Re: pressure hull design (Fwd)

["Marsee Skidmore" <heyred@email.msn.com>]

John,

I'm asking Santa for the Nash book. Thanks for the recommendation. Can you
provide Ray with the info that he needs to list it on the "Sources" page? I
understand your reluctance to use available hemispheres, but I'm curious how
you intend to fabricate yours. My boat is an unscheduled, unbudgeted paper
project until I win the lottery, but I intend to start with the pressure
hull, and build in both directions. I'm designing for an operating depth of
1000 ft, but my experience is with deep divers. As a result, I have a bias
toward a spherical pressure hull supported by an exoskeleton, with a bare
minimum of equipment enclosed in the pressure hull. Hydrostatic testing of
the pressure hull is a must. I've contemplated towing it out to deep (>1000
ft) water, tying a rock to it, and depending on some Tom Swift release
device to get it back to the surface. However, this violates my "Don't take
it into the water if you can't afford to lose it" SCUBA training, so I'm
less than thrilled with this approach. I may be a Polyanna, but I'm assuming
that there are testing facilities that will rent pressure chamber time for
components that are smaller than a pressure hull.

Joe (from Marsee's computer)

-----Original Message-----
From: John Brownlee <jonnie@chronic.lpl.arizona.edu>
To: personal_submersibles@psubs.org <personal_submersibles@psubs.org>
Date: Wednesday, September 16, 1998 4:54 PM
Subject: Re: pressure hull design (Fwd)


> Hi Marsee,
>
> My point in suggesting that someone who did not know where to
>find/derive such a formula probably needs a little more than a formula in
>a book to be safe. For example, if you don't know a little about the
>mechanics of materials and structures, the Nash book (which incidentally
>DOES treat surface irregularities and tolerances, more later) will be
>largely useless to you. It is wonderful, but very rigorous with a good
>deal of multivariable calculus in the beginning. For those who don't need
>this sort of rigorous analysis, don't bother. Desigining in a huge safety
>margin is always a good insurance policy.
> The Nash book is VERY rigorous, and more or less shows how to set
>up analyses for all the hull geometries you can name. More importantly, it
>has an amazing bibliography, pointing to many papers on both the theory
>and experimental trials on the buckling of imperfect shells under
>hydrostatic pressure. It does not address tolerances per se, but presents
>a general way to address geometric imperfections of many kinds. Indeed,
>the chapters on spheres are pretty interesting that way; the emphasis is
>not solely on the size of the imperfection (the tolerance), but the
>-nature- of it. For example, considering the crush depth for a sphere
>which has been flattened by a clamp on opposing sides is very different
>than for one which has a dimple on one side. The bits on reinforcing rings
>for cylinders and cones are also pretty cool.
> Don't get me wrong, the Nash book is NOT for everyone. But, if you
>suffered through senior-level analysis or calculus, it's a chance to
>exercise the math muscles and learn a lot about the mechanics of pressure
>hulls. It is largely geared towards someone with a strong engineering, or
>at least mathematical, background. If you never took much calculus, or
>don't want to get your 'hands dirty' in the hull design process, don't
>bother. Hire a licensed engineer to do the analysis for you, etc. Also,
>much of his emphasis on high pressure regimes (like hundreds of PSI) is
>serious overkill for the 100' boat.
> With regard to whether or not forged vessels intended for holding
>compressed gasses and the like are good enough, I must confess I don't
>know exactly. There's a lot of unknowns there, but I suppose with the
>proper bent you could find out. I'd just rather build the thing from
>square one using known steels, etc. myself, but that's how I've operated
>all along. I am sure that there are several COTS products which work fine,
>but somehow the ol' propane tank leaves me a little on edge, as I'd like
>to dive deeper than 200' in the best of all worlds.
> I should qualify all this by saying that I have probably in one
>form or another designed and built everything BUT the hull for my own
>boat. Being a computer/optics wonk I started with what I knew and went
>back to school for what I didn't. Started with life support, of all
>things, and built from there. Of course, working at a university helps;
>when I knew I needed to know more about statics, I could sit in
>on a class over at the engineering college and pick it up.
> This raises an interesting question: in designing, you obviously
>start with the hull (well, you -START- with a spec, but that's different
>than what I mean) but in BUILDING, does everyone start with the hull and
>move inwards? Has anyone done it 'backwards,' like I have? Moreover, does
>anyone test their components hydrostatically during construction?
>
> John
>
>John Brownlee
>Lunar and Planetary Lab
>University of Arizona
>jonnie @ lpl . arizona . edu
>
>