Comments by "Gregory Wright" (@gregorywright4918) on "IJN Hiyo - Guide 334" video.

It makes sense given their cruise liner origins, although it broke with IJN tradition to try to vent below the flight deck off to the sides. Most of the other nation's carriers brought the smoke up through the island, though the angling was cute.
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The Shokakus had an extra 2,000 tons and 100 feet in length, which meant a much bigger hanger deck, thus a bigger air group. They had triple the SHP, and engines designed to be run often at the max, compared to the Hiyos breaking down so much due to being pushed too hard. The Independences were less than half her size, but built on the hulls of Cleveland-class cruisers with 4 shafts and 100,000 SHP.
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She was often delivering aircraft to southern bases, and she still had the speed to evade most subs, though breakdowns would plague her and lead to more hits.
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There's a reference in one or two works to them being based on the SS Bremen, just scaled down, but that could explain the high-pressure boilers. Most works I've seen say they were replaced with Kampon boilers, the standard IJN set. Would be interesting to see design sheets for the original cruise liners. Laid down 30 Nov. 1939, sold to IJN while building 10 Feb. 1941, launched 24 June 1941, commissioned 31 July 1942. The time from sale to launch was only 4 months, so they were probably not converted very much before launch, just finished for hitting the water, but that is usually when the boilers and engines are sealed in. Might be enough time to upgrade the boilers. That would make some sense as it would mean the boiler staff would be familiar with Kampons. The longer time from launch to commissioning would be the changes to rip out the cruise liner fittings and install hangar deck, flight deck, island and gun sponsons. Of the XCV you listed, the Leviathan had the same speed but was much bigger (and German-built, with higher-pressure boilers), while the other two were similar sized but only about 20 knots.
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It did make for a tough 1942, but mid-43 things turned as the new-builds came online.
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They are 7,000 tons bigger, but only have half the sets of geared turbines and shafts and one third the SHP compared to the Hiryus. They lasted longer, but that was by skipping the first three "major" naval air battles and suffering engine problems in the fourth.
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@AsbestosMuffins Interesting what-ifs on both sides, but note the general IJN propensity for under-armoring their ships and aircraft. They got better range and more bang for the ton, but could not take the hits like the better-protected USN ships.
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You can do that when you know they will only steam on land-locked lakes where the most dangerous antagonist might be a drunk Canadian moose.
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