I'm working on the design for a dry-hull model submarine. My plan is to cast the hull shape that I want out of aluminum because this type of casting is cheap, easy to make, and because aluminum has a good strength to weight ratio. I want the hull to be able to withstand an external pressure of at least 173psi(the pressure at the bottom of the deepest lake within driving distance of my house) but, with a safety factor, I'd prefer that it can withstand an external pressure of 250psi. The submarine will be cylindrical in shape with an inside diameter of 6-8" and a length of 4-5'. The problem is that I'm not entirely sure how thick to make the hull walls. I've done a lot of googling but what I find are a lot of references to pipe or pressure vessel design where the pressure vessel is subjected to internal pressure not external pressure. The trouble is that this type of pressure vessel will exhibit a completely different failure mode than the one that I'm trying to prevent. Consequently, I have a couple design decisions in front of me and I'm not entirely sure how to approach them. 1) How do I calculate the minimum hull thickness to get the strength that I'm looking for? 2) Do I need to place re-enforcing bulkheads on the interior of the submarine to prevent buckling and, if so, how to I calculate the spacing between them? 3) If I pressurized the interior of the submarine to match or exceed the external water pressure(to allow the computer to detect leaks and ascend to the surface) how would this affect my previous two design questions? Any comments, thoughts, or suggestions are greatly appreciated!