RE: [PSUBS-MAILIST] Hull Support size

[Alec Smyth <Asmyth@changepoint.com>]

Graham,

Its an iterative process in which you are seeking a balance between things
like the properties of the material, the spacing of the reinforcements, the
geometry of the reinforcements themselves, the thickness of the plating,
etc. There are 3 distinct failure modes, and the objective is to prevent any
one of them ocurring much earlier than the others. For example, if you had
massive supports but used paper-thin plating, that would be an inefficient
design. Or if you had massive supports but they were 10 feet apart, same
thing. Or massively thick plating with diminutive reinforcements. You get
the idea. The only caveat I would mention is that one of the 3 -- general
instability -- calculates with less precision than the other 2, so you would
want to increase you safety margin for that specific failure mode.

I took a quick look at the program you mentioned in an ealrier post. I have
not had the time to compare the formula with my references to see exactly
what its calculating, but since it is one formula, it will at best be
calculating only ONE of the three possible failure modes. Do yourself a
favor and calculate the other two as well! 

My recommendation is to take the spreadsheet you can get off the PSUBS site
and plug in a first set of numbers for what you think might be ballpark
scantlings. Check the results for the three failure modes. If some failure
mode occurs way earlier than others, add a row to the spreadsheet with
modified scantlings. In this way, by adding rows you go zeroing in on a
balanced design. Of course, you'll also have to keep an eye on weight and
displacement as you do that. The strongest hull is a solid, but its not very
habitable and tends to sink.

Anyway, you'll need some literature to interpret the failure modes I keep
referring to. I'll send you that off list.

As always, please bear in mind you are not getting this from a professional
engineer, just a hobbyist. The literature I'll send you has some pretty
interesting math in it. If you don't find it clear, I would recommend going
with a proven design like a Kittredge, in the interest of safety.


Have fun,

Alec



---Original Message-----
From: Graham K [mailto:cryogenicz@dsl-only.net]
Sent: Monday, June 24, 2002 6:13 PM
To: personal_submersibles@psubs.org
Subject: [PSUBS-MAILIST] Hull Support size


Is there any way to calculate your support beams for your hull?

Regards,

GK