Re: Visibilty / Safty or Safty / Visibilty ?

[Phil Nuytten <72020.572@compuserve.com>]

Subbers:
        I read Jon Shawl's response to Rick Lucertini's expressed desire
for max viz . . .I must admit that I'm a little puzzled. Jon's  points are
well taken with regard to safety . . and like Motherhood, the Red Cross,
and apple pie . .safety is  hard to discuss in anything other than a total
acceptance mode without appearing  to be advocating improper practises and
general bad joss!
The specific references to fibreglas hulls, flat sided nozzles,( as being
generically unsafe) are  incorrect, however. There is no magic or black art
to the business of external pressure vessel design . .it uses the most
basic engineering principles  imaginable.An oaken barrel is 'safe' to some
depth . .(deeper than one would think - according to John Lethbridge, 1715,
and Robert Stenuit, 1988 !) Fibreglas hulls are fine - many have been made
and used safely to some depth (Searay,etc.) - even plywood is OK!! My old
friend Jim Helle built a plywood sub many years ago, as a young engineer,
and it worked fine - He went on to found Helle Engineering,an international
supplier  of deep-water communication and other electronic systems. The
point is, you can make a sub out of damn near anything that is impermeable
to water . .it's just that some materials and shapes are more efficient
that others. Sub design, like life, is a series of trade-offs and
compromises. Soft iron is cheap and easy to work but is heavy, titanium is
expensive and horrible to work with . .but very strong, light, and
corrosion proof. Aluminum is about in the middle - about the same as mild
steel, pound for pound, but acts like a giant anode in the presence of more
noble metals, etc.,etc. The cheapest way of increasing bouyancy is is to
increase diameter, but then you are trading in-water bouyancy for increased
air weight and decreased depth. 
        My personal reccomendation is not to 'strangle the baby in the
crib' ( as songwriters say) . .if you want to use a propane tank, fine . .
.figure out what pressure it will tolerate safely, or have some-one do it
for you. Chunk of steel pipe - great!- a readily available tube in some
suprisingly high carbon steels! Just find out what it's material
characteristics are . .for sure. Let your imagination run wild! But run the
scheme by your favorite local engineer - if you don't have one . .go to the
nearest college or university and ask the structural guys what they think..

        Viz?? yeah, I feel so strongly about it that I have several subs
that are damn near all plexi . . speaking of which , that is a much ignored
fabrication material for hulls . . segmented rings cut out of 1 or 2 inch
sheet and bonded with staggered joints is a  viable way to go and can be
polished to good optical qualities . . and on and on. There are some tricks
to sub design but they are just ways of being more efficient, of 'tweaking'
the last foot safely out of a given material or design . .but they don't
replace basic, simple,  engineering principals and tables -which are as
close as your nearest library.
If you don't have asub, get one!
Phil Nuytten