Catamaran composite beam design

Discussion in 'Multihulls' started by groper, Apr 29, 2012.

  1. groper
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    groper Senior Member

    Just trying to figure out the best way of building the beams in my cat...

    I want to infuse these foam cored shear webs on a flat table. I would also like to incorporate the uni directional schedule into the top and bottom edges of the these shear webs as shown in the attached drawing to act as the tension/compression spar caps, rather than build the caps seperately and bond them (tape them in) to the shear web for simple reason that its more work...

    However, im not convinced that this is an appropriate way of engineering it because all the spar cap fibres cannot be located at the extremities of the shear web, theres a difference of 100mm (width of the UNI tape) between the outer most spar cap fibres and the inner most spar cap fibres if i lay them flat on the shear web as shown... is there a better way of designing this process to achieve the same end result in a 1 step infusion easily?

    On the rear beam, there is a 45deg change in direction from the horizontal bridgedeck, down into the chamfer panel on the inner side of the hull. Can i simply overlap the fibres as shown in the drawing to achieve this change in direction or do i need to feather them out further around the intersection?

    Or is there a better/easier way of doing all this period?
     

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  2. waikikin
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    waikikin Senior Member

    lay the uni into the connective bulkhead tabbing/landing area to inner skin of underwing & hulls & cabin/beam.
     
  3. groper
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    groper Senior Member

    Yes, thats the way ive seen it done, however im trying to avoid that extra work if possible by incorporating the uni into the panel and infuse it all in 1 shot... i wanted to ask whether this would be acceptable or a flawed design?
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    groper,

    You need to address this in 2 parts. Firstly the design and secondly the fabrication.

    With regards to the design...you state there is a 45 degree change of direction. Given that these members shall, as you point out, be in tension, this change in direction shall increase the load in those fibre by sqrt2, or 41%. Thus once you work out, from the loads what structure is required, can you achieve the method of fabrication you desire and maintain structural continuity and integrity?

    If you can demonstrate to yourself yes, then it is easy. If you can't, then you need to go through the design spiral again.

    Wrapping say 4 layers of fibre around 90 degrees is silly, yet wrapping them around say a lazy angle of just 10 degree is easy. So, to get the desired change of section you require, you must first address the structural continuity aspect and then see how to maintain this and how easy/difficult is it to do what changes, if any are required and can you do it.
     
  5. groper
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    groper Senior Member

    well again, i cant quite interpret your words fully AdHoc, but as to the fabrication i have the ability to fabricate it any way i choose and all tools and materials are on hand... What the question relates to, is finding the easiest way of doing it to save myself hours of labour... does that make sense? Why do it the hard way when i can do it the easy way...

    However im not a composites engineer, and i dont understand enough about the effect of moving the spar cap fibres to where they are shown in the drawing, as opposed to where they normally would go on the top and bottom of the shear web... thats what im trying to understand... to me, it seems that every induvidual uni fibre in the tension/compression caps, would have a different strain based on their proximity to the edge of the shear web. Done in the usual spar cap fashion, all the fibres would have the same strain for all intensive purposes. Doing like i propose, this is no longer the case and i dont know how to compensate for this effect?
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Yes, that does.

    But this doesn’t.
    You can’t just guess/assume or “I reckon”…you need to design it. By that I mean, establish what the loads are. From that you can calculate what scantlings are required. If you don’t understand, then you should not be ‘dabbling’ as such. Seek the advice of a NA or composite engineer.

    And this is why you’re unsure. If you don’t know what the loads are to begin with, then how can you design the structure accordingly?

    Which is my point. 1) design then 2) fabrication. It seems you're putting the cart before the horse..
     
  7. groper
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    groper Senior Member

    i did not ask what the loads are;

    im asking whether the fibre orientaion and placement was a vaild solution to the simple bending problem- not how much fibre do i need and where...

    The torsional and buckling loads are handled by other parts of the structure, this is a simple transverse bending problem - and the loads can be arbitrary for discussion purposes... does that clear things up?
     
  8. Richard Woods
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    Richard Woods Woods Designs

    I agree with your concerns. As always, I assume you want an efficient design, not just a design that is strong enough but is also overweight.

    I think you will have to bite the bullet and make proper flanges. That way the material is as far from the neutral axis as possible so results in the stiffest beam

    I assume you will be glass-bonding the bulkheads/beams into the hulls. In which case what you have sketched should be OK

    But of course I'm not the designer of your boat....

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  9. groper
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    groper Senior Member

    Thanks Richard, you seem to have understood exactly what my concerns were... the different distances of the UNI fibres from the neutral axis.

    So, to reengineer the scantlings for this new fibre placement, i need to use a shallower beam depth (or distance between caps) to figure the new scantlings? But by how much, do i use the distance to the closest fibres to the neatral axis, or midway between the closest and furtherest, or...???

    I realize its going to be heavier this way, but the reduced labour and increased mechanical properties of the infused laminate rather than hand laid, should partially make up for this...
     
  10. Richard Woods
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    Richard Woods Woods Designs

    It's a trivial design exercise to compare a beam with long deep flanges and one with wide shallow ones. To keep the same beam stiffness you keep the same inertia, assuming the material properties are the same. I would expect that a beam such as you are proposing will be much heavier than a conventional box beam with the main laminates on the upper and lower surfaces

    I doubt whether you are calculating so accurately that there will be a difference between infused and non infused laminates. And building a boat quicker won't make it lighter!

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  11. groper
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    groper Senior Member

    This is still part of a box beam, the drawings only show 1 shear web for each beam... the beams are comprised of 2 shear webs each, sandwiched between the bridgedeck and upper deck for the front beam.

    The rear beam i could use a conventional cap on the top edge which forms a seat on the rear deck... and i guess i could include the bottom cap in the bridgedeck laminate, im just looking at other options to find the easiest method...
     
  12. groper
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    groper Senior Member

    Hmm... if i use the center line of the UD tape to reconcile the total beam height, i loose 50mm on both edges which reduces the total height of the rear beam to 400mm. This resulted in a deflection of 70mm as opposed to 45mm for the 3.3m beam in cantilever with end load = 2 tonnes. So a big difference for the same weight... to acheive the same deflection, i need to increase the tension/compression caps laminate weight by 60%.... looks like i might have to do it another way for the rear beam...

    Front beam is a different story with much greater depth, the difference is minimal...
     
  13. catsketcher
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    catsketcher Senior Member

    Why uni the shear web

    What I don't get when designers draw this is why they put the unis on the shear web at all. Considering the bridgedeck floor and cabin or top and bottom of the box beam is a big flat area.

    Do the designers think that the huge top and bottom flange will be elsatic enough not to take the load and let the skinny little pieces of uni take it instead? I can't see it. For my mind the load will be transferred to the top and bottom skins anyway as they are so stiff and will load up before the unis can take much load.

    So leave the unis off the bulkhead and put them on the top and bottom flange. Do the skin next to the bulkhead to reduce shear load on the flange. This should be easy to incorporate into the top and bottom laminate. There are secondary bonding problems anyway but as the load will be taken by the stiffest section of the beam it is probably better to wet on wet the unis onto the top and bottom flanges and then secondary bond these onto the bulkhead with good coves and tabs.

    Saw a friend stuff his crossbeam with unis as specified by the plans. He cut out the top of the balsa core and laid in unis. Sure the unis could take some load but the ability to accept shear loads from the web would be low as the area the uni could bond to the bulkhead was only 25mm deep max. The well known designer probably had forgotten about max shear loading of glass laminates. Super stiff uni reinforcement but little opportunity to feed it into and out of the 25 mm deep groove. I see this on other well designed boats too. Keep asking questions Groper. With the exception of Richard here few designers have a background in naval architecture or engineering. They still produce great boats but some things make me scratch my head.

    cheers

    Phil
     
  14. BobBill
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    BobBill Senior Member

    Very interesting topic, to me. Been struggling with way to fold akas on a Malibu Outrigger and still keep costs and so on low.

    Original design calls for 12 foot ply akas, made with 4 pieces of .75 marine ply...simple, cheap.

    Below is pic of beam with hinge I picked up from proa thread somewhere. Looks to be best for application and could work with aluminum or carbon.

    I have thought about other ways and other materials, but keep coming back to simple wood...still interested in alternate ideas.

    One does not want to have a failure...
     

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  15. waikikin
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    waikikin Senior Member

    I think you just mentioned it, the structure is already in place to incorperate uni ei bridgedeck & underdeck/seat. Phil's point on the replacement of core in the bulkhead is very valid in regards to squibby bonding area in between the bulkhead skins. I don't see the much extra labour in installing the uni in way of the bulkhead tabbing or interleaved with the tabbing, your gunna be laying glass there anyway. Jeff.
     
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