Kevlar canoe

Discussion in 'Boatbuilding' started by AK44, Sep 24, 2019.

  1. AK44
    Joined: Sep 2019
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    Location: Canada

    AK44 Junior Member

    Hello,
    I'm looking for technical advice from people having experience working with composites.
    I plan to make kevlar canoe. The plan is for 18' canoe 3 or 4 layers with vacuum epoxy infusion.
    But I recently got over pretty inexpensive kevlar sheets 3 ft x 5 yd pieces.
    Which means it will be made from many pieces.
    How is from your experience, how much it may affect hull strength?
    Is it worth to consider making it from pieces or better not to take a risk and go with one piece per layer?
    Thank you for your time!
     
  2. jehardiman
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    jehardiman Senior Member

    More important than the material is....what is the weave?
     
  3. AK44
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    AK44 Junior Member

    It's actually good question. Unfortunately I don't know wave because it surplus from other guy client and they do not have info.
    What waive should it be?
     
  4. AK44
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    AK44 Junior Member

    Or better say, what wave won't work?
     
  5. jehardiman
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    jehardiman Senior Member

    Should be linear or satin weave if you are going to use it for structural shapes, otherwise the resin will be the only strength. Most typical Kevlar "fabric" is designed to exploit some other property of Kevlar (i.e. cut resistance, fire retardant) while still giving good flexibility and draping.
     
  6. AK44
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    AK44 Junior Member

  7. jehardiman
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    jehardiman Senior Member

    In building a lightweight canoe or other narrow open shape, it would be preferable to use a fabric with greater linearity and less crimp. Ideally, there would be continuous stem to stern fibers without crimp (Crimp is the bending of the warp fibers over the weft fibers). So from a strength to weight perspective linear>satin>plain. However, while plain weave has lower mechanical properties (due to the crimp and more resin fill), it does build thickness faster and has slightly better cross axis impact properties. As long as the Kevlar fabric is intended for composite construction and you stagger the butt joins, it should be acceptable, if a litter heavier. A quick Google gives the following summary of fabrics
    Woven Fabrics https://netcomposites.com/guide/reinforcements/woven-fabrics/
    or you can use the old Gibbs & Cox Marine Design Manual
    https://ia800205.us.archive.org/20/items/marinedesignmanu00gibb/marinedesignmanu00gibb.pdf
     
  8. AK44
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    AK44 Junior Member

    As far as I understand in case of satin warp should go along the canoe hull.
    And cross axial impact won be affected much?
    Am I correct?

    Sorry, but I still cannot get what is linear weave?
    I have not read yet Gibbs & Cox Marine Design Manual, but woven fabrics not listed it.
     
  9. jehardiman
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    jehardiman Senior Member

    Realistically, the hull is going to be a minimum of 4 layers 0,+45,-45,0 where 0,+45 and -45 are the angles of the warp to the main stress axis (for a canoe generally along the keel and gunwales). With plain weave this layup won't have much change on strength. The outermost ply is always going to be the heaviest loaded, whether in tension or compression. If you are really worried about impact from the outside of the hull, you could try a <outside> 0,90,+45 -45 <inside> layup.
    Linear or Unidirectional fabric is a fabric where there is a preponderance of warp fibers with few or lighter weft fibers. These fabrics are very sensitive to stress axis, but provide better performance if used properly.
     

  10. AK44
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    AK44 Junior Member

    Thanks for advice it really make sense. But also as far as I understand for plan weave orientation does not matter?
    The plan is for now: first layer of fiberglass, second layer of full length kevlar, third layer of partial kevlar sheets, 3/8" foam core complete bottom and ribs on sides and on top of foam kevlar sheets. How all of it sounds for you?
    Also foam density - will 4lb works for the application?
     
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