RE: [PSUBS-MAILIST] Hull Calc Spreadsheet

[Alec Smyth <Asmyth@changepoint.com>]

You're certainly right that certification agencies use lower safety factors.
Per an earlier post of Carsten's the Germans even use variable factors
depending on the design depth, and for deep diving subs the failure/normal
factor was 1.73.

Problem is, I'm an amateur and I'm building the sub in my garage. So my tool
for determining out-of-roundness, for example, is a Stanley tape measure. I
am quite sure that the pros have far more control over material and
fabrication tolerances. But even if I did get everything built just right,
the theoretical calculations are still only predictions. The actual failures
have been found to deviate from calculated values quite significantly. As a
general rule, the simpler the geometry the more reliable the calculation.
This is the reason I went with a really simple hull geometry in the first
place. If I had compound curves, or large openings, tapering wall
thicknesses, etc. I would not have the means to calculate them. And I try
not to use what I cannot calculate.

Bear in mind also that the strength of shells does not decrease in a nice
linear way when you lose accuracy. Speaking figuratively, if your geometry
is off by lets say 5%, you won't find the collapse depth is off by 5% too.
Chances are the collapse depth can be down by orders of magnitude. 

There are lots of components on my sub that only present an inconvenience if
they fail, and for those I don't mind saying I've skimped and scrounged as
much as possible. But a pressure hull failure is altogether different. 

Ultimately its all about taking calculated risks, and the bottom line is the
safety factor will represent your own personal degree of risk aversion, as
well as the functional objective (e.g. tourist sub vs military). I don't
think there is a universal "right answer" on this one. 

rgds,

- Alec

-----Original Message-----
From: Dewey Mason [mailto:bayan_darkeyes@yahoo.com]
Sent: Wednesday, October 31, 2001 7:26 PM
To: personal_submersibles@psubs.org
Subject: RE: [PSUBS-MAILIST] Hull Calc Spreadsheet


Hey Guys,
I am wondering why there seems to be such an VERY high
level of excess in every calculation and spec ya'll
build into your subs. The scientific arena does not
use so high a factor for a error margin. With good QC,
and high quality workmanship, a 100% margin should be
well above the likely point of failure. The USSR used
a much lower factor that this, and not one hull
failure has occurred (that anyone heard of, anyway).
It seems to me that an absurd error margin and
excessive layers of redundancy only up the final cost,
and slow the production of otherwise serviceable subs.
I DO belive in being safe, but also being sane. A sub
that has five layers of redundancy, and a safety
factor of three times calculated peramiters, and then
only sailed into waters no deeper than the operational
max, is a toy, or is wasting a huge amount of
potential.
I would not say anyone is WRONG in their choice of
safety features. I am simply asking, why SO MUCH usage
is being given up in the name of safety? In our
designs, we are currently looking at a one third
reduction of calculated max depth as operational, with
a test depth of ninety percent of max. We will need
the capacity for the work we will be doing. Also, what
factors do the various certification agencies use? The
insurance companies?
Just my thoughts on it, for what they are worth.
Dewey
T.D.P.F.



__________________________________________________
Do You Yahoo!?
Make a great connection at Yahoo! Personals.
http://personals.yahoo.com