Shear calc help

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Nov 9, 2018.

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  1. fallguy
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    fallguy Senior Member

    Please also look at the mast section pic from Monday 7:42. It has a hardware channel on the bottom.

    That channel I have considered filling. It will collect seawater and rot the ally fast, especially on the inside flats.

    When I built the insert mould, I taped over the channel as the mast was the plug.

    Would you fill it? It makes for a nice drain. I plan a cover over the sockets, but the boat will be subject to freeze thaw. I could partially fill it as well and just get rid of the sides of the T so to speak. Then I could even key the beam into the remaining channel by wrapping the beam a bit loose with the plastic, but this hampers the freeze thaw problem. Any water or snow thaw mighy get in and destroy the socket.

    The fitment could be improved, but only on the bottom as the taper of the socket is required for release.

    The boat is headed to the Great Lakes early in its lifetime; later some ocean if I survive long enough.
     
  2. fallguy
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    fallguy Senior Member

    Here is a picture of the socket section and fit. I can easily improve the fitment on the bottom as advised. Thank you Groper.

    You can see the taper for release here. There is some looseness, if you will for the neoprene.

    Another possibility would be to rebuild the strapping. It would cost about $700.
    733CB48C-DA04-4D6A-9749-4DC2A9478587.jpeg
     
  3. groper
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    groper Senior Member

    yes its a shame you have the mast section the way you have it - i would have done it upside down from how youve done it... This way the taper of the mast section could be in a molded fit for perhaps 80% of its surface area and still allow removal. The way you have it - you will only get maybe 20% contact area... I can see a lot of movement in your design there, and it puts most of the total stress concentrated on the very small bolted area. Remember you have torsion aswell as bending in that mast beam...

    That entire laminated glass channel could be molded around the plastic film covered mast upto just shy of the widest part of the section. You could put some some flat flanges either side of it held in place with coved plasticine to complete the molded peice before layup. Unfortunately it involves building it all again and yes - new straps...

    Another option would be to block the existing setup with machined blocks to cradle your mast within the channel and take up all the gap. When you remove the mast- the blocks come out with it. However doing this is likely even more expensive than rebuilding the whole thing...

    Probably not what you want to hear... but you now have something to think about :)
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    I don't understand imperial units at all...all chinese to me. But using your M16 s/s bolts, i get a max torque of 76Nm.
    This gives a max preload in the bolt of 2440N.
    With a standard M16 washer the area is 479.9mm^2
    This gives a pressure of just 5.1MPa.
    This is insufficient to cause issues on the laminate.
    If you're concerned about the nut loosening - use a split pin to hold it.

    Your biggest concern is the gap. Not addressing this significantly effects the results of above and many other aspects of the attachment too.
    You need a direct load path from the mast to the structure, via the straps/bolts. As it is, you don't have that.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Dan,

    Good advice here from Groper.

    And once done, then satisfy yourself that the load from the mast that is now transferred via the bolts is insufficient to cause issues with the "gap filler" what ever you decide it to be. And then that same load into the surrounding laminate from the gap filler.

    Many people tend to forget bearing stress and shear stress of the structure the bolts are attached to. If you had an M16 bolt in 20mm thick ally and try to pull the bolt perpendicular to the shaft of the bolt, i.e to tear the bolt through thickness of the ally, is the result any different from if the ally is just 4mm thick? Then using the same example if the bolt is 2 diameters from the edge or just 1/2 a diameter from the edge?

    How you arrange the load transfer using a bolt is very important. As Groper notes, the torque etc is pretty much irrelevant at this stage. Load transfer is the elephant in the room.
     
  6. fallguy
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    fallguy Senior Member

    I measured things today and a one inch riser block 2x2 would get us close. All the loading would be spread across the same 3.3xx square inches on both sides of the plate. I think I will wait until the other socket is done and the sockets are both finish bedded with the neoprene to see if they are all a common size. I would have to plastic wrap the neoprene liner around the mast to avoid making the fitment too tight for my silencing.

    The remanufacture of the straps would be costly and still imprecise.

    Anyone want to throw out a torque spec?

    25 ftlbs?

    Still not sure I understand the laminate rating vs the bolt, but it looks like the bolt would fail first and that is not intuitive.
     
  7. fallguy
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    fallguy Senior Member

    Thanks for all the input. I'd rather hear from people now versus a hearing it in heavy seas.

    These beams below are lashed on with inner tube strips. Cost was zero. They are just safety outriggers; water temp is 39F.
    75683A1D-465C-4ABB-A726-981C732625E3.jpeg
     
  8. groper
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    groper Senior Member

    Getting nowhere fast...
    i think i understand your initial question now - you basically want to know if the glass channel youve made from 9 layers of 600gsm glass will be strong enough?
    if this is the case, not enough information has been given. We need to know the global design specs such as vessel beam, maximum displacement, and the number and position of structural cross deck beams etc.
    Alternatively - we could use the mast section properties and design to exceed its properties. ie- the aluminimum beam will fail before anything else - and we can assume the beam has been sized correctly by the designer RW (which he likely has)
    From this - we can work out the stress on the bolt head and thus the lamainte as a function of the maximum leverage applied via maximum bending, summed with torsion, summed with bolt torque to arrive at a final perpendicular load on the bolt head.
    We also need to know more about how the glass channel is fixed to the hull and surrounding structure adjacent to the bolt heads... do you have any drawings of these designed connections?
     
  9. fallguy
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    fallguy Senior Member

    Nope.

    I trust Richard on all the above.

    I came here to get a torque recommendation. I got good advice about closing the gap, but still need a torque spec is all.

    The mast section meets or exceeds specs.

    Sparcraft S830 is the section.

    sparcraft_catalogue_2016.pdf https://drive.google.com/file/d/0B32prmcZNtQLNnhNbkZkQXRJYTg/view

    Let me know if further info is needed. Most appreciated. I would prefer to avoid posting any drawings here to protect RW work.
     
  10. groper
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    groper Senior Member

    The bolt torque is not all that important.
    Provided the load can be transferred thru appropriate connections, sufficient torque is only that which provides no movement of the connection. If the connection can move, it will fatigue the materials and eventually cause damage and possible failure. If the connection moves - even with lots of torque - then the connection design is poor and should be reconsidered...
     
  11. fallguy
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    fallguy Senior Member

    So I will fix it up and run at 25ftlbs.
     
  12. BlueBell
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    BlueBell "Whatever..."

    Nylock nuts though, right?
    If it's a disassembly point.

    Crazy thread.
    I'm glad we got all that worked out.
    I concur btw.

    I'd be curious how RW shows the connection.
    How you've done it?
     
  13. fallguy
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    fallguy Senior Member

    I did some reading suggesting nylock was more prone to galling, especially in ss.

    Read the bullet on prevailing torque lock nuts.

    https://www.fastenal.com/content/feds/pdf/Article - Galling.pdf

    I plan to avoid them as I have had personal experience with nylock galling; especially since this is not a one and done. In fact, I purposely read up on the galling issue when considering nylocks. I think I will double nut them all ober std lock washers.

    I am building extremely close to all of RW building plans with very little changes. He is an exceptionally good, seasoned designer who is also willing to accept other methods (if I may). He has been infinitely patient with me as a foam rookie. My wife and I are pleased with the way the boat is coming together; despite my badly failing right hip.

    Further, I am grateful for the support of the forum.
     
  14. fallguy
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    fallguy Senior Member

    4E7F943B-8230-47FB-A639-C24B302372B7.jpeg
    I felt it right to post the solution. I built glass shims all about an inch thick and bonded them in under loading with the beam and straps. The one pictured still need some milled fiber to bond it and fillet around. I realize the thread is old.
     
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  15. Ad Hoc
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    Ad Hoc Naval Architect

    Many thanks for the update.
    Nice to see the simple solution.

    Hope the boat build is going well so far...??

    Never too old to add more info to the thread... :)
     
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