Mast compression post specifications

Discussion in 'Sailboats' started by Matthew Dunk, Apr 19, 2022.

  1. Matthew Dunk
    Joined: Apr 2019
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    Matthew Dunk GILow

    Hi all,

    Our Kelly Peterson 44 was originally designed with a keel stepped mast. At some time in the past a very neat and professional job was done to convert the mast to a deck stepped tabernacle system.

    The only bit I do not like is that they used the lower section of the original mast as the compression post. The mast itself has in mast furling so this means we have a pretty huge mast section in the cabin when a much smaller steel tube section would work just as well.

    How much smaller is my question.

    Can anyone point me to some numbers or formulas for calculating an appropriate tube section/wall thickness etc?

    Matt
     
  2. TANSL
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    TANSL Senior Member

    You have to calculate a series of forces before arriving at the formula to define the mast. The attached file tells how to do it. If you have doubts, send me the details of your boat, sails and mast and I will do the numbers for you but I think the attached file explains it quite well.
     

    Attached Files:

  3. Matthew Dunk
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    Matthew Dunk GILow

    umm… I think there may be a misunderstanding. I’ve got the mast, I’m just wanting to replace the compression post.
     
  4. TANSL
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    TANSL Senior Member

    This is what myo thought you wanted to solve. Apparently, as you say, I've had a misunderstanding. I'm sorry.
     
  5. gonzo
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    gonzo Senior Member

  6. TANSL
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    TANSL Senior Member

    This is true only if the column is in compression.
    But what needs to be answered is how to calculate the compression to which the column is subjected.
     
  7. Blueknarr
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    Blueknarr Senior Member

    Since the current aluminum mast section seems to be adequate.
    Use another tube with at least equal capacity.
     
  8. TANSL
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    TANSL Senior Member

  9. Matthew Dunk
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    Matthew Dunk GILow

  10. Matthew Dunk
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    Location: Adelaide, Australia

    Matthew Dunk GILow

    An engineering friend has suggested that I use the peak load that can be exerted from the fore and aft stays and the stays on one side of the mast, Simplisticly this would be the sum of their breaking strains.
     
  11. TANSL
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    TANSL Senior Member

    @Matthew Dunk , when you know the compressive force acting on the column, you can use Euler's formula (the one applicable to your case) to determine the geometric properties of your post, assuming you already know the mechanical properties. As long as you don't know the compression force, Euler's formula, or any other, will be of little use to you.
    If there were no stays to hold the pole, it would suffer bending forces that, with the stays, are greatly reduced. These forces generated by the wind acting on the sails are transformed by the stays into compression forces on the mast. I therefore agree with your friend the engineer. But we still have the unsolved problem, what is the amount of that compression force? The file attached to my previous post answers this question, a question that people, turning the wheel a thousand times, refuse to answer. And please tell your engineer friend not to get to the braeking point. You have to stop well before, let's say halfway through the tension corresponding to the breaking point.
     
  12. Matthew Dunk
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    Matthew Dunk GILow

    Yes, the sum of the breaking strains was the worst-case scenario.

    But I’m not keen to tell him what to do. He has been sailing the world for 20 years in a 45 foot aluminium cutter he designed and built himself, with multiple circumnavigations and the North West passage under his belt. Somehow I reckon he knows more than me.
     
  13. TANSL
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    TANSL Senior Member

    I admire sailors as experienced as his friend and, seriously, I would have liked to have been able to sail as much as he did. But, when it comes to calculations on the strength of materials, I prefer to follow the recommendations of structural technicians even if they have not sailed around the world.
    A thought, not very deep. Why do you think that nobody asks Lewis Hamilton, probably the best current driver in the world, to design a car? He has driven all over the world, at tremendous speeds, but no one thinks he is the best at designing cars.
    Forget my words in the previous post, please, and don't say anything to your friend.
     
  14. Matthew Dunk
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    Matthew Dunk GILow

    Well, your analogy of the racing driver would be relevant EXCEPT:

    This guy did design the boat.
    This guy did build the boat (he welded while his wife rolled the aluminium.)
    The boat was so successful that multiple circumnavigating sailors have asked for the plans and built copies for themselves which they have also sailed around the world.

    Oh, and he was a senior engineer for BHP who made so much money doing his job he was able to retire in his early forties and sail the world for the next thirty years, stopping briefly to design and build what might be the ultimate cruising yacht. (And still going.)

    So, yeah, the analogy is kinda broken and I’ll stick with his logic.
     
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  15. rnlock
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    rnlock Senior Member

    I'm pretty sure that the sum of the breaking strengths is a good, conservative value with a safety factor. I suppose, in reality, one could omit the strengths of the downwind stays. Unless someone was going nuts with the turnbuckles.

    I'll bet the existing mast section is total overkill. An aluminum compression post could probably be much thinner, considering that the unsupported length is much less, and all the loads are axial. It might make the calculations more accurate if the joints at the end of the post were "pinned", i.e. were not able to transfer torques to the post. OTOH, if the post was well supported, without adding any bending loads, it could probably be smaller. I could see rounding the ends of the post and having them fit into some sort of sockets attached to the hull.

    If it was convenient, the ends of a post with pinned joints could be much thinner than the middle, though the calculation might be a little more fraught. Maybe second year strength of materials rather than first. All lost in the fog after many years.

    There's a bunch of stuff on Euler's column buckling theory that's easy to find on line. There are also pages with info on how to calculate the moment of inertia, and even calculators which can tell you what it is, given the dimensions. I'm not so sure about tapered sections, but I'm sure that columns tapered at the ends, at least in the case of pinned ends, will require less material than ones with constant cross sections.

    It's more about the stiffness than the strength, which favors wider sections if you want to keep things light. At least until the walls are so thin that you poke them and they buckle. It's possible for some people, if they have a good sense of balance and can keep the load centered on the middle of the can, to stand on an empty aluminum soda can. The least poke at the can makes it collapse. I've seen this, I just don't know how much weight a can can handle. Years ago, it was clearly more than 140 lbs. The lightest post would probably be made of balsa, but you wouldn't have room to walk past it.

    If you really can't figure out the math from the references on line, tell me a load and specify a metal tube, or series thereof, and I could run the numbers. At least for uniform cross sections and if you're going to pin* the ends properly.

    *Doesn't have to be literal pins.
     
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