Unidirectional Carbon or Weave for Torsional Stiffness

Discussion in 'Materials' started by Ratty, Oct 15, 2016.

  1. Ratty
    Joined: Sep 2016
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    Ratty Junior Member

    Hello All,

    This is my first post, although I've been lurking on this site for years.

    I have made a 16 foot (4.8m) trimaran sailing canoe. The main hull (approximately 2ft square) is made of 3 layers of 5 oz Kevlar.

    Although it doesn't oil-can much (I added some 45 deg biaxial/mat to the sides), it is lacking torsional stiffness that shows up when under sail.

    I am thinking of adding a single layer of carbon on the inside to help this problem and to add some more stiffness to the bottom so that feels more solid while standing in it.

    My question is, would it be better to use uni-directional carbon or plain weave for this added layer?

    I believe the load path is mostly fore-aft for this slim hull so better to use unidirectional carbon along this axis. I also think this would make it more torsional stiff because the fibers would be perpendicular to the torsion shear.

    But if it won't help that much, I'd just go with plain weave since its cheaper and looks better.

    By the way, the main hull is constructed of 2 identical 8 foot (2.4 m) sections that bolt together through plywood reinforced bulk heads. I did this so that it could be nested and I would only have to build half of the male mold.

    :Ratty
     

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  2. upchurchmr
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    upchurchmr Senior Member

    If you want torsional stiffness, you need two plys of unidirectional, one at +45 and the other at -45.
    A single ply will not help.
    If you use plain weave, lay it in at 45 degrees.

    Oil canning would be helped more by stiffeners although any additional thickness will help.
    Putting in a thick or box section gunwale would also help. A small perimeter deck would help in the same manner
     
  3. SukiSolo
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    SukiSolo Senior Member

    The problem you have is torsional stiffness of the hull, induced by the float beams twisting the hull. The best shape (geometry) to resist torsion is a tube. To that end you need to beef up the inwhales and create some small bulkhead frame or maybe hog (triangulated) frame. These two frames, with maybe a middle one, between the two, need to be linked either by stringers or as suggested an internal hog. One case gives you a 'box' which you can X brace, the other a triangular prism effectively. Both will fight torsional twisting better than adding another skin.

    You could do it with a skin but it would require real thickness, say 6-10mm + internally, either foam, timber whatever and glassed over. It is another way of creating the box effect but slightly heavier.
     
  4. Ratty
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    Ratty Junior Member

    The 45 degree layup makes sense, but all I can find with a quick search are unidirectional widths of 12 inches. Would I just butt these next to each other or overlap them? Overlapping might create a diamond pattern of stiffeners.

    The gunwale does have vinyl box section extrusion, but it's not too stiff. Wish I could have found an aluminum one.

    Thanks for your help!
     
  5. Ratty
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    Ratty Junior Member

    I think I get the basics of what you are saying, make a cross-braced prism similar to a WWI airplane frame.

    I was planning on adding aluminum C channels for seat rails, these could act as stringers to the secondary bulk head frame.

    Thanks!
     
  6. upchurchmr
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    upchurchmr Senior Member

    You can just butt the sides of the unidirectional 12" "cloth". No overlap needed.

    For oil canning you need full length stiffeners parallel to the gunwale (approximately parallel).

    Search for "shear stress" in a torsion beam. You are trying to put the fibers in the same direction as the maximum stress - and you have to get it in both directions.
    I don't exactly know what you are referring to in a WW1 plane, but structure was not well understood back then - I wouldn't use that as an example.

    Adding a deck/ side deck/ boxed gunwale is probably the most effective strategy, along with bulkhead frames.
     
  7. Petros
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    Petros Senior Member

    Welcome to the forum,

    I agree with Sukisolo, you would have to add a lot of skin thickness to make an open channel torsionally stiff. Layering up more skin will not help much.

    Beef up the gunwales (or inwales), and add frames. Actually the best way would be to put horizontal cross bracing between the gunwales, but that would reduce the utility of the hull.

    You basically have half a tube, if it is not a structurally continuous full tube, than you need to have the structure where the "tube" is cut in half to carry the loads to the other side so there is a continuous load path.

    I have built a lot of very light skin on frame canoes, kayaks and small trimarans, without fairly stiff gunwales, out rigger beams tend to put a lot of distortion in the hull.
     
  8. srimes
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    srimes Senior Member

    Are the seats rigidly attached? Maybe make a couple of them bigger. For this design I'd look to the seats to provide the shear support that gives torsional stiffness. Either seats or partial decking.
     
  9. gonzo
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    gonzo Senior Member

    Bulkheads where the crossbraces are will help stiffen the hull. Also, if you can deck over some sections, it will create a box section.
     
  10. Ratty
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    Ratty Junior Member

    Currently they are not rigidly attached, they are just pieces of wood that sit on a 3/4" lip/strake formed about 4" down from the gunwale. I always intended to do something better than this.

    I was thinking about adding a rectangular tube (say 1" x 2") along these edges with some cross bracing to create something similar to a truck ladder frame. The seats could then be just run between these 2 rails.

    Only problem is that because the hull begins to narrow 25% of the way from the ends, this frame would have to end short.
     
  11. upchurchmr
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    upchurchmr Senior Member

    Torsional stiffness is needed "most" of the whole length.
    Isolated seats won't help much and will concentrate the forces trying to resist twisting over a small area - might be a natural place to break.

    You need to do something with the gunwale - its a free edge with no lateral stiffness. Your seats could hang from the box on the gunwale - if you have outriggers you won't have much concern about the seats being too high.

    If you do a box support, you will probably need to build it in place which means it could be built to match the curve.

    Build the box reinforcement, then see if it seems stiff enough. The "ladder" frame would limit your access to the center of the boat if you overdo it.

    Have fun.
     
  12. redreuben
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    redreuben redreuben

    As others have said the gunnels need stiffening and a fore and aft stringer with some ribs would help too ! It will take a lot of expensive laminate to stiffen that hull.
     

  13. Ratty
    Joined: Sep 2016
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    Ratty Junior Member

    Thanks all!

    I will focus more on structure instead of simply materials.

    :)
     
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