Re: [PSUBS-MAILIST] New Guy from down under needs help

[Jon Wallace <jonw@psubs.org>]

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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