The opening establishes the I-400 as the largest submarine built before the nuclear era. Its safe diving depth was only about 100 metres, roughly 82 percent of a hull more than 120 metres long. That geometry meant an excessively steep descent could put part of the boat beyond its safe depth before the entire hull had levelled. The comparison is a useful reminder that extreme size can create handling limits as well as extra capacity.
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Size imposed an unusual diving constraint
The class's record-setting dimensions left little margin between its length and safe diving depth, making dive angle a practical safety concern.