RE: [PSUBS-MAILIST] Pressure Hulls

[Alec Smyth <Asmyth@changepoint.com>]

Actually I didn't use any particular page of Busby as a reference for the
theory. I got my formula if I recall correctly from Submersible Vehicle
Systems Design (Allmendiger). All I did was pick a few spherical-hulled subs
in which Busby listed hull diameter and thickness as well as the collapse
depth. Then I ran the numbers and compared results. My numbers were way too
high every time. 

More later when I get a chance to do this exercise again using this formula,
perhaps this evening.


thanks,

Alec

-----Original Message-----
From: Captain Nemo [mailto:vulcania@hawaii.rr.com]
Sent: Monday, July 01, 2002 3:46 PM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Pressure Hulls


Alec,

Ian would be the man to ask.  He presented the formula; I was just trying it
out.

One likely  area for error might be in determining variable U.  In my
example, I used boiler plate steel with 0.4% carbon content, with a Tensile
Strength of 44K and a Yield Point of 30K psi.  If I'd used milder steel, say
0.2% carbon, those numbers would be about 30K and 22K psi, respectively.
That would have an effect on the result.

One thing I wondered about while setting up the equation was what they
really meant by the Minimum Yield Point of the steel; and is it less than
what's commonly called Yield Point?  If MYP were less than YP, that would
have an effect on the result.

According to STRENGTH OF MATERIALS, Singer 1960: "Ultimate Compressive
Strength for ductile materials like steel may be taken as the Yield Point
which is slightly GREATER than the Proportional Limit in Tension."  In this
case, according to the table of material strengths I used (from MODERN
WELDING by Althouse, Turnquist, and Bowditch 1980) YP is LESS than TS.  That
seems to contradict Singer's basics, so again, I'm wondering.....

Another factor effecting U is whether the steel was hot-rolled or
cold-rolled.  Changes the numbers substantially.

What pages did you reference in Busby?   I'll go check them out.  If they
cite particular sub hulls, they may have used another formula, or the
properties of the steel used may be a consideration.

About the possibility of inordinately high safety factors for spheres
necessitated by sensitivity to out-of-round conditions: I don't know.
Sorry.

Pat





----- Original Message -----
From: "Alec Smyth" <Asmyth@changepoint.com>
To: <personal_submersibles@psubs.org>
Sent: Monday, July 01, 2002 8:34 AM
Subject: RE: [PSUBS-MAILIST] Pressure Hulls


> Pat,
>
> About a year ago, I looked up the formula for spheres (can't remember if
it
> was the same one you just used though) and ran some numbers based on
> examples of sperical-hulled boats in Busby. The formula was simple enough,
> but the pressure ratings I got were way higher than the ratings given in
the
> book, which left me a bit puzzled. Its hard to mis-calculate with such a
> simple formula. D'you know whether common practice might be to use an
> inordinately high safety factor with spheres, given they're so sensitive
to
> out-of-roundness? I'd pull out my Busby and run some numbers with this
> formula to see how it jibes, but I'm at the office. So I really should get
> back to work! But I'll try doing this later.
>
> rgds,
>
> Alec
>
> -----Original Message-----
> From: Captain Nemo [mailto:vulcania@hawaii.rr.com]
> Sent: Monday, July 01, 2002 2:05 PM
> To: personal_submersibles@psubs.org
> Subject: Re: [PSUBS-MAILIST] Pressure Hulls
>
>
> Dewey,
>
> I wasn't worried about that; just clarifying that mine was an exercise
with
> the formula, not an estimate of your particular hull's capabilities.
>
> Pat
>
>
>
> ----- Original Message -----
> From: "Dewey Mason" <drmason2001@yahoo.com>
> To: <personal_submersibles@psubs.org>
> Sent: Monday, July 01, 2002 7:25 AM
> Subject: Re: [PSUBS-MAILIST] Pressure Hulls
>
>
> > Hey All,
> >   I assure you I think highly of you all, HOWEVER,,,,
> > I will double check ALL math on my sub, I promise. Not
> > that I don't belive you, just that I don't breath
> > water, and feel no desire to be smooshed inside a
> > steel ball. Sounds unpleasant.So you need not worry
> > that I will kill myself and then blame you.
>
>