fiberglass over cold-molded

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

  1. Hernandiz
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    Hernandiz Junior Member

    Hi,
    I'm working on the scantling of a 40ft sailboat using ABS offshore Racing Yachts. Construction will be of plywood-epoxy. I read on a thred that it should be applied as cold-molded. if I do so a outer fiberglass skin is not included as structural...
    Is it corect?
     
  2. rasorinc
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    rasorinc Senior Member

    A single layer of light fiberglass over wood is meant to waterproof it. It is not structural.
     
  3. apex1

    apex1 Guest

    That is not a clear advice "should be"?! What does your designer / NA specify? Do you try to make the calculations yourself?
    Why should a member of the hull structure be excluded from structural calculation? Especially if it is of noticeable weight and strength? So, to your question.... NO
    rasorinc... he did´nt say single layer. And even if, see my statement above.
    Regards
    Richard
     
  4. Hernandiz
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    Hernandiz Junior Member

    Thanks rasorinc.

    Richard... who told you I would exclude a member from structural calculation?

    I'm talking about about an aditional fiberglass skin on the exterior side of a plywood hull.
    By the way I'm the designer...
     
  5. apex1

    apex1 Guest

    You.... told me, you did´nt say you would include the outer layer in your calculation Mr. designer!

    Regards
    Richard
     
  6. yachty4000
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    yachty4000 Junior Member

    My understanding is that ABS treats glass sheeved boat as a normal boat with a high density core so you end up with a very strong over engineered boat. The worse thing you can do is one layer of high tech laminate on wooden structure as you effectively end up with a the laminate taking the load. Sheaving with one layer of chop strand mat to bond a wooden structure seems sensible (SP Systems have some good fact sheets on this) I have done this on strip planked restoration projects. I am not brave enough to interpret ABS for you and if it is a racing yacht I would start looking at ISO12215

    Isn;t this what Gudeon Brothers (West Epoxy) book is all about cold moulded construction could be worth a read?
     
  7. Steve W
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    Steve W Senior Member

    So are we talking about a cold molded boat or a plywood boat? either way the typical glass, dynel or polypropylene/epoxy sheathing as rasorinc said should be considered as non structural, it is mainly there as a waterproof membrane and to give a uniform surface for the high grade paint surface that everyone thinks they need. If you are designing for cold molding the Gougeon book is a good rescource as is Modern wooden yacht construction by John Guzzwell.
    By the way,you would not sheath with chopped strand mat ever as csm is held together with a binder which is designed to be dissolved by the styrene in polyester resin and under no circumstances would you use polyester to sheath a wood boat,you must use epoxy for this and while it will wet out csm it does a poor job of it as it has no solvents,also you would end up with a poor resin to glass ratio,actually the best fabric for this purpose is probably Dynel or polypropylene.
    Steve.
    Steve
     
  8. Landlubber
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    Landlubber Senior Member

    Can only agree with you Steve W....if the skin is for waterproofing purposes, use epoxy and dynel....it is tenacious with plywood.

    I helped build a boat in 1972 that way, she is still alive and kicking well.
     
  9. apex1

    apex1 Guest

    Sorry Steve, if we are talking about a cold moulded boat the outer layers (plural), are often applied, and calculated, as part of the structure (in our boats at the bottom only). Ply is a different subject of course.
    I agree with your statement about mat and abt Epoxy.
    But what was the question? Cold moulded with Ply? Outer skin ?
    And who did ask?
    Regards
    Richard
     
  10. Steve W
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    Steve W Senior Member

    Apex, my confusion stems from the thread being titled "fiberglass over cold molded" but the ops post states "construction will be plywood epoxy".on re reading it i think hes going to cold- mold but with plywood strips instead of veneers. Ive built or been on the build team of more than a dozen cold molded boats back in the 70s and almost all were sheathed with 1 layer only of dynel which is only about 4.2oz,certainly not calculated as a structural member.A few used glass cloth but only 4 or 6oz.
    Regards,Steve.
     
  11. Hernandiz
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    Hernandiz Junior Member

  12. PAR
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    PAR Yacht Designer/Builder

    Dynel or Xynole for that matter are not going to impart strength to the hull shell, though they will add considerable weight, water proofing and most importantly, abrasion resistance. The abrasion resistance comes from the high resin to fiber ratio.

    Your original question about sheathing lacks definition. Basically a thin sheath (as has been stated) will impart little strength, unless a high modulus fabric, only providing abrasion thickness. This assumes the wooden, molded hull shell, withstands the loads and the sheathing just offers protection.

    On the other hand a thicker sheathing or a hull shell engineered with the molded potion, being part of a laminate (sandwich or other wise) is a different matter entirely. In these cases, yes the sheathing does impart some to quite a bit of the hull shell stiffness, depending on laminate engineering.

    Secondly, mat is a waste of resin in an epoxy lay up. Mat is a bulking material used in relatively low modulus resins, such as polyester and completely unnecessary with epoxy. The mat will give up and break fibers long before the epoxy, making it useless in an epoxy laminate. The use of mat will dramatically increase laminate weight, thickness, brittleness and not significantly contribute to the strength of the laminate.

    To answer specifics about scantlings, I'd need to know considerably more about the structure you're working with.

    Plywood can be applied in a number of ways. Molding plywood is a fairly common method to get compound curves and one I employ often. Hull shell stiffness can be accomplished through a few different techniques, depending on the design brief and ultimate goals interested in attaining.
     
  13. apex1

    apex1 Guest

    Steve, I understand the confusion, naturally, the description of the choosen method was insufficient (and still is).
    As long as we are not equipped with a serious information about the building method and the purpose, the "outer skin" is meant to provide, we can throw a dice to answer the original question yes or no.
    On our larger designs (50´to 103´) we apply 300 gsm glass / Epoxy in two layers, on the whole hull, one additional of 220 gsm underwater and finally one layer of Polyethylen of 120 gsm on the bottom. And that is very much within our structural calculation.
    The enquirer told us, that he is "working on the scantlings... to ABS rules". If I take that as a fact, my answer to his question is NO!
    I do not assume, someone who is able to calculate scantlings to classification rules, is asking for the influence of 60 gsm strandmat on structural strength!
    But now, that we know the enquirer is the designer, we can expect some specific particulars about the design.
    Regards
    Richard
     
  14. Hernandiz
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    Hernandiz Junior Member

    Look a the pics of my last post. this is a similar construction.
     

  15. apex1

    apex1 Guest

    I did, but could not find any data of the construction method, or the way (layers) the boat is sheathed.:?:
    But to be a nice guy, I will give a proper answer. The boat shown, obviously is a simple Ply sheet construction with some outer glass / EP skin to protect the wood (abrasion and water intrusion). As far as I can see from the picture, it seems to be a bit more than only a thin tissue, but I assume it is not calculated as a load carrying member of the hull. The multi chine ply would carry the loads without the assistance of the skin. Although even if you apply 3 layers of Ep to prevent the wood from water intrusion, you add noticeable strength, more naturally, with the inforcement of fiber. But as mentioned, this is not taken into account in the scantling calculation.
    So, now my reply to your first question is YES.
    Regards
    Richard
     
    Last edited by a moderator: Feb 27, 2009
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