Structural panel bulkhead joining question.

Discussion in 'Fiberglass and Composite Boat Building' started by groper, Dec 26, 2012.

  1. groper
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    groper Senior Member

    Ok i have a problem... i need to know whether a structural foam sandwich panel of a hull, needs to be butt joined and taped together before being taped to a bulkhead or whether its sufficient to allow the join in the panel to fall directly over the bulkhead and tape it to the bulkhead like this;

    [​IMG]



    Or does the panel need to have fibres under the bulkhead aswell?
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    I wouldn't recommend those options at all.

    Here's the preferred/best way of doing these joints.
     

    Attached Files:

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  3. Ad Hoc
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    Ad Hoc Naval Architect

    Groper

    It also depends upon the size of the boats the loads the materials, their properties and whether you can obtain consistent coupon tests to demonstrate this; the QA side is essential. Here is how we designed a typ WTB joint to hull for a 18m composite catamaran recently. This was all designed/built to DNV approval and survey. As you see, it is one of the 3 methods I posted above. Every method has its pro's and con's. You need to evaluate which is best for your application.

    Typ Class Approved WTB Joint.jpg
     
  4. groper
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    groper Senior Member

    Ok thanks for that, but it doesnt really answer the question directly...

    The specific issue, is a butt join of the hull sandwich panel occuring directly over the bulkhead and whether the hull panel needs to be first butt joined and taped together, before coving and taping to the bulkhead. OR does the inside hull skin require continuity of the fibres underneath the bulkhead?

    So in reference to your above drawing, can the inside FRP hull skin be broken where the flexible jointing compound is shown?
     
  5. waikikin
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    waikikin Senior Member


    Make the inside skin continuous, continuity of fiber is important! There may be some applications where the drawing is acceptable but your structural sandwich to hull or deck isn't one of them..Jeff.
     
  6. afteryou
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    afteryou Junior Member

    I would never do it that way. It is my very humble opinion that the hull is the hull and the bulkhead is the bulkhead there for the two should be made separately. My point of view was gained from years of repair not new build/design. Aside from trying to repair something that was built like that (think water), I would also be concerned with flexing causing delam and possibly impact damage being worse than if it had been built in the traditional manner.:)
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Perhaps you missed it:-

     
  8. groper
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    groper Senior Member

    Ok my intuition was telling me the same, but just wanted to clarify if it were possible or not... the problem relates to not being able to join the hull panels before offering them upto the bulkheads due to logistical constraints.

    i guess a solution would be to build a joining/bonding flange along the edge of the bulkhead first, which includes the missing fibres of the internal skin. Then offer up each individual hull panel and bond it onto the flange, and finish by taping over the external skin once both hull panels are in position.
     
  9. groper
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    groper Senior Member

    Tunnels, the problem relates to building a boat from pre made FRP sandwich structural panels... so imagine the topside of 1 hull, made from several smaller foam sandwich panels similar to the individual plates of a steel or aluminium hull...
     
  10. groper
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    groper Senior Member

  11. TeddyDiver
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    TeddyDiver Gollywobbler

    Dunno how the designer has calculated the longitudinal strength and stifness of the hulls in this case but I'd use some longitudinal stringers along the whole length of the boat..
     
  12. michael pierzga
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    michael pierzga Senior Member

    Joining a panel with a plywood scarf or glass sandwich butt joint overtop of a bulkhead or deckbeam, as shown in your scketch, is bad engineering .

    The bulkhead becomes a fulcrum and exerts force on the weakest link in the deck panel..the joint or scarf.

    http://[​IMG]


    Change the position of the panel joint
     
  13. rwatson
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    rwatson Senior Member

    Surely the designer is the person to contact about it.

    I think Tunnels point about getting all the inner and out glass in place before any bulkheads makes a whole lotta sense, especially as you have a join on the same spot as the bulkhead.

    The reinforcing on the inner hull prior to tabbing also seems like the totally logical thing to do as well, and fits with the diagrams provided.
     
  14. rwatson
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    rwatson Senior Member

    yeah yeah - settle down you poly pusher, just state the facts and leave the personality out. get some sleep for %%$R^ sake :p
     

  15. groper
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    groper Senior Member

    The Z-joins are only held together by applying epoxy bonding paste and pressing them together - thats it! The product is called duflex, and many mutihulls are built from it right now, all over australia.

    Yes, its balsa core comes in any thickness you need, with any laminate you need. Typically, 40 foot multihull topsides panels will be 13mm duflex with 800gsm dbias, Z-jointed all the way along the boat - so about 8 vertical joins, you can see them in the pic i posted earlier. The panels strength is reduced from the z-join, similar to a weld in aluminium. The reduction is engineered for in the design.

    My initial question spawned from the fact that if this type of joint is sufficient despite reduced strength, then i figured perhaps other alternatives might also be acceptable provided any strength reduction was engineered for? :idea: The reason, is to allow assembly in a different manner, which may be more labour efficient...
     
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