Re: How catastrophic is catastrophic?

[shawl@torchlake.com (Jonathan Shawl)]

Hi Dave and all,
Maybe someone should start a C-Subs (Concrete Subs) web site. ;-)

> DaveIrons@aol.com wrote: Rick Ed and all
> Judging from the photographs, I've seen, of concrete pressure hulls
> that have been tested to failure.  When concrete hulls fail under
> pressure a large section suddenly implodes.  However, if you test a
> steel hull to implosion the results may be just as catastrophic.

True! Remember every hull will go thump at a certain depth, no matter what you
build it from.

> Concrete hulls have more compressive strength than steel.  The
> crush depth of a concrete hull would be about twice that of a steel
> hull.

Uh,... that depends on a lot more than just the material you are using. It's
like comparing apples to oranges.
One has to consider the bending stresses also, and if I remember right
concrete is not as good as steel in this area. The main reason a steel hull
fails is from bending, not from a lack of compressive strength. I just can't
imagine a concrete sub .5 " thick being 4 times safer than my .25" thick steel
sub. For a simple example take a 1" x 1" x 2' (2.54cm x 2.54cm x 60.96 cm)
steel bar and the same size concrete bar and try to break each one. Support
each bar at it's ends and stand in the middle.  Ok, I still have a steel bar
and several pieces of concrete. Get my point? Correct me if I'm wrong on this,
and include some real figures for comparison. Does any one have some real
design data on using concrete verses steel on pressure hulls?   How thick do
concrete hulls have to be to hold up under the bending stresses caused by the
imperfections in roundness and hull penetrations? In the Popular science
magazine. that had the concrete military sub,
http://popularmechanics.com/popmech/sci/9812STMIP.html
It looks like the hull is several feet thick with equally thick ribs and
internal structures. I can see why that might work on a big sub but I'm not
too sure about concrete being very practical for a mini sub.

> The possibility of a concrete hull collapsing at depth is seems
> highly unlikely.  Stachiw estimates that operational depth is about
> 3,000 feet.

Please give the dimensions of the hull in the above example. Have any real
concrete subs been built and used successfully?
Steel hull subs have gone much deeper than 3,000'.  Alvin's first hull was
steel.

> For a sport submarine in maximum 200 feet of water
> that is at least a 15 to 1 safety ratio.  Unlike a steel hull, the cost of
> building a 3,000 foot concrete pressure hull is reasonable.

Again, that all depends on many factors other than just the material. Maybe
you are comparing a $450,000 commercially made, tested, ready to dive steel
hull sub, with a backyard built concrete sub?
For my home built, steel hull, mini sub, the cost of the hull was a free 500
gallon propane tank. I can still get them for $100. Even if someone gave you
the concrete how much will the forms cost to build or buy?
I would think you would have to cast the entire hull in one piece to avoid
week spots. Wouldn't this require a complete inside and out form? To me that
sounds like building two subs and filling in between with concrete. Some one
would have to show me some figures that show how concrete is cheaper, and
cheaper than what exactly.

> The maximum safe depth, would be depth rating of the fittings.  I
> think the viewport would be the limiting factor.  The acrylic
> viewport would probably crack long before the concrete pressure
> hull would show any signs of stress.

Maybe not...
Take a look at  the Alvin site, at
http://dsogserv.whoi.edu/ships/alvin/alvin.htm
It uses acrylic view ports because acrylic is a proven safe material to use
even at great depths.
:Alvin, Max. operating depth  4,500 meters (14,764 ft.) Four conical acrylic
plastic view ports, each 3.5
inches thick with a 5-inch inside diameter and a 12-inch outside diameter, are
set at different points in the hull."
http://dsogserv.whoi.edu/ships/alvin/userman/pressure_hull.htm

> A more likely mode failure would be damage on the surface.
> Damage to the hull would be visible as a crack.
>
> David Irons

That's a good point. I have heard of more than one story about people spending
years building a concrete or fero-cement boat, only to have it ruined by
moving it to the water. As far as moving damage goes, I think I would rather
repair a dent in a steel hull than a hole in a concrete hull!
I'm going to stick to concrete for footings and sidewalks.;-)
Happy subbing to all,
Jon shawl