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For scale, the submarine is 2,200 tons total, so it’s a little over 3% overweight.

Which is more than enough to put it out of balance, depending on where it is.

Apparently the fear is much buoyancy issues than balance, the extra 70t means even with empty ballasts the sub might not be positively buoyant.

That's why this could be solved by lengthening the hull, increasing displacement and thus compensating for the extra weight.