/42 which was only 23 days before she sailed for the Aleutians (in all likelihood it had not even finished builder's trials, but she was needed for operations in the Aleutians).
I tried asking the experts about this and received some answers at http://www.j-aircraft.org/smf/index.php?topic=8933.0 and http://propnturret.com/tully/viewtopic.php?f=2&t=833.The general view was that Junyo could do 26 knots on a good day as she made 26 knots on trials but might not be able to do more than 23 knots under some conditions. I think that the IJN ran trials some time before commissioning ships, so I guess that Junyo was fully ready for service in June 1942. For example, Yamato ran speed trials on October 30, 1941 and was commissioned on 16 December 1941.
 
By the way, anvbody knowing if they had catapults? Launching torpedo bombers at 23 knots without catapults could be difficult, at 18 knots it was in fact impossible.
No IJN carrier used catapults in WW2. They tried one on Kaga pre-ww2 but it was a very bad design. They had started the design from the catapults used to launch floatplanes which have to be carried on a trolley and forgot that the aircraft that they were planning to launch had wheels! I think that they eventually used rockets to assist in launching aircraft, which might be rather too exciting. However, it is worth pointing out that the IJN was ahead of the rest of the world in landing aids http://www.ussessexcv9.org/pdfs/Japanese Carrier Operations.pdf.
 
IJN aircraft carriers in WW2 were equipped with catapults, but the aircraft un use did not use these, as all were light enough to get off the flightdecks, without assistance of the catapults, even when loaded with ordonance. Only the proposed successor of the B6N "Jill", the B7N "Grace" and the N1K1 "Shiden" heavy fighter were propably too heavy to do so, but these never operated from a carrier in the first place, as there simply were no left in operational service, by the time they came around.

A fully laden M1K1 in a fighterbomber role could carry some four AP bombs of 250 kg, where the smaller A6M type rarely exceeded a bombload of a single such bomb. Simmilarly the proposed B7N could carry a torpedo, like the Jill, but alternatively up to two tons of ordonance, where the B6N could only carry 1000 kg at best.
 
Catapults were included in the specification for various IJN carriers but were not actually installed. For example, here is a picture taken as a Japanese Navy Type 97 Carrier Attack Plane ("Kate") takes off from the aircraft carrier Shokaku, en route to attack Pearl Harbor, during the morning of December 7th 1941 that I found at http://warphotos.basnetworks.net/World_War_Two/Pearl_Harbour_Raid_-_December_7th_1941/83/bound_for_pearl_harbour/. There does not seem to be any catapult visible.

bound_for_pearl_harbour.jpg
 
Actually, at least according to her table of movements from the terrific combinedfleet.com, she was in the IO, but not a part of the Kido Butai.

There is no mention that she was attached to CarDiv 1, 2 or 3. The quote above is also found in several other sources, all of which I believe use combinedfleet.com as the source. So Ruyjo was in support of, but not part of, the main strike force.

In July of 1942 she was, temporarily, assigned to CarDiv 5 in July of 1942, following the diasaster of Midway, but was detached a month later for recon operations. On 8/23/42 she attacked the U.S. carrier force in the Battle of Eastern Solomons in conjunction CarDiv 5 but while under separate command by Commander CruDiv 8 and was sunk by aircraft from VB-3 & VT-3 the following day while operating independently from the Main Body.

For various interpretations of "operated" and "with."

Remember that the OP did not have Ryujo with Kido Butai, but rather operating directly with Junyo and Zuiho to attack Midway's defenses.
 
The Japanese lose another deck. The battle was more than a upset. It showed that, even at the early point in the war that the Japanese basic combat theory for carrier combat had serious, and, as it turned out, fatal flaws.

These included the regrettable (from IJN perspective) habit of A6M pilots of removing items they believed added weight to their aircraft (like radios), the belief that the carrier itself was the best place to site the primary AAA defenses, and the weakness of the IJN primary AAA weapons. It also provided confirmation of the difficulty that IJN fighter had in destroying U.S. aircraft once their fairly small supply of 20mm ammunition was expended.

From the American perspective it was also the battle that illustrated that, while a single F4F was clearly inferior to a single A6M (assuming equal skills for the pilots), a pair of Wildcats could outfight nearly any number of Zeros if the USN pilots kept their heads about them. The validity of the pre-war decision of the USN to emphasize deflection aiming and approach was also confirmed (it was this skill set that made the famed "Thatch Weave so effective). The relative value of the USN carrier escort concept, the advantages of active fighter direction, and the stunning advantages that air search radar provided to a carrier formation were proved as well.

It demonstrated the remarkable lack of survivability built into the IJN carrier designs and exposed the IJN's failing in D/C, something that it never fully overcame. It also illustrated very nicely the problems that fully enclosed hanger decks presented in the era before readily available fiefighting foams.

It also showed the remarkable ability of the Yorktowns to absorb bomb hits and keep on fighting. Unfortunately it also showed the weakness of the design to underwater damage, something that would be repeated in the Solomons campaign to the American's sorrow.

You don't explain how they lose another deck. With Ring leading Hornet's planes off on a wild goose chase, American airgroup strength isn't overwhelming against two decks. With two decks, Yamaguchi Tamon can mount better retaliatory air strikes and maintain better CAP over Zuikaku and Hiryu. The strike that sunk Hiryu was very hodgepodge. I can see the IJN leaving with two decks remaining and I can see Yamaguchi getting them both sunk. I can also envision Spruance losing a deck to a larger strike than the ones that Hiryu managed to mount - especially if Tomonaga can sink Yorktown with a larger first strike.
 
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