Cat Scantlings

Discussion in 'Multihulls' started by TealTiger, Mar 26, 2012.

  1. Richard Woods
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    Richard Woods Woods Designs

    In my previous post I copied part of my crossbeams comments seen in full on the FAQs page of my website

    What I didn't include was my comment on Team Phillips. The largest podcat yet built.

    One reason (maybe the major reason) why it was abandoned was because the pod started to break away from the beams. This was due to the twist/torsion between the hulls. The front beam was very far aft and there was nothing to stop the bows lifting and twisting the boat. So there were big loads on the cuddy and it began to break free. Originally the idea was to have that part flexibly mounted, but in the event that never happened

    That's why I say the fore/aft beam placement is very important if you want to keep the hull together

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    It is not placement of the beams per se. That is easy. It is the establishing the load s to apply that is the critical aspect. Since once you have the loads, whether the beam is at the extreme ends or 1/4 way in or 1/2 way in from the bow/stern doesn't matter. Why....because if you know the loads, you can design for it.

    If Team Philips had problems they underestimated the loads they expected to experience. To design the scantlings, it is simply reduced to a force x lever. The 'lever' being the location of the beam. But the force...ahh...that's the tricky part.

    If one is going to design for 'light weight', then they had better be 100% sure of the loads they expect, otherwise failure shall occur far sooner than they think.
     
  3. groper
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    groper Senior Member

    ok we have talked about the type of loads the beams are subjected to, can we back this up with considerations and practical application for beam construction methods of composite bridgedeck catamarans?

    In most of the australian built modern sailing catamarans, the designers are using large bulkheads which span between the hulls... on an average 40 fter, they will use about 4 of these bulkheads with the biggest/heaviest one located under the mast for obvious reasons. A heavy schedule of uni directional is placed under these bulkheads between the bulkhead and bridgedeck, and also layed over the top of the bulkhead (which may be under the topdeck for example) This forms a large I beam -like the original poster was asking about. And it seems its torsionally stiffened by using structural furniture attached to one side of the beam to form a box or perhaps the front anchor locker webs where the bridgedeck rises to meet the topdeck etc. Rear beams are often formed into seats on the back deck, again forming a box beam to help with torsion loads etc... you get the idea...

    The discussion id like to raise relates to the way in which these beams are constructed, and what alternatives there are.

    For example, is it nessesary to run the Uni flanges directly over/under the bulkhead? Can the uni flanges be laminated to the sides of the bulkhead along the top/bottom edges so that the bulkhead can be completely prefabricated and faired before installation into the boat?

    Ive also seen instances where the core material is removed from the top/bottom edges of the bulkhead after the laminate is applied to both sides, then uni tape is wetted out and folded over on itself and inserted into the void between the skins along the edge. This leaves no lumps to fair out later, save any taping in, although it would seem problematic/easy to trap air bubbles when inserting all the uni into a deep cavity...

    Are there any other effective methods of building these beams with consideration of ease of building, labour reduction = reducing the cost of construction?
     
  4. Autodafe
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    Autodafe Senior Member

    I hadn't heard the part about the pod breaking up Richard. That's interesting.

    Speaking from a position of considerable ignorance about the specifics, it sounds like a good illustration of the point in my previous post:
    You can make a light/weak pod, but it must be flexible, or you can make a stiff pod, but it must be strong.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Either I'm being very stupid and don't understand the brevity of your question, or you have already answered it above.

    If the flanges are not attached to the bulkhead..how does it form a box?
     
  6. groper
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    groper Senior Member

    No, i simply mean the the location of the flanges on the bulkhead itself... flanges on the side edges vs flanges directly under/over...

    You see, if rebated edges are put along the top and bottom edges of the bulkhead, and the heavy uni schedule is run in this rebate, it would simplify the construction of the beams as they could easily be infused on a flat table and come out completely flat and fair which eliminates alot of labour. The normal method i see being used today, is to hand lay this uni on the bridgedeck, then drop the bulkhead on top, tape it in, then fair everything out... $$$...

    The box is still maintained by the structural furniture which is attached to the side of this main beam/bulkhead which stops it from twisting...


    This is typical of how the box is formed on the majority of these modern composite cats...
    [​IMG]
    [​IMG]
     
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  7. Ad Hoc
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    Ad Hoc Naval Architect

    Again, you've answered your own question.

    So long as the box is maintained and that the load paths are the same and the scantlings are sufficient for either case, a box is a box. You need to satisfy yourself that the bending/shear stresses you obtain at the edges is being taken by the amount of material you have selected to use by your preferred production method.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Ahh..i see you have added some pix now too, nice :)

    But without seeing the scantling dwgs hard to say, since, as noted above...you need to ensure you have a proper load path. Those pix don't tell the whole story. Only the construction dwgs will tell me that.
     
  9. groper
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    groper Senior Member

    Ok... does anyone care to share other effective/elegant designs for these types of beams?
     
  10. CatBuilder

    CatBuilder Previous Member

    Wow, sounds so much more complicated than what's on my plans that I have to reply.

    My beams have all of the uni in the full width bulkhead lamination itself, so it's all done infused on the glassing table. The full width bulkhead is then taped in with biax coves and has a second full width bulkhead very close by (or the forward anchor locker as pictured) to create a box beam which isn't going anywhere and also acts like an I beam, but with the flanges of the "I" just moved into the space between the two bulkheads.

    You guys actually make a real "I" shape with uni between the beam and bridgedeck? Seems like a lot of extra work, or maybe Kurt Hughes is a really good designer. :)
     
  11. waikikin
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    waikikin Senior Member

    Typically the uni to the lower flange is laminated in situ of the bulkhead landing/tabbing area- from hull to hull across the underwing to other hull - prior to the bulkhead installation, for the uni across the top it's simpler to incorporate the uni into the bulkhead tabbing(interleaved to deck inside skin side) once the cabin & or deck skin is in place. The aft bulkhead/ beam structure often needs for access(too small to "get in") reasons to be fabricated with flanges in place with tops or fairing bonded on. Crowther & Schionning cats(in my exp) would have roving or tape incorporated into the top & lower edges of "connective" bulkheads in place of the core material, this could be quite time consuming to install.Jeff.
     
  12. groper
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    groper Senior Member

    indeed catbuilder, its further complicated if the beam is curved instead of straight... then you cant laminate the uni schedule into the bulkhead on a flat glassing table... is has to be done in situ after the bulkhead is bent into shape or its all done in a mold... lots of nice cats have the rear beam curved like this...

    example;
    [​IMG]
     
  13. waikikin
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    waikikin Senior Member


    Hey Groper, that looks too groovy to be real. Just needs me in the pic reeling in a tasty meal................
     

  14. CatBuilder

    CatBuilder Previous Member

    Wow, I have none of the above. The only uni is in the flat panel of the vertical faces of the box beam. The box is made, then anchored to the bridgedeck. No extra laminations are made on the bridgedeck and it's all roved and coved into place.

    My bridgedeck is a monster though.... Very thick core and 2300 gsm on each face of it.
     
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