Pressurizing a hull to make it more stiff

Discussion in 'Boat Design' started by SeaSpark, Apr 9, 2006.

  1. Thunderhead19
    Joined: Sep 2003
    Posts: 506
    Likes: 3, Points: 0, Legacy Rep: 21
    Location: British Columbia, Canada

    Thunderhead19 Senior Member

    I see what you're getting at. Something like the reverse of a submarine. Pressurizing a balloon, for example, (or a condom) does make it stiffer, but it makes it a lot weaker. Take a condom and fill it with water (not so much that it stretches), and drop it. It probably won't break, and it has only become marginally stiffer (and thats with water in it, not air). Now fill one with water until it streches a fair bit. It will only be a tiny bit more rigid, but if you drop it, it will burst. The impact loads that a boat sees are many times greater than those seen by a boat hull. With aircraft design Aircraft fuselages are not pressurized to make them stronger of more rigid. When Federal Express buys old passenger planes to use to ship cargo they de-pressurize the passenger cabin to make them stronger ( compensating for the degree of fatigue that the airframes have undergone in their lifetimes from constant pressurizing and depressurizing.

    What you're suggesting is definitely achievable.
     
  2. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 206, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    Not in this forum, and the results of the investigative FEA showed that the pressure needed to fairly high to be worthwhile and that other methods worked out lighter and cheaper such as adding more fibre.

    From an engineering viewpoint it's a non-starter for rigid materials.
     
  3. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 206, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    Gareth

    Small hulls rarely fail from anything if built to sensible scantlings ( exempting racing hulls). What kills them is the unplanned loads following which you will often get tensile failure of parts of the structure (as well as buckling).

    Ships for example often fail in tension when the deck/topsides structure prove inadequate .

    Also be aware that a lot of buckling in ship structures is often not a catastrophic failure in itself but leads in itself to related tensile failure.

    Thats why we model everything in those expensive computer packages these days. Its all horribly complex.



    Cheers
     
  4. DanishBagger
    Joined: Feb 2006
    Posts: 1,540
    Likes: 46, Points: 0, Legacy Rep: 523
    Location: Denmark

    DanishBagger Never Again


    Nah, you're kidding me ;)
     
  5. SeaSpark
    Joined: Mar 2006
    Posts: 593
    Likes: 17, Points: 0, Legacy Rep: 96
    Location: Holland

    SeaSpark -

    Investigative FEA

    Finally, some numbers.

    Thanks MikeJohns, still curious how this FEA analysis has been done.

    By the way i posted a picture of the lattice mast from your gallery in the " Mast aft rig" thread http://www.boatdesign.net/forums/showthread.php?t=11164 and followed the 2 other lattice threads in this forum. Do you have more information to share on this subject?

    There also is a remark about the A-mast alternative. Do you have experience with this?
     
  6. grob
    Joined: Oct 2002
    Posts: 216
    Likes: 5, Points: 18, Legacy Rep: 53
    Location: Cotswolds Waterpark, UK

    grob www.windknife.com

    "Hulls rarely fail in tension" OK I worded that pretty badly, the point I am trying to make is that while pressuring a rigid hull is not going to be the answer to all hull problems there are situations where it has some merit. i.e where you are trying to protect from faliure due to local buckling. Local buckling (bezier buckling) is found in structures with relatively low stiffness and thin walls.

    The eqn for bezier buckling of a thin walled tube is:- stress to cause buckling=.25*E*t/r

    where E=matl stiffness, r=radius of a tube, t=thickness of tube.

    If you plug a typical carbon mast dimensions into this you will see it is too stiff and thick walled to suffer from local buckling. A rotomolded catamaran hull is a different matter it uses a cheap low stiffness matl and is a relatively large diameter with a small wall thickness. e.g 0.25*1000MPa*6mm/150mm=10MPa. 10MPa is well below the normal faliure stress of the matl. Therefore unlike a carbon mast it could benifit from being Pretensioned.

    If you want to see how this works in practice, get a large plastic thin walled bottle of fizzy drink and take the lid off to releive the pressure, then put it back on and give the bottle a squeeze, its not vey stiff!. Next give the bottle a shake and try and squeeze it again, it is now much stiffer. This is a rigid structure made stiffer by pressure.

    The downside may be that while local buckling would not be a catastrophic faliure as you say (they will often spring back into shape when the load is removed), pressurising a hull may indeed put you into the realms of catasrophic faliure if you have not done your sums right. Think of the fizzy pop bottle experiment taken to the extremes. I think this is why people have stayed away from this method of hull stiffening.

    All the best
    Gareth
    www.fourhulls.com
     
    1 person likes this.
  7. granite
    Joined: Oct 2005
    Posts: 25
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: UK

    granite Junior Member

    The catapult is a catamaran with inflatable hulls, there is beam to stiffen them and a fiberglass bow section fairing

    http://www.catapultcats.com
     
  8. SeaSpark
    Joined: Mar 2006
    Posts: 593
    Likes: 17, Points: 0, Legacy Rep: 96
    Location: Holland

    SeaSpark -

  9. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 206, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    SS

    If you want FEA tips you need to access tutorials on pressure vessels.

    As for lattice masts, yes lots of information, we design them; mostly for larger sailing vessels we charge $400 Australian per design . They work out very cheap to build and no heavier than the equivalent Al extrusion. You have the steel maintenance work ongoing but with modern epoxies that is minimal.

    A-frame spars suit modern lightweight materials but you have to allow for the compression on the mainstay trying to spread the frame so you need cross members and then the whole structure starts to get a bit too much windage.

    I'm not keen on aft mast designs either, too much windage aft in the bare rig so running off is dangerous in the situation that it shoulkd be safest (in a severe storm).

    cheers
     
    1 person likes this.
  10. SeaSpark
    Joined: Mar 2006
    Posts: 593
    Likes: 17, Points: 0, Legacy Rep: 96
    Location: Holland

    SeaSpark -

    Reputation

    Mike Johns, your reputation in the trial we are having has just gone up.
     

  11. tspeer
    Joined: Feb 2002
    Posts: 2,257
    Likes: 226, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.