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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
    206 posts, 29 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
    1,065 posts, 162 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
    8,426 posts, 1,858 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
    771 posts, 302 likes
    Hunter 41 DS
    Un 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
    8,426 posts, 1,858 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
    771 posts, 302 likes
    Hunter 41 DS
    Un Port Huron
    I am sure this would restrict adjustment while underway, not impossible but certainly impractical.
     


  7. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    8,426 posts, 1,858 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
    771 posts, 302 likes
    Hunter 41 DS
    Un 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
    206 posts, 29 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
    6,342 posts, 284 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
  11. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    8,426 posts, 1,858 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.
     


  12. Scott T-Bird

    Scott T-Bird

    Joined Oct 26, 2008
    3,275 posts, 413 likes
    Starwind 27
    US Barnegat, NJ
    I can see the value of a chart but I can't decipher what this chart means. I assume the bottom numbers are in rig tension (% of breaking strength). I don't follow what "V" & "D" are. I don't follow "half turns". I adjust backstay simply by feel right now. It would be good to have a system. Mine is masthead with split backstay. I simply pull line in on the blocks to pull the adjuster down the split shrouds and draw the shrouds together. I don't really have a baseline except to draw it down far enough to create 'moderate' tension when sailing in 'moderate' conditions. It would probably be a benefit to measure the loos gauge tension at various settings so I know where the baseline should be and how to adjust more precisely to conditions. I suppose I could then mark the shrouds for the correct positions of the adjuster.

    I do notice that when sailing upwind in stronger conditions (20 knots of so) the leeward shrouds are slightly more slack than normal as I increase forestay tension with the adjuster. My spreaders are slightly swept back. Uppers and lowers are fixed to the deck at the same location (just one set of lowers).
     


  13. Scott T-Bird

    Scott T-Bird

    Joined Oct 26, 2008
    3,275 posts, 413 likes
    Starwind 27
    US Barnegat, NJ
    I agree, with higher windspeed I adjust the backstay and the improvement at the helm is vast. It really helps to balance the helm and improve pointing.
     


  14. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    8,426 posts, 1,858 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    Sorry we switched gears here. The OP was asking about forestay tension on a boat with a non-adjustable backstay. So.... this data is about adjusting RIG tension.

    V=vertical shrouds, D=Diagonal shrouds. 1/2 turns are on the turnbuckles, it allows us to adjust from one relitive position to another without having to break out the gauge.

    As we discussed earlier above, adding backstay tension (to a true fractional boat) will ADD to the amount of forestay tension created by the rig rig tension, with a slight decrease in rig tension.
     


  15. danstanford

    danstanford

    Joined Aug 2, 2010
    206 posts, 29 likes
    Beneteau 323
    CA Cobourg
    Thanks for the information Everybody, I have two follow up questions, one of which was part of the original query.

    Can I, and should I, attempt to create this bend with a furling mast? I have one affirmative in the responses above, anybody else with experience?

    Assuming the answer above is yes, what is the best way to add adjustment? I do run a bimini with the split backstay cables running through it....
    Thanks in advance,
    Dan
     


  16. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    8,426 posts, 1,858 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    Sure you can. The question is, why would you want to? If you have a furling mast and sail, then I'd assume the sail is cut very flat and with very little luff curve. So bending the mast might make your sail shape worse.

    Remember that you don't need to bend the mast to add forestay tension with the rig tune. If you adjust right you can add lots of tension with very little or no addition bend.
     


  17. Scott T-Bird

    Scott T-Bird

    Joined Oct 26, 2008
    3,275 posts, 413 likes
    Starwind 27
    US Barnegat, NJ
    Sailboat specs Beneteau 323 indicate that you have a masthead rig. Is that correct? No to bend, yes to add adjustment for forestay tension.
    Here is how I did mine ... and how it looks in action. I used a backing plate below the deck.
    The new configuration pinches the bimini when in operation. I finally just cut the fabric to show where the new opening needs to be and am having a new bimini made.
     

    Attached Files:



    Last edited: Oct 24, 2017
  18. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    8,426 posts, 1,858 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    Its fractional. 15/16th, so barely.
     


  19. Rick D

    Rick D

    Joined Jun 14, 2008
    6,476 posts, 141 likes
    Hunter Legend 40.5
    US Long Beach, Shoreline Marina, CA
    So, that has always puzzled me. What is the point of a 15/16th rig?
     


  20. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    8,426 posts, 1,858 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    All else being equal, the best fractional is rig is the one that is fractional enough to allow the benefits of the design (mast bending).. the higher it is, the bigger the headsail can be, and the higher the spin head is. But you lose leverage on the forestay to bend the rig. Fract boats started out at 3/4, then went to 7/8ths. For a long time now best is thought to be 9/10s for racy boats. We can bent the mast on BlueJ just fine, but we have a powerful backstay adjuster. You do see higher ratios for some models, most cruisers who are less likely to miss the the full range of trim adjustment.
     


    Last edited: Oct 24, 2017

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