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The Japanese merchant marine suffered heavily from submarine attacks during WW2, partly because they were largely unescorted. In addition to holding back as many ships as possible for the decisive battle, the IJN designed ships which were poorly suited to being ASW escorts and didn't think much of the job in general. As others have remarked this is strange for an island nation which depends on overseas trade, especially since they had been Allied with the UK in WW1 and presumably heard something about the U-boat offensive and it's effects.

So let's say there is a hardworking group of officers tucked away in the back of IJN HQ who are at least aware of the potential issue and decide to do what they can about it. They can't change naval policy very much or decide the construction of many ships - the decisive battle is very important, after all. But they can try to come up with a design that can be built in civilian yards and is suitable for escorting merchant ships, thus releasing destroyers and cruisers for real warrior's work. They can probably get a few examples built, and start working out how to employ them.

What might they come up with?
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