fiberglass over cold-molded

Discussion in 'Class Societies' started by Hernandiz, Feb 26, 2009.

  1. Hernandiz
    Joined: Feb 2009
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    Hernandiz Junior Member

    Thank you for that answer Richard.
    You all were right my question was't clear. English is not my first language and I'm not always clear enough.
    In fact I'm familiar with steel corstruction, not wood and at first glance I tried to applied the formulas for sandwish construction. But 2 years ago I participate to the construction of the boat you saw on last pics.This is a mini transat design by Jean-Pierre Magnan. The hull were 1/2 oukoume with 12oz fiberglass roven outside only. If there is no fibreglass inside it can't be sandwish but 12oz look oversized to be un-structural.
    So I try to find how to apply this kind of design with ABS rules.
     
  2. Steve W
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    Steve W Senior Member

    Apex ,i can see why with total of 600gsm on the topsides and 940gsm on the bottom you would include it in the structure,thats a lot heavier than a typical sheathing although its on a larger boat than i was thinking of.I agree 100% with your last post.
    Hernandiz,even though the fabric skin is not structural you should do your own test samples with the various fabrics,say, glass, dynel and polypropylene on a piece of plywood and then pound them with a ball pein hammer. My preference is for dynel for the following reasons
    1/ it drapes really well,better than glass.
    2/ it consumes more resin which would be a bad thing if it were a structural skin but is exactly what you want in a waterproofing skin.
    3/it absorbs impact better than glass.
    4/its more abrasion resistant than glass.
    5/contrary to what some have said its not heavier than a glass skin in a typical application as dynel is about 4.2 oz whereas in a similar application you would likely use 6oz glass so while it uses more resin its about the same weight.
    I have not used polypropylene cloth but i believe its similar?
    Steve.
     
  3. apex1

    apex1 Guest

    You are right, 12oz IS adding sufficient strength to be noticed in the calculation (at least as a safety factor).
    And do´nt mix "design" and "diseno", design is "construccion", "edificio".


    Regards
    Richard
     

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  4. apex1

    apex1 Guest

    Steve, nono it is 300gsm total in two layers, than add the rest! Alltogether it sums up to 640 at the bottom (fabric only).
    Dynel is a acrylic fibre (and a good choice for the intended use), whereas I pointed to PE (Polyethylene or "Spectra", "Dyneema" etc.), wich is the strongest fibre available in composit construction. PP (Polypropylene) is a different stuff, and as far as I know, the Aussies $ is made of it (which holds me back from a deeper investigation about the application in yacht building).:D
    Regards
    Richard
     
  5. Hernandiz
    Joined: Feb 2009
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    Hernandiz Junior Member

    Thanks for the advice!
    Dynel is about the same final weight but what about the final cost:?:
    (compare to fibreglass)
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Dynel and Xynole will absorb two to three times the resin of a regular weave cloth of the same weight, possibly more. Given the two, Xynole is the better choice. My tests show Dynel to be around 3 times more abrasion resistant the cloth of the same weight. Xynole about 6 times better then cloth of the same weight. Again, much of this is the resin to fiber content. Both of these materials drape over compound curves much better then regular weave cloth, which is a real benefit, that must be traded with the additional weight and the fact that they will not contribute to the strength of the laminate (too stretchy).

    Dynel costs slightly more then good quality regular weave cloth. I haven't priced Xynole recently, but it was about twice that of regular cloth.
     
  7. Steve W
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    Steve W Senior Member

    Par, thats interesting about the xynole, ive got to try it sometime,thats a polyester i think. you know,we all say that these various fabrics dont add strength but while they may not add structually they sure add significantly to the impact resistance in small plywood craft,this was shown in a rather dramatic way when my son and i were building ourselves a couple of stitch and tape kayaks,we reduced the plywood to 3mm instead of 4mm and then instead of glass taping the outside seams we sheathed the whole outside with dynel.When we were not working on them we hoisted them to the ceiling of the shop to get them out of the way with a tackle system.when taking one down i dropped it from 20ft on to concrete,it did no damage at all,i was suitably impressed,if it had just been sealed with a few coats of epoxy it would have sustained considerable damage,in this instance i had treated the dynel as a structural part and reduced the scantlings accordingly.The finished boats at 17ft 10 with doubled bulkheads so we can break them down into 3 pieces for storage weigh in at 42 pounds.
    Steve.
     
  8. tom28571
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    tom28571 Senior Member

    I have fairly extensive data on testing of various sheathing materials but can't get the table to post in a readable fashion. Suffice to say that Xynole is ahead of the other synthetics and way ahead of fiberglass on abrasion resistance. No single layer of material other than biaxial non wovens add significant stiffness to a wood panel.

    The fabric is much more determinant for abrasion resistance than the epoxy.

    The abrasion tests are in the September/October 2002 issue of Boatbuilder magazine.
     
  9. yellow cat
    Joined: Mar 2009
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    yellow cat Junior Member

    check this www.multihulldesigns.com kurt hughes comments is my greatest inspiration so far, i ordered his vhs video but a dvd can probably be purchased.
    i saw combo (wood/fiberglassed ) boats in the past, delamination due to dissimilar structural stresses was a problem . Epoxy seems to be doing well, it's holding on some small projects. I would use fiberglass for impact resistance but not stress added resistance, delamination could occur. Fire retardant mesh can be added inside the epoxy. I picked up some roxul mineral insulation and will saturate it partially with epoxy, could be an option to fiberglass with many advantages. a mesh is also mentionned in west-system web site, its for airplanes also .
     
  10. apex1

    apex1 Guest

    Sorry yellow cat, I am shure you are a bit besides topic.
    What please is "combo"?
    Delamination is absolutely no problem! It just does´nt happen.
    Epoxy does´nt "hold on some small projects" only, it holds on every size of hull. And makes it stronger, to which extend so ever. Have a look at my Gallery there are some "small projects" shown.
    And we are talking about Epoxy / glass application only in this thread!
    The advantage of the glass / Epoxy application is mainly the better stress resistance, secondly the better impact protection! And how would you like to apply a glass layer for impact resistance but not for stress resistance? Does your layup "know" what was your intention?
    What is a "fire retardant mesh" good for, below the waterline? Just to weaken the matrix?
    Sorry but your post was total crap!

    Regards
    Richard
     
  11. yellow cat
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    yellow cat Junior Member

    delamination and fire retardant

    sorry i was not clear. The delaminations occured with polyester resin and fiberglass, not epoxy, at least so far. The fire retardant is for inside engine room fuel storage areas and kitchen. If a mineral fiber is combined with epoxy on one side of a sandwich panel and fiberglass is on the otherside i will not be surprised for structural gain.
     

  12. apex1

    apex1 Guest

    If you read the thread, you will notice that we have been asked for a outside sheathing of a plywood hull with glass / EP.

    And "at least so far" is nonsense. Delamination does not happen. period
    The fire retardant you promote, is (sorry) nonsense too. There are well proven systems and even Paints available, light, cheap, and easy to apply.
    A partly soaked mineral fibre would reduce the positive properties of both materials, being expensive and heavy.

    Regards
    Richard
     
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