The pressure per square inch is the same no matter what the area. Larger area = Larger force = Same PSI. The thickness of the hull matters, but the strength of the material matters even more. Normal steel has a yeild of 32,000 psi. High strength steel is 51,000 psi. HY-80 has a 80,000 psi yield. So 1/2" of HY-80 would be stronger than 1" of normal steel. As stated above, the US hulls were thinner but were made of a stronger material (I would assume, especially late war when Germany was running short of speciallty materials). Also, every engineering design has factors of safety. You rate something for a force when in reality it could take 5 times that force without yielding (example - any type of rigging equipment) The US subs were rated for 400' but one managed to go down to 1000' and survive (don't remember what boat, its posted around here somewhere).
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