Bowsprit - Load on solid glass

Discussion in 'Boat Design' started by ghoti, Jun 7, 2022.

  1. ghoti
    Joined: Jun 2022
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    ghoti New Member

    Hi,

    I am sure this is a simple question but my google-fu is failing me. I am designing a simple bow-sprit that that bolts into the existing bow roller channel. Using the calculator provided by Selden provides an estimate of 3.5kN on the tack, assuming unsupported section = support section 3.5kN pushing down on the deck.

    Glass of the deck in that area is fairly well supported ~10mm solid glass and ~800mm from toe-rail to toe-rail. Spreading this across 100mmx50mm feels about right but this this project has sparked some interest, how can I determine what load can be supported by the deck. I'm sure this is 101 type stuff but I'm trying to learn. Any tips?

    Thanks,
    Alex

    Bowsprit calculator : Seldén Mast AB https://support.seldenmast.com/en/services/calculators/bowsprit_calculator.html?id=4605&cmd=submit&pp_refocus=&pp_offset=&lang=en&RM=40&WLH=1200&HGR=14000&USL=500&DBS=500
     
  2. gonzo
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    gonzo Senior Member

    It is a rather complicated question. You would need the design and scantlings of the boat to calculate all the loads. For example, the failure mode may be the deck to hull joint.
     
  3. ghoti
    Joined: Jun 2022
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    ghoti New Member

    Gonzo, it's a good point but the sprit will be loading down onto the deck, not trying to rip it up.

    From the diagram in the link I am interested in the interactions at R1 and understanding what kinds of loads a 10mm sheet of solid glass can take. At this point I would not even have a guess of the order of magnitude so really just trying to get in the ballpark.

    Ultimately I would like to specify a sprit that fails before the deck.
     
  4. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    To give a serious answer it would be necessary to know the structure of the deck in that area.
    (Frankly, I wouldn't know what to do with failure mode. I'm not saying that it shouldn't be considered, I don't know, that's why I would like Gonzo to be so kind as to explain how to take it into account)
     
  5. gonzo
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    gonzo Senior Member

    The link is not opening for me.
     
  6. rangebowdrie
    Joined: Nov 2009
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    rangebowdrie Senior Member

    What is the purpose, extend anchor/extend headstay for a jib? Are you planning a cantilever loading?
    If for sail carrying there should be a bobstay and the bowsprit is, (should be,) in compression.
    Perhaps I've misunderstood?
     
  7. ghoti
    Joined: Jun 2022
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    ghoti New Member

    Thanks All,

    The purpose is to extend a tack point for an Asymmetric spinnaker beyond the pulpit. This won't be used for code sails so not planning on using a bob-stay, this will be a cantilever design.

    I have attached a screenshot of the Selden calculator which estimates the loading on the tack for those where the link is unavailable.

    Screen Shot 2022-06-08 at 9.05.52 am.jpg
     
  8. gonzo
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    gonzo Senior Member

    That only shows the vertical force. What are the horizontal forces?
     
  9. rangebowdrie
    Joined: Nov 2009
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    rangebowdrie Senior Member

    I see now.
    Not being an engineer, how does the upward force at the forward end translate?
    In other words, which connection, (the stem head attachment or back on the deck,) becomes the fulcrum point?
    It "seems" as though the stem point would be the strongest in tension and would be the fulcrum, thereby making the after end putting the deck in compression?
    Perhaps any horizontal components translate into sheer loading for the fasteners?
     
  10. gonzo
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    gonzo Senior Member

    They don't translate. They are independent force vectors. To study and calculate forces, a "Free Body Diagram" is an easy simplification. You can look at examples online. However, for a proper analysis, you need to consider shock loads which can be much larger than the static loads.
     
    rangebowdrie likes this.

  11. rangebowdrie
    Joined: Nov 2009
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    rangebowdrie Senior Member

    Ok, I'll have some studying to do.
     
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