Hull, trim, mine protection, and acoustic coating

The I-400's double-bubble hull provided strength beneath the aircraft hangar. Its special trim system was meant to hold the boat nearly stationary underwater near the launch point while aircraft returned by dead reckoning, but it was noisy and crews doubted they would use it. Demagnetization cables running from stern to the bow planes were intended to reduce mine danger and perhaps permit operation near or inside enemy minefields. I-401 later received a German-made snorkel, while the class used German-made periscopes brought through the blockade. Japan's anti-sonar coating drew partly on information exchanged with Germany but required domestic research and different materials, so it cannot be described as a simple copy.