Composite Tang/ bracket construction methods for fwd crossbeam??

Discussion in 'Multihulls' started by Ismotorsport, Apr 1, 2012.

  1. MikeJohns
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    MikeJohns Senior Member

    Catbuilder

    You need to be aware that structures don't offer such a simple response.

    Even in that picture the fwd parts of each hull will have moved independently of each other.
    Under any differential load they are bound to move independently. It's a straight forward structural problem. The catch is making sure that if you add a cross beam that you either avoid transferring a moment into the beam Or you design the interface and the beam to take that moment.

    There is no doubt that if the beam is fixed a moment will occur and will lead to failure unless it's properly accounted for. It's simply the elastic response of the structure.
     
  2. CatBuilder

    CatBuilder Previous Member

    But the point is the 99% majority of the load is from the forestay and that is what the tang is designed to. Same as a chainplate.
     
  3. MikeJohns
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    MikeJohns Senior Member


    I was just worried by your comment that the hulls don't move relative to each other. It's important that you understand that they do exactly that. So be careful.

    If you pin the cross member you'll avoid a moment.
     
  4. powerabout
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    powerabout Senior Member

    if you could make the stucture so stiff the hulls dont move relative to each other every A class builder would love to know how to do it
     
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  5. idkfa
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    idkfa Senior Member

    that's easy!!! :rolleyes:

    Make them large and curve ends into the hullls, it is easy.

    silly bits of Al tubing need not apply!
     

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  6. idkfa
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    idkfa Senior Member

    Waikikin, I agree, the advantage the traditional bolt-on approach has, is that its failure can be engineered. It can part company with the boat taking only the smallest allowed pieces. Whereas a uni lay-up will take the bow off as the bulkhead comes out! Obviously requiring more energy. Or to make the traditional fitting that strong would mean it would be reallly, really heavy with backing plates.

    So no, one should give failure lots of thought, but think the uni lay is the way to go.

    IMHO
     
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  7. Ad Hoc
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    Ad Hoc Naval Architect

    Well said, But this simple message is falling on deaf ears!
     
  8. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Then again... :eek:

    www.youtube.com/watch?v=tuRyLH3yhXk

    (sorry for the thread disruption :( )
     

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  9. idkfa
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    idkfa Senior Member

    Yep, the tang came loose, darn!
     
  10. Gary Baigent
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    Gary Baigent Senior Member

    Yes, the bow fell off - with absolutely no reason to do so?
    When the first Bamboo Bomber and Great Barrier Express lightweight catamaran designs appeared in Auckland, the owner of the BamBomb Superbird sought expert engineering advice regarding the attachments of the forebeam to hull bow sections ... and that expert advice, concerned with movement in those areas, advised a pin arrangement which allowed pivoting and therefore would stop the connection tearing the bow sections and bulkheads apart, which he was adamant would occur. So that arrangement was implemented ... but the down movement allowed was not enough ... there was a savage hard spot at the inner gunwhale edge at the bottom of the movement ... which of course resulted in the alloy forebeam cracking up, almost completely round its circumference.
    Superbird was exhibited at the Auckland Boat Show - with the torn alloy areas discreetly taped over - which was observed by mischief makers and promptly unwound - to the embarrassment of the owner with everyone peering at the cracks.
    The other BamBomb (see jpeg) had glassed sleeves that went over the beam and down to the ring frames - absolutely no problems.
     

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  11. Charly
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    Charly Senior Member

    Hey I just wanted to bump this thread.

    Ismotorsport, what did you decide? Have you built anything like in the picture? Sure would like to hear about it and see photos if you have. Anyone?

    Thanks
     
  12. Ismotorsport
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    Ismotorsport Junior Member

    Charly,
    I have instructions for the crossbeam tang from engineer but It has not been built yet. as this tang is part of a bulkhead that attaches to hull before the crash bow section I did not need to install it yet. As soon as the second hull is complete I will add the Tangs and the crash bow sections to assure symmetry.
    Example shown on another design....
    [​IMG]
     

  13. Charly
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    Charly Senior Member

    Oh. Ok I see. Well, thanks for the reply. Awesome pic btw.

    My circumstances are a little different. I am building in plywood, but have decided to use as much composite tech as I can for the deck hardware, connections, chainplates etc. I got a layup schedule from Kurt Hughes for the chainplates on my build, but am afraid to start until I talk with him and others a little more about it.:)

    Also, I really like the hole in the bow (bowhole?) :), in the first pic you posted. To me it just makes sense to keep that section of the fordeck uncluttered, get the angle of pull from the bridle down lower, etc. If anybody here has done that, Id sure like to hear some feedback. Thanks!
     
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