how much tabbing for stringers?

Discussion in 'Fiberglass and Composite Boat Building' started by midcap, Apr 9, 2014.

  1. midcap
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    midcap Junior Member

    I am redoing my stringers shortly and I was planning on using 6" 1708 tape followed by 12" 1708 tape to tab them in. I would then use a single layer of 1708 to encapsulate the top of the stringer overlapping the tape by a few inches but not all the way to the hull. Do I need 3 layers of tabbing or is 2 fine.

    It's a wellcraft v20 and the stringers are about 18' tall 3/4" ply.
     
  2. gonzo
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    gonzo Senior Member

    Two will be fine. However, the layers should start large getting smaller. That means the 6" tape goes last. Otherwise there will be gaps and bent fibers over the edges of the tape.
     
  3. midcap
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    midcap Junior Member

    wow, thanks for that info. I am glad I asked I would have done it the other way.
     
  4. Westfield 11
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    Westfield 11 Senior Member

    Gonzo, I am confused. How would gaps and bent fibers form when one lays a wide piece of tape over a narrow one? If you do it wet on wet the top layer will hold the lower one flat. If you do it wet over dry you just use a scraper to knock off the stray fibers and use enough resin to fill any gaps. Are you thinking that he would let the first layer cure and then just slap the next on top without cleaning up the edge?

    It sounds like you are advocating the opposite of standard practices, why?
     
  5. gonzo
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    gonzo Senior Member

    Standard practice in the industry is to lay the bigger pieces first. All the tests show that if you put the small piece first, there will be an edge that the next bigger piece needs to bridge. At that edge the fibers will make a sharp bend but will still leave a gap. The bent fibers are not aligned so have reduced strength. Also, it is easy to leave bubbles at that area which causes diminished qualities on the laminate. Using a lot of resin to fill the gap produces a resin-rich laminate which is weaker than one with the correct ratio.
     
  6. TANSL
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    TANSL Senior Member

    I do not think you're right, gonzo. Each piece must fully cover, and exceed about 25 mm (50 mm?) to the previous one.
    The gap that occurs (to be avoided) may well have no importance, if that gap is not between successive layers. Since then fill in a gap with resin not help much.
    But, as I am no expert builder or repairer, maybe I'm wrong.
     
  7. gonzo
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    gonzo Senior Member

    There was a series of articles on Professional Boatbuilder that had all the data to back my claim. They have an online publication available.
     
  8. Westfield 11
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    Westfield 11 Senior Member

    Thanks, I will take a look. It is a very interesting idea and I do get your point about bridging over the previous layer and leaving a void to be filled by resin.
     
  9. TANSL
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    TANSL Senior Member

    This is the first reference I found. The CS, all have similar standards.
    Any of your references we can examine?. Thanks
     

    Attached Files:

  10. midcap
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    midcap Junior Member

    After looking at that picture, it makes sense what gonzo said
     
  11. TANSL
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    TANSL Senior Member

    I think it's not exactly what gonzo explains. The figure on the left seems to confirm what I say. The figure on the right says that the overlapping layers are alternately on one side and the other of reinforcement. To be honest, I think this is a solution that does not give the reason to gonzo n or me.
     
  12. waikikin
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    waikikin Senior Member

    Both methods are "recognised" practice................ Most "survey"/ "rule" building requires overlaying wider subsequent layers.
    Repair strategies & for faired out/ground work the narrow second etc can have benefits, I think especially for epoxy repairs with stitched fabrics only probly & "better" bonding even more-so, if it's a poly repair/tabbing using torn choppy edges maybe not so much bridging issue...........
    Both methods work, one is supported by "rules" the other by practice, both work.....!
    A link to the PBB article would be great.
    Jeff.
     
  13. PAR
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    PAR Yacht Designer/Builder

    This has been an on going debate for some years (decades). It's done both ways, but most find that installing tabbing or any fabric layers is best, if you to go from big to to small, for the reasons Gonzo has stated. In the case of bagged or infused work, this is less a concern as the gaps get well filled, crushed down and though resin rich, not as bad as hand laid laminates.

    If doing the work by hand, just make up a couple of test patches, one with the smaller to larger layers, and the second the other way around. You'll quickly find you'll never get rid of the bubbles, puckers and resin rich areas at each transition where the larger piece has to conform to the smaller one below it. Additionally, when it comes time to fair the surface, a grinder will cut through the larger over smaller fabric at these transition points, where as the larger to smaller laminate, the grinder will just trim the edges of the subsequent layers. From an engineering point of view, this is a huge consideration as the moment your grinder cuts through one or more of these layers, you've defeated the whole point of staggering the layers in the first place and this is why you'll find most professionals, with actual hands on wet fabrics experience, will prefer to lay bigger to smaller pieces in a laminate. In the production shops, they just don't care and figure "it's good enough", which for the most part it is, but when these same "laminators" build their own boats, in a garage or someplace, they do it from large to small, because they know the difference and it's "their" boat now.

    The image attached clearly shows the result of the small to large overlaps, especially when it's knocked down with a grinder (dashed lines). Not a lot of debate once you look at this image.
     

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  14. TANSL
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    TANSL Senior Member

    I think when one say " this is so " , one is obliged to give any data to support his claim.
    That said, I think this discussion is very interesting. Based on it, we could make two different lines of argument :
    - First line: what 's always been done , is it still correct?
    - Second line: the practice may say one thing but what engineering says ? .
    Regarding the first-line times I can not say much because my experience as a builder is null . Just watching reality I can give a pair of examples:
    1. Years ago all was cooked in copper cauldrons and now that is forbiden .
    2. For years Classification Societies offered us formulas that calculate the minimum thickness of a laminate. And that was it. Now they tell us that a laminate with sufficient thickness can not be right , there are more things to analyze.
    Fortunately every day fewer people is doing things " as they have always done." This has allowed us , for example , enjoy the wheel. On professional deformation , I usually doubt almost everything and seek improvements in what I design . For example , try to get the structure of minimum weight and / or cheaper to build , years but were not done.
    In the second line, yes I can talk, without giving lessons to anyone. I just will expose some ideas for the experts confirm or disprove:
    • The connection of the plate with a profile to be studied, taking into consideration the shear forces that occur in the bonding surface therebetween.
    • A laminate composed of several layers, is a diverse body which does not work the same way in all its thickness.
    • The distribution of stresses in the bond thickness is not uniform. Increases towards the "outside". The outer layers must endure more stress than internal.
    From all this I can (???) conclude that if the last layer is the weakest and less wide, surely we are going to have a problem. And the wider contact surface, the better.
     

  15. midcap
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    midcap Junior Member

    the figure with the small strip last just looks like it all flows better.
     
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