foil structural calculations?

Discussion in 'Sailboats' started by sigurd, Apr 16, 2007.

  1. sigurd
    Joined: Jun 2004
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    sigurd Pompuous Pangolin

    How would I go about it, trying to learn to guess where and when for instance a dinghy daggerboard foil will break?

  2. nflutter
    Joined: Apr 2007
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    nflutter Junior Member

    test it! make a quick model and load it up till it snaps. use balsa or something. do it 5 times. thats a pretty good way of learning whats going on.

    you can use computers too, if your keen. software like solidworks or ansys will tell you in pretty good detail. they'll also tell you how it will perform hydrodynamically. its a drawn-out process though.

    or just remember that for an evely distributed load, bending moment decreases exponentially as you go down from the base to the tip. so that means theres no chance of it snapping at the tip, but heaps of chance where it joins the boat.

    keels are different, where the load is on the end. the moment is the same all the way along, and that means they would be just as likely to snap in the middle or at the end etc.

    T-foils and other complex shapes are more difficult, but you can pretty much cover it with those two rules. just dont forget to think about what happens when the loads are reversed, ie when you get negative angle of attack on foils during a big crash. if your just guesing you have to design for all possible scenarios.

    to guess when, you have to know how strong it is, and that probably means materials testing, because foam and carbon and wood are all pretty non-linear and unexplored. again just comes back to making the laminate and loading it until it deforms.
  3. sigurd
    Joined: Jun 2004
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    sigurd Pompuous Pangolin

    Ah, the good old destruction method. I should be used to that by now. :)
  4. nflutter
    Joined: Apr 2007
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    nflutter Junior Member

    yeh who wants to do maths when you can get the same outcome from breaking stuf!
  5. raw
    Joined: Jul 2006
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    raw Senior Member

    Some errors in the above post that I think need clearing up.

    A keel or centreboard is essentially a cantilevered beam with either a distributed (lift) or point load (person righting or bulb) or both.

    A keel with bulb will be subject to hydrodynamic lift and the point load at the same time, However, when bending moment from the bulb is maximum. the lift will be zero as the boat heel is 90 deg.

    The bending moment due to lift is a distributed load that is a function of vessel speed, span of keel and blade area planform.

    The bending moment due to the bulb mass is a funtion of the bulb mass and the span (depth) of keel.

    The maximum bending moment in all the above cases occurs at the hull joint. If the board or keel is of constant section construction, then failure will occur here. Period.

    For a bulb keel, the bulb bending moment at heel = 90 deg will far outweight the hydrodynamic lift in almost all cases.

    Please note that I have simplified a little in some of the comments. I have also ignored grounding loads. I am not going to cover how to assess materials and section shape here either, as I don't have the time.

    There is plenty of maths behind it and it will be covered in any mechanics text book (university level), though may be a bit confusing for the casual reader.

    There is also find some information in the ABS guide to racing yachts which may also help you.

    Hope this makes sense.

  6. tspeer
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    tspeer Senior Member

  7. PI Design
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    PI Design Senior Member

    I'm not sure where you get that from. For a cantilever with a uniformly distributed load, the bending moment (BM) grows proportinal to the distance from the free end squared, i.e. for a beam 2 units long, the free end will have 0 units BM, 1 unit BM in the middle and 4 unit BM at the fixed end.

    For a point load it grows linearly i.e. 0 BM at the free end, 1 BM in the middle, 2 BM at the fixed end.

  8. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    It will break at the root.

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