Mast Compression Post Question

Discussion in 'Sailboats' started by Jetboy, Apr 23, 2015.

  1. Jetboy
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    Jetboy Senior Member

    It seems like I generally see relatively small mast compression posts as compared to the mast extrusion. It would seem to me that the post is taking the same load as the bottom of the mast. Why are the compression posts not required to be stronger?

    And is there a rule of thumb as to how strong the compression post needs to be relative to the mast?
     
  2. gonzo
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    gonzo Senior Member

    It has to do with its length. The section is adequate to withstand the compression without buckling, which is the limiting factor.
     
  3. TANSL
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    TANSL Senior Member

    All this is obvious, do nothing and need not be a scientist to say

    What do you mean by this?

    Another question: What do you think, Gonzo, about different jobs, different loads, supported by the mast and the compression post ?. Can you explain it to jetboy?
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Stronger than what??...i think you need to understand the question better first.

    You are mixing up terminology which doesn't help in your understanding. Strength is related to a materials' mechanical properties. Thus if the mast and post are made of the same material, it has the same strength, which is related to its "E", Youngs Modulus.

    BUT, and this is the important part, the shape and thickness of the section is different, which relates to its inertia, or second moment of area, the "I". This is commonly called its "stiffness".

    The two attributes together is a measure of the structural stiffness EI....
     
  5. Jetboy
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    Jetboy Senior Member

    Let me explain better.

    I often see a decoratively turned wood compression post with a minimum diameter of often less than 1.5" supporting a deck stepped mast made of 6160 T6 .010 wall 3.5" OD mast extrusion. I would anticipate the wood compression post failing prior to the aluminum mast - especially after it has weathered for 20 years.

    Would something like a 1.5" .025 wall 6160 T6 aluminum tube be equally if not better choice for a mast step?
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Naval Architects/Engineers, don't anticipate. We calculate.

    If the post was not stiff/strong enough do you still think it would be there??
     
  7. Petros
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    Petros Senior Member

    the buckling of either mast or support column is the design criteria. Buckling, particularly of thin walled columns, is a complex behavior and not entirely predictable, so the equations have been tweaked over the years to accommodate varying conditions, but the primary factor is the free length, moment of inertial of the cross section of the column and the modulus of elasticity of the material.

    the equation looks like: F = n (pie)^2 E I / L^2 (1)

    where

    F = allowable load (lb, N)

    n = factor accounting for the end conditions (1.0 for free pivot)

    E = modulus of elastisity (lb/in2, Pa (N/m2))

    L = length of column (in, m)

    I = Moment of inertia (in4, m4)

    As you can see, the predicted load of buckling goes with the square of the length, all other factors are lineal, or constants.

    The cabin post is much shorter than the mast, and therefore far less likely to buickle. Also, a hollow tube like an aluminum mast, is more likely to buckle than a solid wood column.
     
  8. SukiSolo
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    SukiSolo Senior Member


    Well, a square inch of good Sitka Spruce will take around 1.5 tons compression before failure from compression load, though that does ignore column length and buckling. You might find some research on Euler would help on buckling.

    How much compression load is the rig actually delivering to the mast base?

    Not forgetting that wood is poor in compression compared to tension and beam stiffness, however suitable dimensions (and timber type) are well up to the task.
     
  9. Jetboy
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    Jetboy Senior Member

    Can you see me down here from your high horse?

    We all know it's all based on approximations. I do not believe that you can even accurately estimate the range of dynamic loading a mast can experience. Even if you did have a full set of data for example with the range of every input that a mast experiences like main sail size, main sail cut, main sail materials, jib size, jib cut, jib material, and every type of other head sail to calculate the wind, then modeled every sea condition, and every type of boat movement like uncontrolled jibes, etc. the universe of dynamic loading is nearly impossible to actually "calculate". Then you'd need to do an FEM on the mast under every one of those maximum conditions because Euler's load is based on a point load on the end of a column. That is nothing like what an actual mast experiences. You need to integrate Euler's formula along the entire length as each consecutive length of mast experiences greater load from top to bottom and it's both a non-linear load increase and a dynamic one that changes with various conditions. And if you're close enough and use a reasonable safety factor trying to precisely calculate the real instantaneous loads would be cost prohibitive. Not to mention the very basic limitations on variation of structural properties of a material like wood.

    So my ultimate point of this is that I would like to discuss how boats are actually designed. Everyone uses approximations due to the extremely dynamic nature of the loads. I'm asking a question about what approximation is used to justify a compression post that's significantly lower in compressive strength than that of the mast.
     
  10. gonzo
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    gonzo Senior Member

    Many compression posts are attached to a bulkhead. Since the limiting factor is buckling, and the bulkhead helps prevent that, they can have a smaller section. Unlike a mast, a compression post is less complicated to calculate. Are you calculating a particular application or do you want a generic answer?
     
  11. Jetboy
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    Jetboy Senior Member

    Thank you! A few more questions if possible. Would you use Euler's formula as an approximation on a simple 3 stay mast from the bridle down since the top portion likely has no compressive load beyond gravity or are most calculations done on the full mast length? Is there a common rule of thumb estimate for F? Since the mast is not uniformly loaded like Euier's formula models (and Euler ignores lateral forces), how do you make a guess at an approximate load? I understand that shroud tension such that it will heel the boat would give a static maximum, but a dynamic max could see significantly higher instant forces. I'm in the process of designing my column and need to decide if some of my aluminum tube is adequate. Any idea what "n" value is commonly used for a stayed mast? I've read somewhere in the 3-5 range. That would also make sense that the compression post should be more like 8ish from what I've found, but would vary with the rigidity of the cabin/hull.
     
  12. Jetboy
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    Jetboy Senior Member

    Just a generic answer. I'm going to need to decide on what extrusion to use for my compression post in a small 18' trimaran. I have a dozen or so aluminum tubes of various sizes left over from an aluminum offroad buggy frame that I was going to see if any would be useable or whether I needed to think about using something I don't happen to have in my scrap rack.
     
  13. TANSL
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    TANSL Senior Member

    My opinion is that a post compression works, especially in compression, while a mast works, especially bending. Therefore the calculation hypotheses are totally different.
    As it is very difficult, as you say, studying all solicitations acting on the rigging, simplification is generally accepted that sails exert a heeling moment that makes the boat heel about 30 degrees, for example. Coming into the boat GZ curves for this displacement, the heeling arm at these degrees is obtained and therefore total heeling moment. Knowing the position of the sails center of pressure, the force on the mast follows.
     
  14. Petros
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    Petros Senior Member

    the loads on a compression post are actually easy to estimate, the variable compound and impact loads on a mast and rigging is quite different. Most mast sizing is done from tables or charts based on history, rig configuration and size. These sizing methods for real masts out in real sailing conditions are developed from centuries of statistical data rather than scientific analysis.

    On some of the top racing sailboats teams have closely held analysis models by their engineers and designers where they try and accurately estimate mast loads so they can make it as light as possible. Of course these kinds of sailboats often suffer failures when pushing the boat hard in a race, the risk they take when trying to make it as light as possible. On a race boat they also would not for example consider capsizing forces, as you might on a deep water cruiser (reliability is more important than weight and absolute speed). On a racing boat, often if made absolutely reliable, it would be too heavy to be competitive. The idea is to make it only slightly stronger than all anticipated loads. For a deep water cruiser, you want it a lot stronger for any possible laod that actually might happen.
     

  15. SukiSolo
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    SukiSolo Senior Member

    Firstly, wood is perfectly good engineering material. One has to respect it's properties but for stiffness it is the equal of steel and aluminium for it's weight. Yes, different species change the basic nitro cellulose structure density and 'grain' but it is quite valid. As for variation say Balsa (hardwood btw) to Lignum Vitae? Also worth some consideration is the thermal properties of the material, especially as aluminium moves fair bit even over the diurnal range. In cold weather which material would you rather grip?

    I doubt very much than any compression post (even timber) is significantly lower than that of the mast. Other factors as Gonzo has said - ie bulkheads, cross beams, etc can take loads too. Work out hitting the boat from the underside with a lump of concrete and you probably have the ultimate load condition - hitting a solid rock shore in X force wind. Of course a Laser has never put it's aluminium mast through the bottom of the hull.....;) no wood involved......

    Last time I looked, the roof on my house was still there courtesy of 80+ year old timber holding it and the tiles (at 90Kg per sq m) up......;)
     
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