... So imagine yourself at only 2 atm or 33 ft 
    of pressure on your body, but actually diving to 99 feet and staying 
    for a maximum of 205 
    minutes without any deco! Wouldn't we scuba 
    divers love it if we could do that! A 
    hybrid like this would allow us to. I would say it would be worth the extra 
    trouble. What do you think Paul?
  
I'm with you Bill, sign me up for a pressure-limited cabin!  But 
  I am starting to wonder why not just limit it to 1atm. :-)  Once I start 
  thinking about building a crew compartment that handles 3atm and maybe 1atm of 
  overpressure, well it seems like why not just build a traditional 1atm 
  boat?  But this hybrid sub design is fascinating and I'd like to see it 
  sketched out.  
 
  
    Now I would like to know what hull strength I 
    would need for a 1 atm pressure vessel to withstand 2 atm of outside water 
    pressure. Probably have to build it with a safety factor for 3 or so 
    atm
    just to make sure it was safe. I wonder what 
    steel thickness I would need or if a 1/2 inch thick kevlar wing tank 
    would withstand 2 or 3 atm with 1 atm inside it? Any help here 
    fellas?
  
Well that's the rub, isn't it?  My feeling is that many 
  engineering types would be comfortable designing something for themselves to 
  use but would be pretty hesitant to give someone else recommendations on 
  pressure vessel details.  Is that right?  Sorry I haven't been on 
  this mailling list long enough to know what's been done and discussed in the 
  past and I didn't find anything on PSUBS.org or by searching it with 
  Google.