Compression Post Design

Discussion in 'Boat Design' started by CharlieDanger99, Dec 17, 2021.

  1. Barry
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    Barry Senior Member

    Not sure how you got to assuming that additional shelves, which I am assuming Charlie will fasten to the mast base column/panel, will not increase the resistance to buckling.
    For a given set of column parameters, Euler's equation is basically P ( the critical load to cause buckling = a constant K for a given column (in this case the 6 foot ply column) divided by the square of the length of the unrestrained column

    So assume a number for K, which is not indicative of anything, of 1000 appropriate units

    So the critical load for the 6 foot column is P critical = 1,000/ 6 x 6 = 27 units
    With a single support in the middle P critical = 1000 / 3 x 3 = 111 units
    And if it has two supports as drawn in post #13 P critical = 1000/ 1.5 x 1.5 = 444 units

    So restraining shelves, if designed to restrain the buckling, will increase the buckling load by a factor of 444/27 or 16 times

    I was assuming for calculation purposes only that the mast column was not supported by being attached to the transverse frame. Ie with the mast column being attached to the frame will provide buckling resistance in any case. So if the Charlie is concerned about buckling, then a flange, if he has room, attached to column on the opposite side of the frame will more than likely offer any buckling resistance that he feels that he has to deal with.
    Ad Hoc likes this.
  2. TANSL
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    TANSL Senior Member

    In Euler's formula for the buckling of a column there is an important factor, the term "L / r", known as the slenderness ratio. L is the length of the column and r is the radiation of gyration for the column. Let's not forget "r".
    I hope that the picture I have prepared is clear enough to explain my comment (I have not been able to make a better picture). What I mean is that, if the bulkhead buckling collapse occurs as indicated in the picture, horizontal shelves will not help much to solve the problem (I have only painted one shelf for clarity in the exhibition). Of course, all the material that is added will be good, shelves, double plates or whatever, but I think that a vertical reinforcement, probably web+flange, will be much more effective than any other solution.
    In all this I take it for granted that the bulkhead itself is not capable of withstanding the pressure of the mast.
    Pandeo Mro.jpg
  3. Ad Hoc
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    Ad Hoc Naval Architect


    Indeed.. nice summary.
    What is also being missed is that buckling of plywood…is not really buckling, in the isotropic metal sense.

    Since buckling is characterised as excessive deflection, but as you note, this can be easily prevented by reducing the panel aspect ratio.

    So buckling of plywood, is ostensibly down to delamination; which is not the same as conventional buckling, by excessive deflection. As with regards to plywood…delamination is down to moisture and other environmental aspects that initiate “buckling”, or peeling away of the layers. Because when looking at the typ. properties of plywood, the wood used, Okoume, has a compression strength of 27.4MPa, yet in shear it is only 6.7MPa.


    But being a laminate the strength is only as good as the adhesive. Which is circa 0.5-1.0MPa.

    So buckling is not really an issue, it is the shear strength between the laminates…once that is history. Way way before any classical bucking calculations suggest failure.

    As M-about noted, I wouldn’t use plywood for compression either!
  4. gonzo
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    gonzo Senior Member

  5. Barry
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    Barry Senior Member

    You missed my point entirely, Tansl. I was merely addressing your comment "these shelves will not give you any additional resistance against the buckling of the panel."
    The Euler equation can be written to include the radius of gyration BUT as the parameters of this column/mast base are known, it is as easy to use the P critical formula with other terms to create the K constant that I referred to
    Ie you do not have to calculate the slenderness ratio or an individual radius of gyration of the column (as you technically are doing this anyway if you use the single formula)
    I also used a constant K, to make the concept easier to understand, as the length that I referred to was taken as pinned ends with one floating, which we know that is not the case in this instance.

    Again, the point is this, for any column, additional lateral supports increase the stability and hence reduce the chance of buckling. Which is opposite to your comment. And that by halving the effective length, all other things being
    equal and ignoring the effects of the fixing point, ( ie effective length adjustments) the critical load to induce buckling is reduced by the 4 times.

    In this case, the OP's boat has withstood years of NOT having the panel "buckle" so as along as his repair gets the mast base to be as "strong as" what he had before, he will be ok.
    His one shelf or 3 shelf option will increase stability.

    Ad Hoc and others have covered plywood as an undesirable compression material due to the delamination aspect.

    Not 100% relevant is the a "long" column, the radio tower, using wires to provide lateral support. Of course they also stabilize the tower from deflection caused by winds.
    And a building crane which shows stabilizing supports up the side

    Attached Files:

    Last edited: Dec 20, 2021

  6. TANSL
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    TANSL Senior Member

    Why masts have stays is another question.
    It seems both you and I are forgetting or misinterpreting what the other is saying. Well, for my part, that's all I had to say about an element that works in compression, not flexion.
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