how much tabbing for stringers?

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

  1. gonzo
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    gonzo Senior Member

  2. PAR
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    PAR Yacht Designer/Builder

    If you look at this drawing again, no clarification is necessary. If the right hand side (small to large laminate) is employed, you'll have gaps and resin rich areas, shown in red. Additionally, if you grind this area smooth, in fairing, the top layer (in this drawing) or multiple layers (typically the case) will be cut, making their span considerably shorter, defeating the purpose of staggering the laps in the first place.

    On the left side of this drawing, is the accepted way, with the widest span on the bottom, working up to the smallest. This way there's no resin rich gaps or bubbles at the transitions and when faired, you just knock off the edges, not cut through the upper laminate layers.

    The mechanics of this are easy to see, so it's your choice - apply 6" spans of fabric, just to have a large percentage with resin rich areas under it and to have top layers ground away, or use the accepted way, which eliminates both issues.
     

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  3. midcap
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    midcap Junior Member

    makes sense to me. I plan on doing it the way you and gonzo
    recommend.
     
  4. TANSL
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    TANSL Senior Member

    What do you think the attached picture?. Things change, right?
    All I've done is draw things to their size.
    And now, I'll make a couple of questions that only a novice like me could ask:
    - Do you really need to brush that overlaps? (grind this area smooth, in fairing)
    and second, in the same direction but much more technical,
    - What sense does it make to put a material and then remove it by grinding?
     

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    Last edited: Apr 10, 2014
  5. PAR
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    PAR Yacht Designer/Builder

    Your attached image TANSL, is exactly the wrong way, if you don't want blisters, bubbles, resin rich areas and the worst situation, cut fibers when faired.

    I'm not sure what you mean by "brush the overlaps", nor the next portion of this question, can you clarify?

    If you have a fabric overlap and it's faired, the small to large laminate technique will cause the upper fabric layers to be cut. I'm not sure why this is difficult to understand.
     
  6. TANSL
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    TANSL Senior Member

    PAR : sorry , I do, I can understand you but I find it harder to explain my self.
    We're talking , apparently , about two ways of doing the same thing , and probably both , well done, are correct.
    You have no reason to write that what I say is "is exactly the wrong way" because it's not true and you know it.
    Have the humility and generosity to say than , of the two ways , one is better than the other but defending our positions with technically solid reasoning.
    The way I propose is not an invention of mine . It is what I have seen recommended standards with which I have worked and I tried to reason why I think it's the best .
    Another issue : I do not see the need for the overlap is faired . It seems very expensive and not see it necessary, to contrary , I believe greatly reduces the bond strength .
    Your solution , though you do not know , I'm sure that is used because there is some technical reason to support it. You do not have to know it but to me, support full force of the reasoning behind "always been done " is not enough. I would like more explanation .
    Probably your solution in some cases, be more correct than mine and vice versa.
    Apart from your experience, that sure is great, if you give me solid arguments, let me totally convinced and I will learning. Otherwise, not worth you try.
    Cheers
     
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  7. FMS
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    FMS Senior Member

    Maybe if you are vacuum bagging it. I don't have experience vacuum bagging.
    With a hand layup, thick cloth wouldn't make the 90 degree bend tightly. It would fit more similar to PAR's drawing. The cut edge is never perfect. Some fiberglass strands will become loose where it's cut when laminating. Thin cloth that might lay closer to 90 degrees needs to be worked to get any air bubbles out and when I hand laminated anywhere there was a non-flat surface the glass floated on some extra resin which was preferable to leaving a gap or void there from over working it.
     
  8. PAR
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    PAR Yacht Designer/Builder

    If you cut through fabric layers in fairing, any technique is precisely incorrect, if tabbing or overlap spans are of any importance. The same is also true of excess resin/fiber ratios, bubbles and blister creation, directly resulting from this method.

    There are lots of places where an overlap or tabbing stack, needs to be faired. Fabric widths being relatively limited, forces a laminate to be made of multiple layers, which often need to overlap. Some places, like in the bilge, the amount of fairing is limited, but there are plenty of other places, where an overlap needs to be ground down a bit, so you don't use as much fairing compound and the area is actually flattened. There are other techniques, such as cutting the overlap, as the resin hits the gel stage and making a clean butt joint, but this is much more work and very tedious, compared to just waiting for a cure and knocking it down with a grinder.

    Cutting fibers isn't technically sound, so any application method that can avoid this, should be employed. The same is true of excessive resin pools, potential bubbles and blister generation, as a result of said technique.

    Can you get away with it? Sure, most manufactures do it all the time, but then again their laminates are less than ideal, overly heavy and not as strong as carefully applied fabric layers. There's no real savings going with the smaller pieces first, in terms of labor or materials, so no advantage other then what a laminator might grab first, doing a tab in or other portion of the schedule. On the other hand, someone doing a home repair isn't restrained by management or other "rules" so they can take the time, do it right and in the process, save some resin, potential for bubbles, etc. Lastly, on weight sensitive or highly loaded piece, this type of laminate stack isn't acceptable at all and would get rejected out of hand. Could you imagine a carbon mast with this type of stack on the masthead crane, once ground to make fair, it would lose much of it's fiber overlap, so modulus and strength go out the window. You can rest assured it will be done big to small and carefully controlled in regard to resin/fiber, resin pockets and overlaps. On the other hand (again) if it's just a tabbed in pocket for a rod holder, in the side deck of a bow rider, then who cares if you need a couple of extra pieces of fabric, to get the same strength. So, it's a matter of workmanship in some cases, while in others a matter of strength, stiffness and weight, but there's no doubt which is proper and which you can "get away with" if you're not particularly proud of your efforts and strength, stiffness, material use and cost aren't big concerns.
     
  9. tom28571
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    tom28571 Senior Member

    When you look at the possible failure modes of such a joint, it is pretty clear that it makes little if any difference. The same reasoning shows that a single layer of a substantial glass fabric is just as good as multiple layers. The main reason is that the failure mode is peel strength of the glass to surface interface. That is changed very little by adding more thickness to the glass.

    The only increase in peel strength is a result of a stiffer laminate pulling on a somewhat larger area of surface. In other than a peeling action, the tape offers little strength to the joint in a direct pull anyway.

    Various tests have shown that a bending force causes a failure in hinging the vertical part away from the surface at one edge of the filet. On the side of the joint where the hinge break appears the tape offers no advantage at all and on the other side, the force is in sheer between the first layer of tape and the lower surface with the upper layers offering very little added strength.

    Like most builders and designers, I usually err on the side of overkill in critical joints like keels and chines since price of failure is so high. A stiffening bottom stringer unsupported by an above sole or deck may also be critical. Actual strength analysis is rarely if ever done in our boats so we take this safe path to reliability instead. Boats such as those in the AC can't afford any extra weight so they spend tons of money in engineering all excess weight and therefore excess strength out of the boat.
     
  10. SukiSolo
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    SukiSolo Senior Member

    There are two small things I would like to add to this debate. Firstly, please resin the stringer before applying fabric to seal the timber/ply/foam. Secondly try and put a radius (radii) on the edges if possible, easy on the top but a fillet maybe required at the bottom. The more porous 'cloths' or CSM materials are better for resin penetration and less air getting trapped than the tightly woven stuff like plain and Biax, which really needs a vacuum for neat results.

    I come across so many small boats with rotten 'ply' (definitely not marine quality) and cheap rubbish timber 'cores' it makes you wonder how the builders are still in business. A pleasure to find good work which is out there, so well done those who take the time to attend to these details.

    Right now, I have a 2.4m which needs about 250mm Sq (10" sq) core under the mast step and foot steering to be replaced. Easy devil to get at, eh? :rolleyes: Also could not believe the ammount of air in the deck layup of a Finn, but as the gelcoat is now more grey rather than translucent blue you can't see it...;)

    My impression is a lot of core material is placed in and then covered in glass prior to the resin being applied. This leaves the core too dry so 5 years down the line I'm digging it out and replacing it! I love the encased mild steel bolts resined in too btw.

    Along with PAR, if doing a neat repair, I'll often sand back the layers to smooth and then top with a coat of clear or tinted resin. Just depends on the type of cosmetic required for the job.

    Was it really an 18 foot tall stringer? ......
     
  11. fcfc
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    fcfc Senior Member

    The problem is that TANSL image come from ISO 12215:6 current rule. They say with this method that each tabbing layer transfer directly its own share of shear between skin and bulkhead/stringer. With the other tabbing method, shear has to go thru tabbing thickness to be transmitted by tabbing outer layers.
     

  12. TANSL
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    TANSL Senior Member

    With just meant to show that things can be white, black, or just the opposite, I show two figures obtained Lloyd's SSC that said, as appropriate, both solutions are possible.
    When the designer must meet certain standards, he can not do things as he wants (even if practices are widespread throughout the world), but as the rules tell.
     

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