Re: How catastrophic is catastrophic?

[DaveIrons@aol.com]

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

I may do that.

> 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?

Out of roundness is not as critical in concrete hulls.  Because of there
greater thickness to
diameter ratio.  Concrete is stiffer than steel therefore flat spots do not
tend to
progressively collapse the hull as it does in steel hulls.  		

 In the Popular science
>magazine. that had the concrete military sub,

Hull thickness to outside diameter ratio of . 08 will produce a slightly
buoyant submarine. 
A five foot diameter sub could have a 4.8 inch hull.


>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?

No subs that I know of have been built out of concrete.
Seacon a navy undersea habitat was tested for 314 days at 600 feet.

The tests were performed on concrete spheres sixteen inches in diameter.  A
scale model
habitat was tested the full size habitat would be twenty feet outside diameter
the habitat
was designed for an operational depth of 3,350 feet.  The habitat had a two to
one safety
factor built into it.  The implosion depth would be 6,700 feet.

>Steel hull subs have gone much deeper than 3,000'.  Alvin's first hull was
>steel.

Yes, All steel is not created equal.  As I recall military subs have a crush
depth of about
1,500 feet.  I think a two to one ratio is a conservative estimate.  Concrete
can also be
made stronger with polymer additives.  


> 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.  Someone
>would have to show me some figures that show how concrete is cheaper, and
>cheaper than what exactly.

Lets say high strength concrete is $100 per square yard.  A yard of high
strength concrete
weighs about 5000 pounds.  That is about two cents per pound.  Adding large
aggregate
would reduce the cost of the concrete by half or one cent per yard.  For a
mini sub I'll use
two cents per yard.  A 10,000 lb. mini sub would cost $200 in concrete.  The
formwork
would depend on your design.  The concrete should be mist cured over 60 days. 
Therefore the entire hull could be cast in sections.  I intend to use two sets
of forms for
the inter and exterior.  One set of forms would be moved ahead as the first
set is filled. 
Another two sets of forms would be used for the spherical end caps.  I would
estimate for
your mini sub six sheets of 1/2" plywood $100.  Plywood would be cut into
semicircular
sections to act as stiffeners for the plywood forms.  
There is an economy of scale for larger submarines.  The depth rating is
proportionally
deeper for the same hull thickness to diameter ratio.  Concrete is cheaper as
larger
aggregate is added.  

> 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. 

Good point.
One method of moving a very heavy boat would be to excavate an area next to a
deep
channel.  The excavation would be flooded to float the boat and the dam taken
out to float
the sub into open water.  For the mini sub example I would recommend a custom
made
cradle for the sub and a trailer designed for heavy machinery.  The used
concrete forms
would make nice cradle for the sub.  The boat I wish to build would be too
heavy to move
by land.  

However a ferrocement boat that is damaged by moving is probably structurally
flawed. 
The movers probably saved the boat from sinking.

>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.;-)

I'm more comfortable working with concrete.  Polymer reinforced concrete is
about
20,000 psi. about twice the strength of the concrete.  The hole should be
primed with
polymer and the hole filled in with polymer reinforced concrete.  


Happy subbing to all,

David Irons

Subj:	 Re: How catastrophic is catastrophic?
Date:	3/14/99 6:09:15 PM EST
From:	protek@shreve.net
Sender:	owner-personal_submersibles@psubs.org
Reply-to:	personal_submersibles@psubs.org
To:	personal_submersibles@psubs.org

Psubbers,

 Gary Boucher

>I think I am seeing a dead horsie surfacing in a concrete hulled sub,
or, no it may be a, yes, a wire mesh electrolysed hull.
Gary Boucher

Has this subject been discussed before?  I would be interested in any previous
discussions.

Thanks 

David Irons