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Re: [PSUBS-MAILIST] New Guy from down under needs help
Thank you Jon
Touché........Your points are well taken ..in fairness to me, my line of
thought was designing, not the calculable known mathematical geometric
formulae, but to the weakest point and doubling that which I guess,
relatively thinking, works out the same as 4 times the formulae...I'll still
keep my back ups though, to be sure! to be sure!.
Sadly with great embarrassment I did not consider the time degradation of
the acrylic as apposed to the steel so again you win this round.
Thank you ..this most clearly demonstrates the benefit of this forum with
access to the wide range of experience and dedication.
Cheers
Les
----- Original Message -----
From: "Jon Wallace" <jonw@psubs.org>
To: <personal_submersibles@psubs.org>
Sent: Monday, March 15, 2010 3:32 AM
Subject: Re: [PSUBS-MAILIST] New Guy from down under needs help
Les & Anna wrote:
By the way the sub pressure testing requirements bothers me a little
..can you direct me where I might obtain some written data on this.
*I am all for maximum safety it is the major precursor to all my
design.....*....but I have incorporated at least three back up systems
for preventing slipping any deeper than 300 feet, including auto
surfacing beyond 320 feet as a safety factor. With this on board the 3
times pressure factor seems over engineered.This I would consider
precludes a three times safety factor
Les, keep in mind that the math used for calculating strength of material
to resist external pressure is based upon perfect geometry. A perfect
sphere, cylinder, hemisphere, etc. Safety factors take care of issues
resulting from not being able to obtain perfect geometrical shapes during
fabrication. Once you move out of the realm of perfect geometry, the
defects introduced during fabrication become weak spots that degrade the
calculations, perhaps significantly. It's great that you have three
backup systems for auto-depth at 300 feet, but remember that redundancy
provides no guarantees as United 232 in 1989 had three redundant hydraulic
systems which all failed (talk about a bad day) and resulted in the
aircraft crashing. Mechanical things can fail, even if built redundantly.
The nice thing about safety factors is that they never fail. As well, if
your intent is for the vessel not to exceed 300 feet then you could
eliminate the depth-limiting redundancy and auto-surfacing systems by
simply not diving in waters that are deeper than 300 feet. (Operating
Guidelines for Personal Submersibles, Section 3.2, Site Selection)
By the way I think I read in the Stachiw book a seven times factor for
acrylic domes and windows?
Now my logic indicates to me that either they should be both the same
either way because the lesser will fail anyway so the maximum is
obsolete???
Possibly, but not necessarily. I think you'd have to consider how each
component may fail individually due to any defects that might occur as a
result of operations over time, and my guess is that you'd end up
concluding that each components safety factor needs to be considered in
isolation rather than attributing an arbitrary safety factor for the
entire vessel based upon your confidence in one component. The defects
leading to failure of acrylic (crazing, impact, scratches, cleaning
solvents, etc) are likely going to be dramatically different than the
defects incurred over time that might lead to failure of your hull (dents,
rust, fiber unwinding, etc).
Jon
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