Terminate backstay to over head radar arch?

Discussion in 'Sailboats' started by verbryck, May 23, 2019.

  1. verbryck
    Joined: May 2019
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    Location: long beach CA

    verbryck New Member

    Hi there, I'm in the process of installing a stainless steel arch over the back of my Ericson 39. I want the arch to mount solar panels, bimini etc... While I was looking at it the other day it occurred to me how much better it would be if I could terminate the back stay to the arch as opposed to having the back stay run through or in front of the arch down the transom. That is what brings me to this forum. I do not have the engineering background to determine if this is safe and or desirable to think about. I plan to use this boat for coastal cruising. The arch is made from 1.5 inch welded stainless steel tubing. The proposed termination point would be in line with the current trajectory of the back stay and approximately 5 feet off the transom. In any case I hope this is an appropriate place to pose such a query. Thanks for any insight into this question, George
     
  2. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Welcome to the forum.

    It could be done.
    BUT
    You will need to find an engineer who fully understands several disiplines. Most arch guys don't fully understand rigging or hull engineering. And versa visa.
     
  3. verbryck
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    Location: long beach CA

    verbryck New Member

    Thanks for the response do you know of anyone whom I could contact? Thanks, George
     
  4. upchurchmr
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    upchurchmr Senior Member

    Forget it.
    There are so many ways to F... this up you might not believe.
    Starting with the strength of the hull where you attach the arch.
     
  5. verbryck
    Joined: May 2019
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    verbryck New Member

    arch backstay.jpg

    Here is a photo of a boat I found at the marina today illustrating how close the back stay comes to an arch. It seems like it would be so easy to just connect it to the arch and do away with the split stay running down to the transom. This arch is aluminum and is not welded so do not think it would support the load. Any engineers on this forum could explain to me the math I would need to use to figure out the loading for this type of arrangement? Thanks, George
     
  6. upchurchmr
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    upchurchmr Senior Member

    What you want on this forum is the equivalent of a Mechanical Engineering degree.
    No one with the qualification to design this will help here due to the liability when it fails and drops your mast in the ocean - or on your head.
    As Blueknarr said, it can be done - but it will be so heavy it would be silly.
    It will take a lots of material in the arch to get it stiff enough to actually provide anywhere close to the same support for the mast, even before worrying about the strength.
     
  7. gonzo
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    gonzo Senior Member

    Can you post a photo of the arch?
     
  8. JamesG123
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    JamesG123 Senior Member

    Mr. Upchurch has given you the best answer you are going to get here. Its not done that way because its not optimum weight and strength wise, even if it might be convenient for the operator.
     
    Last edited: May 24, 2019
  9. tlouth7
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    tlouth7 Junior Member

    There is no reason it couldn't be done, but what would you gain? Slightly better transom access at the price of excess design effort and more points of failure.

    If I were going to do this I would have a split backstay terminating at the top corners or the arch. Thus the crossbeam acts like a spreader bar on a crane and the uprights are in tension. This relies on the arch being inline with the tension loads as viewed in profile. If (as per OP's photo) the arch is leaning backwards then it will just be pulled forward by the backstay (unless it is made very strong or has stays added which would rather defeat the point). The maths is not terribly hard with the possible exception of joining the arch frame to the hull.
     
  10. JamesG123
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    JamesG123 Senior Member

    The arch becomes a structural member of the craft. It would need to be rooted deeply into the hull, not just bolted on like a normal arch or bimini frame.
     
  11. gonzo
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    gonzo Senior Member

    Arches have more fasteners than the chainplate for the backstay. It is not really that complicated. Post a photo of your arch for better advise.
     
  12. JamesG123
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    JamesG123 Senior Member

    The chainplate fasteners go into the hull where it was designed to take the full stresses of the rigging. Arches (esp. aftermarket ones) tend to just mounted any-which-where on the hull material since their loading is an order of magnitude less.

    You ought to know better than to try to give "internet advice" on something this critical.
     
  13. gonzo
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    gonzo Senior Member

    It is not really complicated to transfer the load. I asked the OP to post a photo to have an idea of the setup.
     
  14. upchurchmr
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    upchurchmr Senior Member

    No it is not that complicated.
    Its just critical to get it right, and the structure where the arch is mounted is really important, same as James said.
     

  15. gonzo
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    gonzo Senior Member

    The OP is not posting photos of the arch. That would make the discussion more relevant.
     
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