Stringer placement... structural difference?

Discussion in 'Boat Design' started by dlpanadero, Mar 17, 2016.

  1. dlpanadero
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    dlpanadero Junior Member

    Hello, I have a question about stringer design/material/placement that I'd love to hear some input on. I'm re-doing the stringer system on my 15' Pathfinder skiff. I'm cutting/grinding the old laminate out now from the bad stringers and removing the foam material underneath. My question is, once I get the old laminate and foam out, does it really matter what material I use to re-build the stringers? I understand the structural support comes of course from the glass, and not the material underneath the glass. So, for instance, would it be a waste of $ to use nida-core to 're-build' the stringers? Then of course the nida-core would be glassed over, I'm assuming with 1708.

    Secondly, if I were to use nida-core, what would be the difference between laying it down flat on its broadside and glassing over that, or laying it down on its edge and tabbing it in? Hopefully this question makes sense. Anyway thanks for your time, and I hope to get some insight/advice! Thanks!
     
  2. TANSL
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    TANSL Senior Member

    Inside the fiber stringers should you put the cheapest material possible, since it works almost nothing in the strength of the reinforcement. Only it serves to shape the reinforcement. If possible, even, do not place any material.
     
  3. SamSam
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    SamSam Senior Member

    I believe that sometimes the compressive strength of the core is counted upon to prevent the stringer from buckling. If it isn't, then the stringer itself has to be beefed up enough to provide that strength.

    There is a vast difference in stiffness between a similar sized board lain flat versus on it's edge. That would be the same for a stringer. The stringer on edge supplies the vertical strength/stiffness to counteract the forces on the bottom of the boat, such as hitting waves etc.
     
  4. TANSL
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    TANSL Senior Member

    Indeed, so reinforcements are calculated.


    Naturally, if not, why we put the reinforcement ?. But also reinforcements form the contours of the panels of the hull and make these, with its recessed perimeter, better withstand loads on them.
    The panels also collaborate to local and longitudinal strength of the hull, do not forget.
     
  5. gonzo
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    gonzo Senior Member

    In these boats, the wooden core is part of the structure. Rebuild the stringers the same as the original, and you'll have no problems.
     
  6. Rastapop
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    Rastapop Naval Architect

    What function do you think a wooden core is filling that another material won't?
     
  7. SamSam
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    SamSam Senior Member

    I looked up what nida core is. Being a honeycomb construction, I don't know if it would function well on edge as a vertical stringer. If that was so, it would then be just an expensive form over which laminate was applied, where inexpensive materials would work as well.

    If you laid it flat it might work, but then what would you do for the deck support?
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Obviously wood has structural properties most lightweight cores do not, accordingly the lamination over the timber need not be as heavy as that over a core that is effectively acting mainly as a former to give shape to the glass. If glass stringers were standard, the boats should have lasted 50 years or more, free of enclosed timber rotting away, and with similar integrity to new.
     
  9. TANSL
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    TANSL Senior Member

    That may be true or it may be completely wrong. It depends on how the designer has considered the core of the stringers.
    No need to put some stringers the same as the previous, simply put ones that have the same strength as the original (the same modulus, for example).
    It is not good to give advice that can lead to a waste of time and money.
     
  10. SukiSolo
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    SukiSolo Senior Member

    So why have these stringers gone rotten? If the foam and glass is up to the job, there should not be a problem....

    I've cut out a few foam core stringers that have gone rotten and prefer to replace with timber. Seen paper rope used as cores, bits of old packing crate (staples included) and other odd stuff. However it's not difficult to rip some half decent dry timber, seal it and bed it and glass over it. Unless the floor of the hull is incredibly uneven, these replacement battens bed down well. If you want to go light, well, WR cedar, obeche, balsa, spruce or pine would do, a little heavier and Doug Fir would be good. (I am aware of the SGs').

    Mostly these foam stringers fail from point/edge loading from the inside of the hull. Such as jumping about with hard shoes/boots, dumping an anchor, or loading sharp edged objects. This damages the glass laminated on the top as the foam has not the compression strength to support the 'not anticipated' load even though up to hull stiffness loads ie wave thumping etc. Sometimes the foam just starts to disintegrate 'inside itself' from flexing, and of course once started it becomes like sand grains rubbing against themselves...

    Your right TANSL, but on small stuff it is rare to find hollow core engineering well done except on specialist stuff IMHO. So I'd follow Gonzo's advice on this 15'er and I'm sure he's right and knows the boat. If the timber is epoxy sealed prior to glassing it will easily outlast many a foam core. Most rotten timber cores I have come across were not sealed properly before glassing in, or have been drilled into afterwards allowing water ingress. Not convinced foam(s) would have done any better, in similar useage.

    Check the quote at the bottom of the page on this skiff Forum....


    http://www.thehulltruth.com/boating-forum/378988-16ft-5-inch-draft-flats-skiff.html
     
  11. Rastapop
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    Rastapop Naval Architect

    As a core those differences in structural properties are pretty insignificant. The difference in lamination will be minor (far less than the extra weight the wood core would introduce), if there's any at all. Much more significant than the core material would be choice of core height.



    Presumably most of the "rotten" foam cores you've seen were also due to not being sealed properly, or to being drilled afterwards.

    The OP isn't considering a foam core, he's considering nida-core, a structural honeycomb.

    Assuming I'm reading your question correctly:
    The first method will give you the intended shear strength of the core, the second won't.
    On the other hand, with the second method you could make the core as high as you liked, and so benefit greatly against bending stresses.
    Personally I'd use it with the intended alignment though (the first method).
     
  12. PAR
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    PAR Yacht Designer/Builder

    In most cases the core is part of the structure. In those cases where it's not, the laminate is considerably heavier over the core, rivaling the hull shell thickness. In these cases you can employ mashed potatoes as the core.

    It's uncommon the laminate carries the whole load, because the stringer material and the labor to install it is much cheaper than the extra laminate needed for a the other. This is also why wood is the most common core, it provides a considerable amount of longitudinal stiffness, decreasing the need for a heavy laminate.

    So, pick your poison well. One uses a relatively inexpensive core with minimal tabbing, while the other uses a lot more fabric and possibly a more costly core too.
     
  13. dlpanadero
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    dlpanadero Junior Member

    This boat is not wood-cored. I'm not sure what exactly the core material is in the hull, but I know there is no wood in the boat. The stringers like I said are simply foam with laminate overtop.

    In any regard thank you all for your responses, this is some great input. In response to how the stringers failed, these pre-1999 Pathfinder tunnel hull skiffs essentially all had the same problem, the manufacturer repaired those that were still under warranty and owned by the original owner. I suppose then it was either a design problem or a defect in the material. Also, the boat has a keel-guard with thru-hull screws, so I suppose these could be leaking and have allowed water to get up underneath the foam in the center stringer. But, the stringers on either side of the center one are bad too, so this wouldn't have been the only problem.

    Thank you all again for all the responses and keep them coming if you wish, it really is great insight hearing everyone's response, and gives me a lot to consider. I'm still in the grinding/removal process. I'll post some photos once i get them on the computer.
     
  14. dlpanadero
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    dlpanadero Junior Member

    And thanks for posting that link... Yeah it would really make sense that the stringer failures in these boats came in part from people not using them in they way they were intended. That is to say, running them wide open and beating them around in even a light chop. That being said, I can hardly imagine Maverick Boat Company would manufacture a skiff that couldn't withstanding a slight beating in some chop. It's a cool boat, I just would rather be fishing out of it than sitting on the deck of it in my driveway grinding away at it.
     

  15. TANSL
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    TANSL Senior Member

    Typically, the laminate support the entire load on the stiffener. A stiffener usually works primarily to bending and therefore their effectiveness depends on first moment of its sectional area. Therefore, all the material which is close to the neutral axis contributes very little to the total resistance. That is the philosophy of the reinforcements in "T" or the bulb tires, removing superfluous material and put a lot of material away from the neutral axis. Something similar (not equal) happens with the pillars or, for example, with masts: they are hollow (I know, of course, there are solid wood)
    When laminated stiffeners are used, either are constructed with a mold or are constructed by laminating on a material, the core, which only serves to shape. In the first case the reinforcement is hollow and the weight saving is considerable.
    It is a very good practice to place some laminate layers, very strong, only at the "crown" of strengthening and making the "legs" of the reinforcement, as thin as possible so as not to add little useful material.
     
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