Composite Tang/ bracket construction methods for fwd crossbeam??

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

  1. Ismotorsport
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    Ismotorsport Junior Member

    How do you build a composite tang/ bracket?
    Looking for information on how best to build a composite tang/ bracket for crossbeam attachment such as the following examples....
    Seems like it attaches to a bulkhead, but it is solid laminate or can you use high density foam core with heavy laminate buildup. Also when installing a pin through the tang, can you just drill through laminate?


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  2. Ad Hoc
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    Ad Hoc Naval Architect

    It really depends upon what load the cross beams are having to withstand. The load from these beams shall dictate what is the best solution. You shouldn't just assume one method in preference for another....it should be ascertained by the amount of load path that is required.
     
  3. waynemarlow
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    waynemarlow Senior Member

    Corr talk about the right thing to create enough spray to firehose the whole boat down ten times over, err anybody thought a bit about stream lining such a critical area.:)
     
  4. Ismotorsport
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    Ismotorsport Junior Member

    It's been used in many other designs in lieu of laminating the crossbeam to hull or aluminum brackets... I was told that laminating joint of crossbeam to hulls will always flex slightly and show stress cracks.. This composite bracket allows some movement without the potential cracking.. So is it solid carbon laminate or cored laminate..
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    Is it like a version of this... But without the tube?
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  5. teamvmg
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    teamvmg Senior Member

    I make bracket and tangs like that out of solid carbon wrapped around and clamped in between strategically sized pieces of timber [Wrapped in parcel tape for release]

    Attached is pic of a gooseneck made this way

    Where a bolt goes through the bracket, you need to line the hole with kevlar because carbon is no good with abrasion [not yet done in the pic]
     

    Attached Files:

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  6. Ismotorsport
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    Ismotorsport Junior Member

    Nice work vmg... How exactly do you line the hole with Kevlar exactly? Do you layup Kevlar laminate on the outside of hole and re drill through Kevlar or roll up Kevlar cloth and insert into hole then drill through that? For a composite tang like the following.. Can you make a solid carbon tang with kevlar line hole that is then laminated to the vertical bulkhead or is it best laminated directly into the bulkhead like a chainplate.

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  7. CatBuilder

    CatBuilder Previous Member

    Good thread! Watching with anticipation as I haven't finished this area of the boat yet. I am doing composite chain plates which are pretty much the same thing, but this looks like another good use for composite parts.

    On my boat, the aluminum tangs are bonded to a watertight bulkhead (first of 2) at the point where the tang is shown in these pictures.

    Seems bonding a composite tang to the bulkhead would actually be easier than bonding an aluminum one, then bolting it to the crossbeam.

    Good thread.... watching with anticipation.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    I can't click on the image to zoom in. However, you have 2 hulls which are free to move independently of each other. As soon as you attach a structural member of XXX stiffness to both of them, the method of attachment is very important as the stiffness of the member shall dictate how it behaves. Since as one hull moves the other shall displace, or move, differently to the other, but will then displace/move differently once the structural member is attached - it is being forced to move like the other (which depends on how stiff the member is). Therefore, you need to know what the load is and how you intend to distribute that load on the tang and then into the surrounding structure. The type of 'fixity' you select for the joint is important too.

    So, without knowing what the load is and how you wish to attach the joint, you could be either over engineering it, too heavy and bulky, or, under estimating it, begin to show signs of cracking and eventually pull free.
     
  9. CatBuilder

    CatBuilder Previous Member

    Ad Hoc, that's not really what the tang is for or how the bows of catamarans work. They are not used to stabilize the bows at all. The tangs are used to take a tension load from the aluminum beam. This tension load originates in the forestay.

    It's nearly an identical situation to a composite chainplate. It's a load in tension only.

    In fact, my boat will be launched without the aluminum beam and motored 1000 miles before it is put on to support the forestay load.

    Structurally, the beam is not even necessary. It has nothing to do with bow movement relative to each other.
     
  10. teamvmg
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    teamvmg Senior Member

    I have made a kevlar tube out of a bit of kevlar sleeve laminated over a wooden dowel [With parcel tape], then cut bits off and drill out the dowel.
    Over -drill the carbon and then bond the kevlar tube in. leave the bolt in to keep it all lined up.
    don't try and drill kevlar - it's a night mare.
    I should have said that the kevlar liner will also insulate the raw carbon from your stainless steel bolt.

    If you are building from scratch, you could lay up the bulkhead and tang as one piece, making the transition from solid carbon to high density foam to your bulkhead foam as the laminates decrease.
     
  11. Ismotorsport
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    Ismotorsport Junior Member

    The structural model dawn for my project calls for a composite tang attachment point like the morelli designs use. I think that laminating a full carbon plate with Kevlar lined hole will be suffice to take the crossbeam loads. The other interesting project would be substituting uni fibers for the wire stays for dolphin striker support like some of the following examples...It seems like these can be built in composite as well??

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  12. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Noted, as that is what it appears to be from those images; assuming the deckhouse is taking the "global load", as such.

    However, as one hull moves it shall move differently from the other; just having one person on the port bow and none on the stbd bow is a simple example of the two moving independently of each other. In which case, no matter what you "think" the joint is being used for, fore-stay beam, it is being affected by the relative movement of one hull to the other. Thus if the joint is fixed in some way, then the joint must withstand this relative movement, unless the joint is free to rotate and a translate at will, like rubber, which I doubt it is!
     
  13. Ismotorsport
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    Ismotorsport Junior Member

    Vmg, that is useful information... Thanks. Perhaps you can share your thoughts on how the crossbeam bracing can be made with uni carbon cloth in lieu of a wire or stay? Could you wrap the uni around the triangle shape in multiple layers.. And could the crossbeam ends be composite tanga like your gooseneck fitting?
     
  14. Ismotorsport
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    Ismotorsport Junior Member

    Composite forestay fittings too...

    Seems like you could go about creating a composite chainplate fornthe forestay attachment as well using this technique..
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  15. CatBuilder

    CatBuilder Previous Member

    I see your point about the importance of the tang attachment type, that makes sense.

    However, the bows moving differently relative to each other really doesn't matter. It doesn't happen.

    My point is, your post is saying the joint has to deal with the relative movement of the bows (to each other). This would indicate a tang attachment that has to deal with both tension and compression loading. That type of movement is nil, so that is not what these tangs are engineered for. They are engineered to take a tension load straight from the aluminum beam.

    The bows do not move relative to each other in a properly designed and built catamaran. If you jump on one, the other will rotate to match the angle you just created, putting that rotation through the rest of the boat. The boat will heel, just a couple degrees if you have more weight on one bow than the other. Also, the opposite hull to the one where you put the weight on the bow will rise up at its stern.

    The bows do not move at all relative to each other. They can only move inward, toward each other, because of forestay tension on the aluminum beam. That's what the seagull striker is for - to keep the aluminum beam from folding in half at the forestay attachment point and pulling the bows inward.

    It's not "what I think", its basic catamaran design 101.
     
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