Beam joint overlap?

Discussion in 'Multihulls' started by revintage, Oct 21, 2019.

  1. revintage
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    revintage Senior Member

    Did a mock up of the main beam joint I plan to use on my small 100kg foiler. Had to cut the PVC tube rings open to slide them over the 90mm tube. The 90mm tube with PVC rings has a good slide fit into the 110mm tube with 0.2 mm play. Will lock with stainless clamps made from 20mm wide 2mm stainless, cut from a tube with 110.3mm inner diameter. One clamp in each end of the overlap.

    Question is how to calculate the necessary overlap of the two alu tubes, any suggestions?

    nybalk2.png joint33.jpg
     
    Last edited: Oct 22, 2019
  2. TeddyDiver
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    TeddyDiver Gollywobbler

    That makes two point loading spots for both tubes. How about a longer PVC sleeve and just a cotter pin instead of clamps? Should be as strong as the tubes..
     
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  3. revintage
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    revintage Senior Member

    Correct! But I want to be able to adjust AOA by turning the inner tube relative to the outer. Thought of using another 5cm PVC sleeve in between the two. Chasing weight ;-) .

    The longest sleeves I can make in one piece is 196mm, and the PVC is 0,34kg/dm.

    Just guessing, but a single 196mm(two times the tube diameter) PVC sleeve, would probably do.

    But how long should the overlap be to get equal strength, if we go by the two point loading instead?
     
    Last edited: Oct 21, 2019
  4. Ad Hoc
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    Ad Hoc Naval Architect

    As the tubes slide you get, as Teddy noted:

    upload_2019-10-22_9-7-13.png

    So you have 2 reaction loads and this creates a couple and thus a bending moment in that short section.
    You need to satisfy the shear area, from the reaction loads and the bending moment created by said reaction loads.
    Then finally do a deflection check.
    But since this shall be subjected to a lot of cyclic loading - fatigue - your design stress level should be appropriately low.
     
  5. revintage
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    revintage Senior Member

    The beam tubes are chosen with decent bend and torsion margin. The original main beam design was 90x2mm, with 1,2x the strength of Doug Halsey´s obviously well working Broomstick.

    Will empirically try 40cm overlap with three spacer sleeves, adding 3kg compared to the original.

    About the stainless steel clamps, one will be placed at the big tube end, but I guess it will make no harm, to add one where the small tube ends. As it is slide fit I don´t see it necessary to split the tube under the clamp.

    About the foil, there will also be a two reaction loads forming a couple.
     

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  6. Ad Hoc
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    Ad Hoc Naval Architect

    Im a bit confused:

    But now you say this:

    So why are you asking how to calculate the overlap, if you're saying these tubes are selected with sufficient margins...and then:

    Doesn't seem like a calculation to me. Very confused... o_O:confused:
     
  7. revintage
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    revintage Senior Member

    Hey Ad Hoc,

    To me, you seem more annoyed than confused, no pun intended;).

    1 Even the smaller tube has ample margin to be used as full length beam tube, the 110mm has 1.5x of the smaller. So sorry I left "full length" out:oops:. The question of how to calculate the overlap still remains!

    2 I asked for help with calculations, the answer you gave was the same as TeddyDiver´s but in other words. So without more than vague and obvious calculation info, I had to use my GFC. Actually your words above gave me the idea.

    Why not take my last idea and do some back engineering:)?
     
    Last edited: Oct 22, 2019
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Not at all.....you seem not to understand what you are doing:

    Well, i simply have no idea if you are unable to calculate that which you seek, when you make statements like this:

    If you have not calculated it previously...how can you make this statement...where is your verification?

    You cannot in one breath say...the tubes have been selected for their bending and torsional margins and then in another say, how to calculate it?
     
  9. upchurchmr
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    upchurchmr Senior Member

    PVC spacers are going to be a sure failure point.
    Or have you calculated that also?
     
  10. revintage
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    revintage Senior Member

    In fact I am. But probably looking at this problem another way than an active professional. It was actually 50 years ago, slide ruler age, that I did my courses in mechanics and haven´t needed to use the now faded knowledge since, as I moved into electronics.

    Sorry you didn´t notice that I refered to a well known, well working design, with the same beam configuration, same foil configuration, same size and same grade aluminium tubing. Simply by calculating J and Z and compare the different tube sizes, I choosed tubes with higher values. The margin figures in a post above.

    You probably misinterpreted and guessed that I had done calcutions for this specific loading case.

    The calculation of "So you have 2 reaction loads and this creates a couple and thus a bending moment in that short section. You need to satisfy the shear area, from the reaction loads and the bending moment created by said reaction loads." was what I needed help with.

    Being retired, I have had the time to refresh my knowledge, so you can leave this thread without bothering anymore.
     
    Last edited: Oct 24, 2019

  11. revintage
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    revintage Senior Member

    While I respect Ad Hoc, your comments make me wonder?

    How on earth did YOU calculate the solid PVC spacers(50mm wide, 8mm thick) to be a "sure failure point":rolleyes:.

    Where is your verification?
     
    Last edited: Oct 24, 2019
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