Why the I-400 had to be so large

At 5,307 tons surfaced and roughly 6,670 tons submerged, the I-400 was comparable in displacement to the Arethusa-class cruisers HMS Penelope and HMS Aurora. Its immense stated range, four diesel engines, and duplicated systems allowed it to cross oceans without a nearby base. The three Seiran aircraft and eight forward torpedo tubes mattered, but the design was organized first around endurance and the ability to appear far from Japan. Its crew of 144 was modest for the hull's size, while its limited torpedo reloads show that displacement did not translate into a proportionately heavy conventional armament. Modest surface and submerged speeds show the tradeoff clearly: this was a long-distance raider rather than a fast tactical submarine.