[PSUBS-MAILIST] Pressurized double or triple layer hull ?? Any idea ?

I'm new here (just join the list this morning) and happy to find a group of people interested in personal manned subs. My name is Benoit Bissonnette and I'm a scuba diver. I'm interested in the underwater life and I'd love to eventually build a sub that would carry me more confortably into our Canadian's cold waters or deeper and longer than a diver would endure. I've read quite a bit on the subject and I have a good idea of what to expect or maybe not...

I had an idea for a design and I'm wondering if you guys can give me your opinion on it. Since I would like to design a smaller sub with a smaller volume (basically giving up the cylinder shape for a more "man shaped" hull to reduce the air volume), I tough of building a solid and structured hull. The reason I'm looking for a shape like this is for more comfort (a sit position) as well as reducing the volume of air that need to be neutralized. I wish I can end up with a lighter sub that doesn't need tons and tons of equipment to move. Anyway, since the geometry of the hull won't be circular, I know that a tremendous stress will be applied on the different element of the structure. I had an idea of building let's say a two or three hermetic layer hull and pressurize each air gap with a positive pressure. If at 100ft you get around 45PSI on the structure, pressurizing the internal air gap at 30PSI wouldn't relief some pressure on the structure ? Instead of having one structure or layer resisting from 0 to 45PSI wouln't be better to have a structure or layer resisting from 0 to 15PSI and 15 to 30PSI at the next layer and finally 30 to 45PSI for the exterior shell. Obviously, this pressure can be monitored and controlled internally as needed. I think this idea might help reach greater depth with smaller structure or eventually make subs lighter.