Submarines that descend to great depths must withstand very high external pressures. since their interiors have to be at atmospheric pressure to accommodate human life, they are like a pressure vessel except with external pressure rather than internal pressure. while there is an external hull that is hydrodynamic, it does not carry the pressure difference. instead, a cylinder shell carries the pressure difference. if seawater has a density of 1027 kg/m3, what should be the wall thickness of the cylindrical shell with a 13 m internal diameter when diving to 400 m, if the normal stress in the steel is limited to 200mpa?