Question

the hulls in submarine have to be specially strengthendend

Answer 1

Because of the nature of the failure.Internally pressurized vessels fail after the stress induced by the internal pressure in part or over a area of the vessel exceeds the materials strength. In contrast, a external pressure failure means the vessel can no longer supports its shape and (usually suddenly) takes on a new lower cross-sectional shape. This could occur at stresses below the yield strength of the material as other factors play an important part, such as deviation in roundness.Such a deviation if roundness, could result in very high induced stresses in the material when subjected to external pressures. This is why the finding of a dent on the “new” Canadian submarine HMCS Victoria (HMCS Victoria (SSK 876)).This dent restricted the operational depth the submarine could go to. Too deep and the hull would probably reach a point of failure and loss of the submarine and (probably) her crew.So the resistance a material has to failure improves its use for submarine construction. The steel used for most of the submarines constructed for the US Navy was HY-80. This was changed to HY-100 for the Seawolf classes and HY-120 for the Virginia classes The Soviets/ Russians constructed submarine using Titanium which had a strength comparable to HY-100.According to open sources, the Virginia class can operate at depth of 1600′, the Seawolf class up to 2400′. That works out to almost 700 psi for 1600′ and over 1000 psi at 2400′. That's a lot of external pressure to resist.