PRESSURE HULLS

[THOMAS KNAPP <t-knapp@home.com>]

     Hello, my name is Tom Knapp.  I'm a new member of the
personal_submersibles mailing list. I have a fascination with submarines
and since childhood have dreamed of building one. Initially, I'm
interested in building a two person, dry, submersible, that can safely
dive to 300ft (give or take).  Recently, I e-mailed another member, Jon
Shawl, with questions regarding pressure hull fabrication.  After
letting me know he used a propane tank for the main hull on his sub, he
advised me to join this group and pose my questions to all the other
members as well, so here goes...
     I can anticipate many hurdles to building a psub, but I see
production of the pressure hull as one of the largest set of obstacles. 
In addition to the cost of procuring (very likely) a propane tank, there
are modifications such as adding a tower with a hatch and view ports,
pressure testing, etc. My main question now, however, has to do with the
internal structure of the pressure hull and who (in the general since) I
should hire to do the welding of any modifications.
     Years ago, I spoke with a local pressure vessel manufacturer (who
wanted nothing to do with PVHO manufacturing). He indicated that a
general rule was that if a port or tower (or secondary pressure hull)
was attached to a larger one, its diameter should be no larger than 1/3
of the larger diameter.  Many photos of small submersibles indicate this
rule isn't followed, which probably indicates they have internal
bracing. Let me know if my thinking is on the right track.  Any
anecdotal information on welding or hiring welders to perform
modifications would also be appreciated.

Thanks very much, 
Tom Knapp (t-knapp@home.com)