Fiberglass layup questions

Discussion in 'Fiberglass and Composite Boat Building' started by PickNasty, Jul 17, 2021.

  1. PickNasty
    Joined: Jul 2021
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    PickNasty New Member

    I am relatively new to fiberglassing and I wanted to get some input from the veteran builders out here. I am replacing the transom on a Mako 22 cuddy with epoxy and coosa, I have two big questions.

    1.) From all that I've read, if going with epoxy, the addition of CSM to the layup schedule does not add strength, only thickness. So the thought is to go with biax cloth instead. Possibly 17oz, maybe heavier, not sure yet. The question is, as far as the layup schedule goes, do I lay my first layer out as the largest and work successively smaller or start small going to larger.

    My thoughts on this are that if I am laying up new glass on old, then I'm relying entirely on secondary bonding and it would make logical sense to put the greatest area of secondary bond down first and then work successive smaller layers until desired skin thickness is achieved. However, I have heard this may be more prone to failure than going successive larger after the first layer.


    2.) Given the stated above, is there a benefit for the transom if the weave of the biax is alternated between layers by 90°? I have seen this done in several project and wondering if this is a technique maybe best used in certain situations?

    The clarification is much appreciated.
     
  2. rxcomposite
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    rxcomposite Senior Member

    No, Biax has only 42% of the strength of WR of the load it will carry. It is good only when the structure is in torsion. Assuming you are going to hang an outboard motor in the transom, this is a cantelever design.
     
  3. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum PN.

    Seeing as you only joined today, you are probably not allowed to post any photos of your project yet.
    But if you do have some photos that you would like to post, you are welcome to email them to me, and I will post them on your behalf.
    My email address is tinsmyth@gmail.com
    There are many experienced fibreglass repairmen on here who will be able to give you good answers to your questions above - but photos always help to explain things so much better.
     
  4. ondarvr
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    ondarvr Senior Member

    There are "good"and then "better" ways to do it.

    With epoxy and biax, or a similar fabric, anything you do will better than what was there originally, or what needs to be done for it hold up for the next 30 years.

    So don't get yourself out in the weeds over the absolute best way to rebuild it, just get enough glass on it and move on from there.

    The order of layups in size does have an impact on ultimate strength, but you are in no way going to be relying on achieving the ultimate strength for it to hold up.
     
  5. TANSL
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    TANSL Senior Member

    I can't quite understand what you mean by that. The ultimate strength of a material is what it is regardless of any layups order. In any case it seems suicidal to be working with a material in the vicinity of its ultimate strength.
     
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  6. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    What boat are you building, or are you repairing?
    Are you talking about laminating the transom? Only?
    If you are repairing a transom (putting new glass on old) why would you need such heavy weights of glass?
     
  7. ondarvr
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    ondarvr Senior Member

    He was asking about whether to put down the largest layup first and the shorter ones next, or the other way around.

    I said it's not going to make a difference, he won't be pushing the layups even close to the point of failure from this aspect of the build.

    When you put down the largest laminate first, then the shorter ones over it, it keeps the fibers in a straight line. Doing it the other way puts many bends in the fibers that can result in a weaker structure.

    This won't become a factor in this project.

    I used to build large high pressure tanks, we had to get the best physical properties we could out of every product we used.
     
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  8. fallguy
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    fallguy Senior Member

    The largest pieces go on first. This avoids air entraining on the overlaps.

    Fiberglass does not get progressively stronger with a shingle effect. The strength is in thickness. The size change is to avoid a hard point.

    If you put the long pieces on last, you also risk sanding through during fairing.

    This is just good practice and to not drive you crazy when you cut through glass fairing or have air bubbles when laminating..
     
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  9. rxcomposite
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    rxcomposite Senior Member

    The order of layup is the most important of all. The one with the highest strength goes outside, the second weakest comes second, third gets the weakest. Strength is a material property and is not related to size or thickness. And, it is not ultimate strength but yield strength, the point where the material yield but not rupture.

    Very roughly, UD, cross plied UD, WR, Biax, CSM, core. You can use any classic lamination formula, Max stress or max strain method, or the 1st principle Max stress = M*y/I. The result will come out the same.
     
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  10. TANSL
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    TANSL Senior Member

    You should repeat these paragraphs from time to time, in other threads, so that we all learn. Thanks.
     
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  11. TANSL
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    TANSL Senior Member

    Well, I think the difference is huge, it represents doing things well or not. The difference is that a laminate formed by the same layers as another that complies with the regulatory values, in a different order, may not be sufficient. It's common sense, placing more resistant material where it is not needed and weak material where the efforts are greater does not seem logical. And, as some say, those are not theories but facts.
     
  12. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    I see an argument between type and dimensions.

    If one type of cloth is used in the repair then size plays into the order.

    If multiple types of cloth/weave then the repair should be done by type/ weave.
     
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  13. rxcomposite
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    rxcomposite Senior Member

    A practical approach without going into any calculations is to assume the original lay up schedule is correct. Cut a sample, burn it to separate the fibers and take note of the fabric type and sequence. Laminate according to the original sequence no matter if it did not follow the protocol. It could have been overbuilt anyway.

    Just take note that a transom is in reality a watertight bulkhead. The inside corners are reinforced by additional layers, up to 1.5X the thickness of the original thickness. all the way from the bottom up to deck to hull connection. In design, the transom thickness is equivalent to that of the adjacent laminate such as bottom laminate (underwater) and side laminate (above waterline) and reinforced by vertical and horizontal stiffeners.

    Primary longitudinals are bracketed where it meets the transom. That includes the longitudinals above the waterline if the transom will carry an outboard.
     
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  14. ondarvr
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    ondarvr Senior Member

    Huge if he were stressing the structure, but he's rebuilding it with epoxy and stitched fabrics, this will be far stronger than the original build of cheap polyester, chop and roving.

    So my reply was, yes it can make a difference, but in this case it's not a concern.
     

  15. TANSL
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    TANSL Senior Member

    Yes, now I have everything clear. It had seemed to me that you were saying a huge and dangerous nonsense. I am sorry.:rolleyes:
     
    Last edited: Jul 19, 2021
    fallguy likes this.
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