Re: New ideas

[shawl@torchlake.com (Jonathan Shawl)]

Hi John, and Subbers,
I hope someone doesn't start thinking about digging up their driveway culvert
to build a sub. :-)
I think I remember reading some place that a cylinder made for external
pressure has to be within <1% roundness to pass ASME code for human
occupancy.  On my subs hull 36 " ID, you would have to be with in .36 inches
total out of round. Example Lets say it is oval and it was 36.18" top to
bottom measurement, and 35.82" side to side, that would be the maximum. Does
this agree with the book you just read? The one thing that I like about mass
produced, machine made, propane tanks is that they are round and meet this
spec. Of course it's a good idea to check it yourself. And they have true
Hemispherical heads. The best kind. I see no one answered my earlier question
about how much hemispherical heads cost. Anybody check into that yet? If
anyone builds the  Kittredge Sub from plans and you have a roll up made, make
sure that the prints that you give the steel fabricator has this roundness
tolerance on it. If you don't and they give you an egg shaped tube, tough,
it's yours. And if you do spec it out and it does not get made to spec. then
you can reject it. I know this first hand from working at a Vacuum furnace Co.
that built vessels. The furnaces that had to have flat sections had very heavy
bracing, and I think they would have been to heavy to float. One box shaped
chamber was about 6'x6'x6' was 2 inches thick with external bracing every foot
or two on every side in both directions, the bracing was 2" 6" welded on edge.
this was one heavy box, and it was only good for 1 atm or 30' depth, even then
you could put a 4' strait edge on the outside and see the curve from the
external pressure. This is why I wonder about the safeness of some subs I saw
photos of that have relatively thin flat pressure structures. It don't sound
like a good idea to me. What do you think ?
As far as wood being easier to work with than steel. If your like me and have
the tools and knowledge for working with both steel and wood, I think it's a
toss up. It's just about the same for me to cut and grind a 1/2 steel plate
into a round rib with a cutting torch and grinder, as it is to cut a rib out
of plywood with a good saber saw and then sand the edges. But then again you
would have to cut more plywood than steel to get the same strength rib, right?

I have a personal liking of welding, as it can be frustrating for me to wait
for epoxy to harden before you move on to the next step.
Jon Shawl

John Brownlee wrote:

>         I just finished reading in the book 'Hydrodynamically Loaded
> Structures' which describes an interesting hull design. The whole hull
> looks like it's accordion folded, with hemi-heads on each end. Turns out
> in testing that it's actually comparable in strength to a comparable
> cyllinder IF MADE WITHIN TIGHT TOLERANCES. That's an important disclaimer,
> remember that different geometries tolerate (or in most cases don't
> tolerate) deviations from perfect volumes of revolution in very different
> ways. The cross-section of the "cyllindrical" wall looks like this:
>
>         /~~\      /~~\
>        / /\ \    / /\ \
>       / /  \ \  / /  \ \
>             \ \/ /    \ \
>              \__/      \ \
>
>         I mean, the damned thing looks like it would fold up along the
> revolution axis like a piece of collapsible Tupperware! But, all the
> finite element models and pressure tests showed that it held up if it's
> made exactly right. Bizarre-O.
>         Now, to read most of your minds and answer your concerns, this is
> not necessarily a great idea, what it is supposed to illustrate is that
> sometimes math and physics join together to confound common sense in
> engineering. However, and this is the REAL point, unless you have amazing
> manufacturing skills, your common sense should always over-ride such
> counter-intuitive ideas. Often such elegant and beautiful juxtapositions
> of opposing forces (such as the accordion fold hull above) are intolerant
> of imperfections which real-world structures are going to have.
>         So unorthodox ideas need to always be approached with caution,
> because chances are someone else already had them and didn't do it for a
> good reason. That's a long way to come to get that out. Sheesh!
>
>         Back to wood, it's been done before rather sucessfully. That means
> that it is possible to create a viewport that handles the cycling fine. In
> fact, if I remember my reading right, there's a lot of good qualities
> attributed to plywood, ease of manufacture not the least among them. Just
> be prepared to have to re-create in wood the solutions people with steel
> subs had solved for them in the 50's, like viewports and penetrations.
>
>                                                 John
>
> John Brownlee
> Lunar and Planetary Lab
> University of Arizona
> jonnie @ lpl . arizona . edu