Flexible non-steamed wood for gunnels?

Discussion in 'Materials' started by sprit, Oct 19, 2013.

  1. rwatson
    Joined: Aug 2007
    Posts: 5,852
    Likes: 290, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member


    You wouldn't be able to bend the timber if you laid up both sides before attempting the bending.
     
  2. NoEyeDeer
    Joined: Jun 2010
    Posts: 983
    Likes: 32, Points: 28, Legacy Rep: 300
    Location: Australia

    NoEyeDeer Senior Member

    I meant longitudinal for the stringers themselves, not for the entire boat. IOW, same thing as you are talking about.

    My point is you are implying a one-sided sandwich (glass only on the inside of the stringer). It would be far more effective on both sides, if reducing total cross section is what you are after.

    rwatson: obviously that would depend on the amount of wood in the stringer. If the idea of laminating glass or carbon on them is to end up with the same stiffness but a smaller sectional depth, then a stringer of the same stiffness would be as easy to bend.
     
  3. NoEyeDeer
    Joined: Jun 2010
    Posts: 983
    Likes: 32, Points: 28, Legacy Rep: 300
    Location: Australia

    NoEyeDeer Senior Member

    I suppose I should explain my thinking a bit more clearly.

    The idea of putting glass on one side of a stringer is not illogical, but what it is trying to do is replace a lower modulus material (softwood) with a higher modulus material on that face. That is the same principle as sandwich construction, and to get the best stiffness/weight ratio should be done as a balanced layup: with the higher modulus material on both faces, not just on one.

    This will be far more effective in maintaining the desired stiffness as section thickness is decreased, and IMO you will want the most effective method since (all else being equal) stiffness is proportional to the cube of the thickness, meaning a little less thickness loses you a lot of stiffness.

    Since you're talking SOF I assume you want minimum weight too, so again you want the most effective method.

    Using unidirectional would be much better than using glass cloth. With cloth, half the glass (and resin and weight) is across the stringer, not along it, and is therefore useless to you. On top of that, due to the woven construction, cloth has crimped fibres that are less effective in stiffening than uni (which has straight fibres).

    Short version: for better results put uni both sides instead of cloth (or uni) on one side. For best results, use carbon. :)
     
  4. upchurchmr
    Joined: Feb 2011
    Posts: 3,054
    Likes: 150, Points: 63, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    NoEye,

    Everything you say is absolutely true.
    I don't really want to complicate the build as much as I do already.
    Glassing one side was a not very completely thought out attempt to improve the situation with less work.

    The actual thought went like this.
    I believe the stringers get broken primarily from impact from the bottom (or side).
    Cedar when bent will probably start failing on the tension side.
    That would be the inside of the boat.
    The fact that my stringers are Home depot wood and I don't cut to get absolutely no grain runout will also introduce significant reduction in the strength in an impact.

    I believe (no actual proof) that the compression side will resist failure to a much higher load than the tension side. Actually this comes from observing stringer cutoffs when I break them to throw them away.

    So to increase the tension side - assuming it is the failure initiation site - I would improve the inside surface strength.
    The easiest way I know is to take a 8" wide board and laminate the cheapest glass I can get (or what I have laying around) which is 6oz. Then rip into 1/2 x 1/2 stringers. Easy to bend in place.

    As you say, equal inertia using two sided and actually engineering the size would bend in place also.

    I have not put pencil to paper to see what is realistically possible.
     
  5. NoEyeDeer
    Joined: Jun 2010
    Posts: 983
    Likes: 32, Points: 28, Legacy Rep: 300
    Location: Australia

    NoEyeDeer Senior Member

    I'd consider uni. It's only around twice the price of standard woven, which isn't many dollars for the amount you'd need.

    If you can persuade some friends to share the cost/materials, even carbon could come in quite cheap. I can get a 5 metre length of 1.27 wide 340 gsm carbon uni locally for around $270 retail.

    In feet/inches, that's around 17 feet long by 4'2" wide and approx 9oz weight.
     

  6. upchurchmr
    Joined: Feb 2011
    Posts: 3,054
    Likes: 150, Points: 63, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    $270 is more than 1/2 the cost of materials for an entire boat.
    This is not a hotbed of Kayak building, even less so for SOF boats.

    There is no one I know making kayaks who I have not started, and they just want a standard design!

    Thanks for the suggestion. I'll probably just continue with the standard for SOF. I already have two boats I want to sell with no one interested.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.