Non-Vitriolic Concrete Submarine Thread

Discussion in 'Boat Design' started by Stumble, Jan 20, 2012.

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  1. Stumble
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    Stumble Senior Member

    To preface this, I have no real interest at this time in building a sub, and honestly wouldn't know where to start if I did. That being said the other threads have devolved into such infighting, and name calling that I don't really feel like getting into the middle of them. That being said, I am really hoping this thread can be a place for honest evaluation of the posibility of building a concrete submarine, and the difficulties therein.

    Now as I see it, there are two major problems that have been raised in building a concrete submarine, though I may have missed a few. Below are some of the possible solutions i thought of when trying to read through the other forums.

    1) Building the preassure hull
    As I see it there is a real concern that hand laying a preassure hull out of concrete is frought with significant difficulty, and requires a level of exactness I wouldn't consider possible for most home builders. To address this concern What would you guys think about using commercially spun pre-cast concrete forms?

    Specifically I am thinking of AWWA Standard C303 compliant tubing. This stuff is available in diamaters of 16-144 inches, and leinghts up to 24 feet (though larger sections are available by special purchase). The C303 standard is an industry standard that mandates a tested strength of 400psi working load (though this is the PSI inside the pipe, does it matter if the preassure is outside vs inside?). These pipes are made by spinning large drums at high speed and pouring the concrete inside the drum, so centrifical force pushed the concrete to the sides.

    There is a huge amount of information, and knowledge on how to work with these pipes, and retain the 400psi minimum. Since this equates to around 27 atm, which is the preassure at around 900 foot, it would seem relatively safe to use these down to a dive depth of say 450 feet. Since you are only using 1/2 of the pipes design preassure, and assuming that the pipe has a normal safety standard of 10:1, this would leave you in the 20:1 safety margin. Which as I see it would be a pretty conservative figure.

    2) Corrosion resistance of rebar
    The indistrial papers I found seemed to indicate that at least in piping of this nature the very high PH of the portland cement (around 13) causes a chemical reaction on the surface steel that prevents rust. I don't know much about this, but AWWA (American Water Works Association) seems pretty confident about this process, and since they are the specialty group of civil engineers in this area it isn't I think unreasonable to accept their determination.

    Finally I have some issues of mine own I would be interested in thought on.

    1) Since these pipes are available in either cylindrical or eliptical, what would be the thought of using eliptical pipe instead. This would I think prevent some of the rolling, and give a natural space for ballast (batteries), As well as allowing for a more natural bilge.

    2) Repurposing the variety of fittings, and flanges used in the concrete pipe business. So instead of reinventing everything, just buy an off the shelf 42" steel T-Fitting used to branch off the main pipe, and installing the hatch in it. This way all the engineering of preassure fittings, and thermal expansion has already been done. Since these systems are commonly designed around that same 400psi figure.

    3) The pric for off the shelf 96" pipe runs around $400/foot. This would put a 50' hull at around $20,000. Which seems pretty reasonable compared to the cost+labor of trying to build a comparable.

    Finally some drawbacks to this:

    1) Does putting the preassure on the outside significantly effect the loading capability of the pipe?

    2) The fittings might cause some fairing issues.

    3) You have to give up the tear drop shape in favor of a cylinder. How much would this effect performance?

    4) The manufacturers of this pipe are probably going to think you are nuts when you call to buy from them.
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  2. hoytedow
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    hoytedow Carbon Based Life Form

    Lake Pontchartrain had a Civil War effort at sub building also. The engineer designer had the same last name as part of my family. I always wondered how/if we are connected.
  3. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Popular Science or Mechanics mag had a story about how

    "terrorist nations" could build cheap bottom laying concrete subs, and how we needed to worry about that.

    I believe the construction of the subs was based in valid engineering.

    Those "Troll" oil rig towers all concrete and rebar, and not that thick (1m), and go down 1000ft.

    How much, if any, of that 1 meter thickness is for other reasons, like supporting the tower's weight, or resisting wave and current forces, or just weight?
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    BATAAN Senior Member

    Plenty of concrete merchant ships built at the end of WW 1. Some actually went to sea and functioned OK.
    Most small deep research subs these days have the motors outside, shrouded and steerable, running in an oil bath so they don't get wet or mind the pressure.
    The batteries too are exposed to pressure, but full of fluid which cannot compress.
    The hypothetical cement sub needs to run control wires at least through the structure somehow.
    Also, what do the ends look like? Hemispherical bulkheads? Just domed?
    I don't think the problem is the pipe itself, that seems not too complicated, it's the other stuff, the ends, and also like hanging the ballast tanks outside, the motors and/or planes, and how do you see? What do the ports look like? Are they in a steel conning tower like thing?
    Do you build steel domed ends with conventional packing glands for wires and hoses and planes and/or external motors and batteries and fit these and fasten them somehow to the concrete pipe?
    Do you put everything inside, including the ballast? That's very 1905.
    Why not make the whole thing out of steel pipe and stop messing around?
    Sure, a cement sub could possibly be built, but from my experience with cement boats, everything about it is difficult except doing the ferro cement work. Fastening the interior in place, making engine beds stay where you ask them to etc.
  5. Squidly-Diddly
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    Squidly-Diddly Senior Member

    aren't there short range underwater radios? control all the workings with

    radio, or even sound.
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Make it out of glass. Don't laugh, it is very, very strong. You just need a glass blower with a powerful diaphragm. Or is a non-vitreous submarine preferred ?
  7. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I'm a big fan of glass for building. Comes in all shapes and sizes and

    it never rots. I'd like to see glass replace concrete for stuff like pre-stressed beams.

    What makes it cheap is it is easy to recover most of the energy used to melt it.
  8. MikeJohns
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    MikeJohns Senior Member

    Structural fatigue from cyclic stresses and internal pressure effects in cracks and small voids will eventually destroy the matrix.

    For a start read the following paper from DNV:

    K. Hove Det Norske Veritas

    Then start thinking about how many dives and to what depth. The problem with reinforced concrete is that it cracks under load, the size of the cracks determine the stress levels and the fatigue cycles allowed. Offshore concrete platforms pump water out that comes through the cracks.

    Eventually after a number of high stress cycles it fails catastrophically, as in your pressure hull stops being one atmosphere and equalises with the ambient external pressure................
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  9. rwatson
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    rwatson Senior Member

    This is very interesting. I was talking to a designer of precast concrete panels, and he was having a moan about structural engineers who made a fuss about a bit of rust on the reinforcing.

    I have read directions in engineering publications that recommend no rust on steel

    Anyway, he said 'I have never seen rust on any steel that has been embedded on concrete'. He said that in the case of breaking up mismade panels after a week or two, the steel was very shiny and clean.

    This ties in with the info you have.
  10. whitepointer23

    whitepointer23 Previous Member

    i don't think that will be a problem. if someone makes a home built concrete sub it wiil probably only do one dive any way.
  11. BATAAN
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    BATAAN Senior Member

    A concrete sub has a great future as a mooring weight.
  12. rberrey
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    rberrey Senior Member

    You could have an inside form the remains in place after the concrete is placed. Threaded dowels made from carbon attached to the inside form that would hold the out side form in place. Flowable concrete pumped into the forms. Only a fool would use steel rebar on a boat more less a sub when carbon rebar could be used. Rick
  13. BATAAN
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    BATAAN Senior Member

    One submersible uses an acrylic sphere with only one opening, the hatch, and controls all machinery with light flashed on photocells outside. Another uses a split dome that hinges open.
    Kittredge sells a nice set of plans for a practical personal submersible.
    And finally, one for sale:
  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    Interesting thread; hope it stays sane. I have a few comments:

    Concrete is an excellent structural material for use in compression. It is also very suited for constructing complex shapes without joints. However the rebar is usually prestressed, mostly to handle tensile loads, and this may be done to accommodate differential expansion rates.

    Many of the potential problems for concrete subs mentioned previously also apply to other materials, such as sealed hull penetrations for electrical and other services.

    I have read that reinforced concrete structures fail due to rust if salt gets at the rebar, which is a risk with bridges and parking lots in cold climates. So for use at sea frequent inspections may be advisable. However, same thing applies to steel . . .

    Elliptical sections to prevent rolling are not necessary, just keep the CoG low.
    Last edited: Jan 21, 2012

  15. bntii
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    bntii Senior Member

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