From what I have observed from living in Russia and an ex-soviet republic, it is obvious the Russians did not get really excited about calculating “strength of materials”. Probably because one never knew if the material produced was going to be consistent in specs. So the mentality was just to throw more material at the project and hope for the best.
I am surprised they didn’t build subs with 3 hulls.
They did, I’m pretty sure. I might be thinking of 2 pressure hulls, and one outer one, but I think they actually built a few with three pressure hulls, and an outer one.