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Thread: Bobstay bolt angles

  1. #1
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    Default Bobstay bolt angles

    Should bobstay ringbolts passing through the stem be drilled parallel to the line of force or normal to the plane of the stem? I see problems with both.

    Bruce Brown

  2. #2
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    Default Re: Bobstay bolt angles

    The only boat I built with a bobstay was about 30 years ago and I'll be darned if I can remember what how I installed that ringbolt. Probably parallel to the water. If it was at a downward angle, it would have to be very long.
    Mine might even have been a tang screwed to the face of the stem. I'll see if I can make it out in old photos.
    I was born on a wooden boat that I built myself.

  3. #3
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    Default Re: Bobstay bolt angles

    There was a thread in September that covered this pretty well. http://forum.woodenboat.com/showthre...bstay-fittings
    If the eyebolt is not in line with the load, the load capacity is reduced drastically.

    http://www.amesweb.info/Fasteners/Fo..._Capacity.aspx
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

  4. #4
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    Default Re: Bobstay bolt angles

    That calculator is a pretty shocking education about how dramatically load capacity drops as the load moves away from being in line with the bolt.

    I just gained a new appreciation for tang-style bobstay fittings. With big bolts.

    Thank you, MN Dave.

    Alex

  5. #5
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    Default Re: Bobstay bolt angles

    You can select an eyebolt size, bigger for oblique angles, that will be adequate. You then need to consider the compressive loads in the wood of the stem exerted by the shank of the bolt to ensure that there is enough bearing area to avoid crushing the wood.
    It really is quite difficult to build an ugly wooden boat.

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  6. #6
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    Default Re: Bobstay bolt angles

    I have seen lots of knarly, crushed stem timbers where bobstay eye bolts have been used, the majority on falmouth work boats, enough to put me off drilling a hole through the stem, and use a tang plate instead, unless using,as Nick suggests, a well oversized fitting. They can come under a lot of shock loads and need to be sized accordingly.

  7. #7
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    Default Re: Bobstay bolt angles

    This would be easy enough to design and fabricate.
    It really is quite difficult to build an ugly wooden boat.

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  8. #8
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    Default Re: Bobstay bolt angles

    image.jpg

    Seen on a BCPC in Ipswich a while ago.

  9. #9
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    Default Re: Bobstay bolt angles




  10. #10
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    Default Re: Bobstay bolt angles

    I guess that this is for the Friendship sloop, yes?

    In which case look for something like this from a chandlery.

    That is from http://www.porttownsendfoundry.com/b...s-whiskerstays Or have one welded up from stainless by your local fab shop.
    It really is quite difficult to build an ugly wooden boat.

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  11. #11
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    Default Re: Bobstay bolt angles

    Looking back at old photos (I sold the boat 25 years ago) I can see that I didn't use a ringbolt, but a stainless steel tang, well bolted to the stem. I went out in some pretty breezy weather and the bowsprit was always still there when I got back to the harbor!
    I was born on a wooden boat that I built myself.

  12. #12
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    Default Re: Bobstay bolt angles

    Quote Originally Posted by Peerie Maa View Post
    You can select an eyebolt size, bigger for oblique angles, that will be adequate. You then need to consider the compressive loads in the wood of the stem exerted by the shank of the bolt to ensure that there is enough bearing area to avoid crushing the wood.
    Quote Originally Posted by skaraborgcraft View Post
    I have seen lots of knarly, crushed stem timbers where bobstay eye bolts have been used, the majority on falmouth work boats, enough to put me off drilling a hole through the stem, and use a tang plate instead, unless using,as Nick suggests, a well oversized fitting. They can come under a lot of shock loads and need to be sized accordingly.
    These might help determine how much to spread the loads to avoid crushing the wood:
    https://www.fpl.fs.fed.us/documnts/fplrp/fplrp163.pdf
    https://naldc.nal.usda.gov/download/CAT86200326/PDF

    More on bobstays:
    https://www.classicmarine.co.uk/arti...wsprit-article
    http://forum.woodenboat.com/showthre...ng-the-shrouds
    https://www.thehulltruth.com/boating...-bow-eyes.html ---> (http://www.toweye.com/)
    These towing eyes (toweye.com) don't look as good to me as the fitting in Nick's post#10, but might be worth looking at for some boats:
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

  13. #13
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    Default Re: Bobstay bolt angles

    According to Herreshoff, the bobstay of a vessel can sometimes be loaded with strains that can exceed the entire displacement of the boat! When you stop to think of this is can be truly a source of concern if one has not taken the time to truly think out a well planned fitting for this critical component of the rigging gang. If the bolt that passes through the stem is loaded at a vector that differs from the line of pull of the bobstay the entire stay load can cause forces that will damage the stem itself and can eventually end up with the ring bolt or fitting failing. When faced with this situation I try to design a system that is of sufficient strength to exceed the boats displacement. This is then anchored into a plate that is let into, morticed, the stem. In addition, a smaller through bolt is added above and below the main bolt thereby spreading the loading into three points of support. A matching plate is placed on the inner face of the stem to back up the spread of the load. This is a factor that is often overlooked by the home boat builder which should be truly addressed with good planning rather than hopeful guessing.
    Jay

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