Anchor locker notch plate and bolt sizing

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Feb 25, 2020.

  1. fallguy
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    fallguy Senior Member

    Main hull connection.

    The brackets would transfer the slamming load to the two main hulls.

    The corecell/epoxy mould is about 50/50 corecell and epoxy by volume. It is a much higher density than just corecell.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Well, that changes the whole picture totally, as this is not a simple load case scenario.

    Since those beams are doing 2 jobs and often at the same time. Thus without knowing the global structural arrangement and load paths to transfer these loads and the assumptions, stating whether the bolts are sufficient has far too many unknowns. If this is a RW design cat, then he should be able to provide with said data.

    Simply too many unknowns and insufficient dwgs to analysis this correctly to provide a satisfactory answer.
    I realise this does not help you, but I cannot provide guesses and hunches when your safety is at stake, doesn't work like that - I need more data/info.

    Can I make a guess/estimate... sure... but that could provide a false sense of security.
     
  3. fallguy
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    fallguy Senior Member

    I have emailed Richard and asked for his input. I believe he is underway sailing somewhere. I understand the reservations. The main beam sockets are truly doing a bit of work with the proposal, but it makes a lot of sense to me versus the cabin breaking off. The entire discussion is good because I am realizing taking the boat at top speed into a head sea is not going to be a good idea which I intuitively understand, but now know would be unwise on the facts.

    Interestingly, I have seen a few videos lately of lashing Wharrams and I am understanding how lashings are sort of a good way to go since they can act like shock absorbers. The method I propose does not have that in it.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    If you get nowhere... I can reverse engineer this, in the sense that in the absence of data thus far - if you provide me with sufficient basic data that I would need, I can offer guidance. But I will need a whole lot more data than currently exists.
     
  5. rxcomposite
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    rxcomposite Senior Member

    You sure? Seems like an overkill. The shearing stress on the bolt dia (area) should be the pulling/shear force divided by area/no of bolts. Then you have to check the shearing stress of the metal plate and the laminate. If the bolt is too strong, it will just shear thru the plate or laminate. You have to consider the 3 types of shear.
     
  6. rxcomposite
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    rxcomposite Senior Member

    It is called statistics. The probability of an event happening. But we, as designers can only guide you on the operating condition/operating envelope you intend to use the boat. Reduce speed when in adverse condition or when vertical acceleration is more than you can bear. Freak accidents do happen but there is where the FoS comes in. 3 would be good enough for design purposes, more if unsure of other factors, 6 if it is a lifting equipment or lives are at stake.
     
  7. fallguy
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    fallguy Senior Member

    yeah, I think they are a bit big

    one of my problems is I don't know how to calculate the strength of the 1/2" plywood core with db1700 each side

    I will calculate what I know tomorrow. A bit tired now.

    I also don't know what, if any, would be a desirable weak point, or if we design all for the loads and see which one loses....
     
  8. fallguy
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    fallguy Senior Member

    sorry, FoS?

    Factors of Safety.... not fullofit

    how to apply it here for me is not so easy to understand
     
  9. rxcomposite
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    rxcomposite Senior Member

    Factor of Safety. Multiply the applied load by 3 (or whatever FoS). For composites, divide the ultimate strength by 3. You are allowed to use only 1/3 of the ultimate strength. The rest are reserve. For metals, divide the yield strength (not ultimate) by 3 (or whatever).

    There may be other definition of FoS and Allowable Load but let us not complicate things further.

    In short, you can apply up to 3X the load you are expecting and still not break (maybe cracked or deformed)
     
  10. fallguy
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    fallguy Senior Member

    For semantics; it would be good to know if the slamming load proposed by AH already accounts for a FoS and which one.

    I will also post my calculations for the stuff I know. I am mostly concerned the tensile ratings of my 5 bolts in the beam are too low and that we might damage them on an impact.
     
    Last edited: Mar 2, 2020
  11. fallguy
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    fallguy Senior Member

    Well, I got an email from Richard and he agreed with the plate idea I think... One outstanding question was still in play.

    I did the calculations on sizing the plate and I came up with the following.

    12 bolts 1/2-13", shear concern, shear rating is 4,598 pounds each versus a need of 53,244 pounds or 4,437 pounds each

    Plate
    12 bolts area, tensile concern or shearing through thickness, tensile calc is diameter of bolt times 12 bolts times thickness of plate. Must exceed 53,244. Steel about 0.130" thick would accommodate. I found some 1/4" 316 I plan to use. It is a bit heavy.

    The nesting bracket and its welds must also not fail. The welds would be in shear, and the bracket would be applying its forces onto the aluminum beam above.

    Since the welds were a concern, the weld area is 12" long each side and welding onto 1/4" bracket and 1/4" plate. I don't know how to calculate the strength needs.

    I also don't know how to calculate the 'strength' of the plywood composites. But, there are 24 bolts acting on the composite and the area of each bolt is 0.25 square inches. So I have a lot of area, but not sure about the strength of 1/2" ply with 17 db each side. I can easily calculate the force if I apply 14.5 psi to 4800 square inches, it is 70,000 pounds force acting on 6 square inches of laminate or 24 bolts. That is 2,917 pounds per bolt if my math is close. I don't really like the idea of being the last 200 pounds on that bolt, but I really don't know the calculation here a bit. I have a feeling if we had a very bad impact that damaged the boat, the damage would be in the plywood core here and not the beam.

    This probably gets into the territory Ad Hoc was avoiding which is 'what should fail if...'

    Anyhow, progressing towards making some beam brackets underneath to keep my bolts from overloading...
     
  12. rxcomposite
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    rxcomposite Senior Member

    For simplicity. use the one with the higher shear strength since in plane shear is measured in units. Attached is the properties of plywood. If the plywood is weak, add a laminate of sufficient thickness. If weaker still, embedd a steel washer to increase shear strength.

    WR/epoxy has an in plane shear strength of 80 N/mm2 (MPa).

    Now calculate if in in plane shear if the 1. whole of laminate will fail. 2. only the pin will fail, or only the 3. area behind the pin will fail.
     

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  13. fallguy
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    fallguy Senior Member

    I don't know which is which for sure or the units they use.

    I am using US doug fir 12mm.

    In plane shear is 2, but 2 what?

    A bolt pressing through the sheet sideways would be a compression force. Assume transverse compression? So 13, but 13 what?

    I calculated the pin already. That was ez.

    Sorry and thanks both.
     
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