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Use backstay to control forestay tension with furling mast?

Discussion in 'Sail Trim with Don Guillette' started by danstanford, Oct 10, 2017. Add this thread to a FAQ

  1. danstanford

    danstanford

    Joined Aug 2, 2010
    148 posts, 6 likes
    Beneteau 323
    CA Cobourg
    My boat has a furling main and a non-adjustable back stay so I assume the mast is not intended to bend. Correct? Assuming this is true, and that I want to use forestay tension as a sail trim tool, should I be looking at a way to put a tensioner on the split backstays?
    There is currently no clutch to bring the jib halyard back to the cockpit winch so I don't have many tools to change the rather round shape of my jib.
    Dan
     


  2. Davidasailor26

    Davidasailor26

    Joined May 17, 2004
    865 posts, 89 likes
    Beneteau Oceanis 37 LE
    US Havre de Grace
    You're correct that with the furling main you don't want any significant pre-bend. But it should still be alright to add backstay tension while you're sailing, provided you let it back off before furling. It's during the furling / unfurling when you don't want too much bend.
     


  3. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    7,111 posts, 1,105 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    Your boat is fractional with swept shrouds. While its a static setting while sailing, rig tension will greatly effect your forestay sag. We adjust ours every time we go out based on expected conditions. We use a fixed number of 1/2 turns from a baseline to simplify.

    BlueJ rig tune.jpg
     


  4. DayDreamer41

    DayDreamer41

    Joined Oct 29, 2016
    444 posts, 133 likes
    Hunter 41 DS
    Un Michigan Port Huron
    With swept back spreaders would not increasing the tension on the back stay change the tension (reduce) on the shrouds?
     


  5. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    7,111 posts, 1,105 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    Yes it does, slightly. That gets factored in during setup.
     


  6. DayDreamer41

    DayDreamer41

    Joined Oct 29, 2016
    444 posts, 133 likes
    Hunter 41 DS
    Un Michigan Port Huron
    I am sure this would restrict adjustment while underway, not impossible but certainly impractical.
     


  7. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    7,111 posts, 1,105 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    Adjusting SHROUD tension is kinda impractical underway, but adjusting back-stay tension is done all the time; we go from full off to rod tight depending on conditions. It changes our shroud tension about 4% of breaking strength over that range.
     


  8. DayDreamer41

    DayDreamer41

    Joined Oct 29, 2016
    444 posts, 133 likes
    Hunter 41 DS
    Un Michigan Port Huron
    Really I would have thought it would be more dramatic a change, just curious as don't even have a back stay to adjust, but the Sapphire is not a thorough bred either.
     


  9. danstanford

    danstanford

    Joined Aug 2, 2010
    148 posts, 6 likes
    Beneteau 323
    CA Cobourg
    Fantastic information Jackdaw, can you tell me what process you used to arrive at the baseline? Experimentation aimed at target speed acquisition? Do you have any idea what the best solution would be for adjusting backstay tension when one doesn't exist to start with?
    Dan
     


  10. Joe

    Joe

    Joined Jun 1, 2004
    6,113 posts, 179 likes
    Catalina 27
    US Mission Bay, San Diego
    I thought backstay tension on a fractional rig was for mast bending... i.e. maisail trim. Wouldn't runner adjustment be more effective for dealing with forestay sag?
    Anyway, I consider an adjustable backstay essential equipment on most sailboats, no matter how you use it. Just because your boat didn't come with an adjustable backstay, don't assume it wouldn't be useful and worth the cost of installation. It was probably an option... the original owner decided not to check the box and pay the extra charge.
     


    Last edited: Oct 11, 2017 at 6:45 AM
  11. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    7,111 posts, 1,105 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    If you look at the chart I posted above, the baseline value is centered on the vertical shrouds set at 20% of breaking strength, a common value for this. In breeze the tension goes up, which also increases forestay tension. In lighter air it goes down. The difference in forestay tension across the range is immense.... at the high setting its very tight; at the low setting the forestay can be swung in a huge arc.

    Note that those values are with backstay off. Adding backstay to any of them will flatten the main yes, but also tighten the forestay and lower the shroud tension slightly.
     



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