Aftmast rigs???

Discussion in 'Sailboats' started by jdardozzi, May 28, 2002.

  1. CT249
    Joined: May 2003
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    CT249 Senior Member

    A fast craft can't afford to point low in terms of apparent wind, because its apparent wind is so far forward that if it points low it will just reach back and forth. It's the other way around - the faster a craft is, the more attention must be paid to reducing drag and creating a high-pointing rig and foils.

    This can be seen very dramatically on a high performance windsurfer or windfoiler, where you can easily switch entire rigs. If you use a rig or setup that "points off the wind" you will just reach back and forth, or maybe not even do that, because the apparent wind is so far forward. To get upwind you need a very high-pointing rig.
     
  2. brian eiland
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    brian eiland Senior Member

    Hi Nick,
    Welcome back to the forums, and this discussion in particular. I do welcome your contributions, particularly with illustrations.

    Here is the problem I have with that diagram/analysis. That one vector component of your backstay force has a vertical ‘upward force’ to it. There is NO way in the real world that a sailboat’s backstay can pull ‘upwards’ on the masthead,….. horizontal at best.

    I wanted to know what that horizontal force needed to be to offset the forestay’s forward force, regardless at first of what compression loads it was placing on the mast itself (that would come later).

    So what forestay forward pulling force do I need to balance? The one presented by the 2 ton forestay load,...it’s horizontal component being 0.813 tons. Thusly, a backward horizontal force of 0.813 tons which translates into a backstay force of 2.25 tons ?
    https://www.boatdesign.net/attachments/dscf9591-jpg.190750/


    If I take your backstay load of something approx 5.5 -5.76 tons, and look at its horizontal component of 2 tons, ….that’s quite a bit in excess of the 0.813 pulling forward ?

    Is my analysis in error?…..where??
     
  3. brian eiland
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    brian eiland Senior Member

    Twin Backstays

    I have always liked the idea of twin backstays,... to divide up the loading that each one carries, and the loads to the hull attachment points. I most cases its difficult to get this 'twin loading', as invariably one side goes slack due to slight migration of the masthead athwartships. I'm hoping to solve this by making the twin backstay(s) one continuous line that wraps around the mast at its upper connection ...or other solution?

    Actually I am hoping to have BOTH of of my backstays (masthead and hounds) capable of performing similarly.

    It might involve a method of wraping around the mast tube itself, or some sort of sheeve or 'loop' solution,....
    https://www.boatdesign.net/threads/attaching-rigging-to-the-mast.68435/
    https://www.boatdesign.net/threads/soft-rigging-solutions.47997/

    Here is what those twin backstays look like on the model yacht,..
    DSCF9605.JPG DSCF9594.JPG

    DSCF9595.JPG DSCF9598.JPG DSCF9598.JPG DSCF9600.JPG DSCF9602.JPG
     
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  4. Nick12345
    Joined: Jun 2020
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    Nick12345 New Member

    Hi Brian,

    I can assure you that the upwards force described in my diagram is most certainly correct. This is because it is created, not just by the tension of the backstay, but by the combination of the tension of the backstay AND the compression load on the mast.
    The following diagram shows your forward leaning mast and back stay only (we'll ignore the forestay for now). If the mast is free to rotate about its base and supported so that it doesn't fall to port or stb, gravity will hold it in place as it leans forward. That is until we start pulling on the backstay. At this point it will start to rotate about its base and the masthead will follow the green arc - and in doing so move upwards! This is were the upwards force comes from. Each line in a vector triangle is always a COMBINATION of the other two sides. Each line must also always represent the direction of the force. The direction for the back stay and mast follow the angle of each component and the third vector which represents the rotational or torque force at the masthead which, if the forces are balanced and the mast head is static, is represented by a line tangential to the arc of rotation - yellow line in the diagram.
    vectors.jpg
    Now that we have established the correct angles for each vector we can give a value to one side of the triangle and find the balancing forces for the other sides. Working out the forestay is done in a similar way. The first to vector angles are the angle of the mast and the angle of the forestay and the third vector angle is again a line tangential to the green arc that also happens to be perpendicular to the mast. My original is correct and would indicate that a 2 ton static load on the forestay will result in some considerable forces of tension on the backstay and even bigger compression on the mast. All of this means more material in these components which means more cost and more weight aloft. This is always going to be the the disadvantage of a raked forward rig configuration. Not impossible though.
    I am assuming you are familiar with the Dazcat Artemis?
    Accessible Expedition Catamaran - The Multihull Centre https://www.multihullcentre.com/multimarine/current-projects/accessible-expedition-cat/
    They use a similar aft-mast, headsail only set up, but the have kept the mast raked aft as this will reduce rig loads dramatically.

    Hope this helps
     
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  5. brian eiland
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    brian eiland Senior Member

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  6. brian eiland
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    brian eiland Senior Member

    I'm still thinking about the modifications I can make to my rig's staying.

    In the mean time I just ran across this posting I made on another forum. It may help to explain why I became SO INTERESTED in this headsail plan,...

    Excellent list of many of his papers in PDF form
     
  7. brian eiland
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    brian eiland Senior Member

    [​IMG]
     
  8. CarlosK2
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    CarlosK2 Senior Member

    To place one sail behind other is a very typical thing to do ...

    the ridiculous idea of a mast leaning forward with an absurd and delirious rig is quite another thing.
     
  9. brian eiland
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    brian eiland Senior Member

    Carlos,...I suspect you have never read back thru much of this rather long subject thread,...perhaps that would be appropriate before adding your criticisms?

    ...for instance this posting of mine, Aftmast rigs??? https://www.boatdesign.net/threads/aftmast-rigs.623/page-7#post-198605

     
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  10. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    ...or you might have a look at this posting,... Aftmast rigs??? https://www.boatdesign.net/threads/aftmast-rigs.623/page-7#post-198774

     
    Last edited: Nov 11, 2023
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  11. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    Structure of the Vessel to Support Rigging Loads

    have a look here,... Sail Loading on Rig, Rig Loading on Vessel https://www.boatdesign.net/threads/sail-loading-on-rig-rig-loading-on-vessel.2293/page-4#post-114487

     
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