Help me understand why this asymmetric lay up was used

Discussion in 'Fiberglass and Composite Boat Building' started by Blueknarr, Sep 21, 2017.

  1. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    I am reinstalling the bow deck on older compilation speedboat,
    Former owner tried to convert to open bow seating.
    I have the original cut out.

    Its original construction from outside inward:
    3/16" Matt or sprayed chop strand
    6 oz twill
    2mm core Matt
    6 oz twill

    What could be the reasoning behind the asymmetry?
     
  2. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    to avoid that, from outside, the warp of the fabric is seen.
     
  3. Blueknarr
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    Blueknarr Senior Member


    Thanks for your thoughts.

    I still don't get it.
    The core Matt is on the inside.

    I understand it's purpose if placed under gelcoat, but nearly 1/4 inch of Matt between it and the surface doesn't make sense to me.
    Inside headliner was carpeted so no need to hide weave their either.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    How did you establish what the lay-up was ? Sounds like it is about 3/8" thick, which seems like extreme overkill.
     
  5. Blueknarr
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    Blueknarr Senior Member


    It is about 3/8" thick.
    Measured cross section at start of repair.
    Thick mat I understand because of its relitive weakness. Inside layer of twill has fewer flesh-hooks than raw mat.
    But what would be the intention of using core Matt so far from the gelcoat?

    I wonder if builder reversed lay up schedule.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Coremat is to get the extra thickness to increase stiffness, presumably.
     
  7. jorgepease
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    jorgepease Senior Member

    yep!
     
  8. Blueknarr
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    Blueknarr Senior Member

    Thanks all for the education.

    I would not have thought of core mat to be an effective stiffener.
     
  9. rwatson
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    rwatson Senior Member

    Its the extra, non compression thickness, holding the two layers of cloth apart, that does the job.

    They could have used dense foam, balsa, instead of matt. Anything to provide distance, therefore stiffness, between the layers of cloth.

    Its like having two layers of trenchmesh in a foundation, if they are on top of each other - no stiffness. If they are separated by 10 cm of concrete, vastly increased stiffness of the foundation.
     
  10. TANSL
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    TANSL Senior Member

    The coremat contributes little to the effective stiffness of the whole. Its main mission is to separate as much as possible the layers that must support the most important efforts (the layers furthest from the neutral axis of the set). The coremat, fundamentally, must support the shearing loads that occur between outermost layers, preventing them from slipping between them.
    The initial laminate is completely symmetrical, which is a very good disposition. Probably, to avoid that the fiber footprint, 6 oz. Twill, be seen from the outside, 3 layers of soft mat have been placed.
     
    Last edited: Dec 15, 2017

  11. redreuben
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    redreuben redreuben

    Couple of reasons, 1) Fibreglass is much weaker in compression than tension so it is "normal" to use a thicker skin outside, more so in foam sandwich but same principle. 2) No engineering, totally made up by what was on the shelf or cheap on the day.

    My money is on option 2.
     
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