Materials

Discussion in 'Materials' started by metin_mehel, Dec 9, 2014.

  1. upchurchmr
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    upchurchmr Senior Member

    Plywood is bad for stringers, because much less of the wood is in the proper direction for the "Major" load.
    Use it all you want if you increase the dimensions to put the same amount of wood in the direction along the length. Be sure to use epoxy to seal the plywood, since now you have lots of end grain exposed to quickly start rotting. And Okoume is known to not be resistant to rot. Great plywood if you protect it.

    Plywood as a stringer will be heavy (if sized right) and rot prone (if not protected). I personally won't be doing it.

    OBTW, Petros's comment above is a good reason to not just epoxy coat wood, but to use some fiberglass to improve the coating - even if you believe the glass adds no strength. Of course, epoxy is much stronger than paint, so unreinforced epoxy breaking down is a bit of a false issue.
     
  2. metin_mehel
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    metin_mehel mech.eng.

    What is a usual stringer timber?
    Okay lets make it easier, Fir Timber...


    Laminated okoume and Fir Timber, Same weight.

    What are the rot resistance, yield strength, modulus of elasticity?
    (Ofcourse low density one will have larger cross sectional results in higher moment of inertia...)
     
  3. PAR
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    PAR Yacht Designer/Builder

    Wow, nothing personal Metin, but you're way out of your comfort zone, to even contemplate making these types of changes on your own.

    First of all, what type of fir are you comparing to okoume? Assuming Douglas fir, the fir will be 25% to 30% heavier then okoume, assuming the same dimensions. If trying to compare okoume plywood with solid Douglas fir timber (of the same dimensions) the fir is still heavier, plus stronger and stiffer than the okoume plywood stringer.

    This would be much simpler if you simply told us what the design was (specifically) and what (specifically) you'd like to change.

    As to the above comment, no one has suggested anything of the kind. Plywood (generally) is 2/3's the strength of a solid timber element of the same dimensions, longitudinally, assuming discussing species of similar qualities.
     
  4. Steve W
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    Steve W Senior Member

    Why would anyone want to substitute okoume plywood for doug fir for stringers, it makes no sense, it is more expensive, not as strong and less resistance to rot. It has already been pointed out that only 2/3 of the veneers are running in the direction you want but that aside, just comparing the 2 species of wood not considering plywood, if you were to substitute solid okoume for solid Doug fir, while the okoume is lighter it is also not as strong so you would need to upsize the dimensions and then end up with a wood that is rated as "non durable" while Fir is "moderately durable". That aside nice clear vg doug fir is close to perfect as a stringer material as it bends beautifully fair, better than most other woods. As Par said, it would be helpful to know exactly what you have in mind.

    Steve.
     
  5. metin_mehel
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    metin_mehel mech.eng.

    Low density materials may have low yield strength or low modulus of elasticity but if we consider same weight the crossectional are increase so as to attain the same weight, will eventually reduce the stress on the member (Mc/I) dramatically because I changes 4th power...

    By the way I have no plan. I am just judging the principles...
    Increasing may knowledge... I have few knowledge about the wood...
     
  6. upchurchmr
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    upchurchmr Senior Member

    Try some of the available documents about wood.
    This was taken from one of the standards.
    http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_05.pdf

    I'd suggest you get the pieces of wood you want and cut out a stringer. Then bend it until it breaks. you can also measure the amount of bend for a given load.

    A personal test to give you a simple answer instead of theory.

    If you really want a stringer for Mc/I optimization, make up a T stiffener.

    Or do the practical test again and change the material and dimensions until you get an equal breaking load.

    This is a very old topic - probably Noah worried about it. After any engineering evaluation you still need to be practical.

    Perhaps with the information in the charts above you could design some sections and show us the results. I'd like to see your low modulus vs high modulus designs.
    Judgement needs to be based on facts and some work not general principles.
     

  7. metin_mehel
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    metin_mehel mech.eng.

    Thanks
     
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