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Increasing backstay tension flattens main?

Discussion in 'Sail Trim with Don Guillette' started by danstanford, Jun 22, 2017. Add this thread to a FAQ

  1. danstanford

    danstanford

    Joined Aug 2, 2010
    133 posts, 3 likes
    Beneteau 323
    CA Cobourg
    I crewed on a boat last night with a rather baggy main and thought about this notion I have read that increasing backstay tension will flatten the main...which this sail needed! Thinking about the geometry, I cannot see how bending the mast will flatten the sail, my imagination says the opposite will happen. Can someone help me understand?
    My boat has no easy adjustment for the split backstay so this is mostly theoretical, and very interesting for me!
    Thanks,

    Dan
     


  2. justsomeguy

    justsomeguy

    Joined Feb 20, 2011
    5,342 posts, 429 likes
    MacGregor, Island Packet 35
    US Tucson, AZ/San Carlos, MX
    Bending the mast takes some or all of the "belly" out of the mainsail, depending on how much belly it has.
    Last night's mainsail could have been past its prime...
     


  3. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    6,873 posts, 1,010 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    You need to think of the mainsail in three sections.

    The outhaul can flatten the bottom 1/3, by pulling it tight.

    On a fractional boat, putting on backstay can bend the mast, pulling the middle 1/3 of the sail flatter.

    The top 1/3 will not change shape much, but that's normal and OK, it twists off.
     


  4. kloudie1

    kloudie1

    Joined Nov 6, 2006
    7,206 posts, 264 likes
    Hunter 34
    US Mandeville Louisiana
    but, like JackDaw says.. fractional rig is what that works on.
     


  5. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    6,873 posts, 1,010 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    Performance oriented masthead boats (like C&C) can also achieve this mast bend with a babystay. Lots of these boats have babystays with 8:1 purchase in place of turnbuckles to bend the mast.
     


  6. danstanford

    danstanford

    Joined Aug 2, 2010
    133 posts, 3 likes
    Beneteau 323
    CA Cobourg
    OK, so the bend in the middle of the mast stretches the middle of the sail toward the bow and thus flattening the sail. Now my geometric mind sees it.
    So this was a reasonable looking sail and I had the telltales flying in light breezes but I couldn't get rid of the luff in the first quarter of the main. Increasing back stay tension would have helped, though I do understand this older sail may have been past its prime too.

    Thanks you Guys! I am going to start racing my own boat next week so please answer the coming questions as I work through sailing better!
    Dan
     


  7. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    6,873 posts, 1,010 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    Yep.
    2370e.jpg
     


  8. Justin_NSA

    Justin_NSA

    Joined Jul 7, 2004
    3,692 posts, 305 likes
    Hunter 30T
    US Wichita, KS
    I think there's a chance of over tightening the backstay and causing problems too.

    Some masts have prebend built in. Are sails made specifically for these? I don't see how this technique would work for them.
     


  9. Davidasailor26

    Davidasailor26

    Joined May 17, 2004
    799 posts, 79 likes
    Beneteau Oceanis 37 LE
    US Havre de Grace
    The sail should be designed for the static pre-bend, but adding more bend can still be done dynamically to flatten when conditions warrant.
     


  10. jon hansen

    jon hansen

    Joined May 25, 2012
    442 posts, 111 likes
    john alden caravelle 42
    us toledo ohio-----sturgeon bay wis sturgeon bay, wis
    Dan, the leading edge of a mainsail is cut in an arc. then when the arc shape is attached to a straight mast your sail gets its fullness,

    to flaten the fullness in higher winds you arc your mast to the shape of the cut in the leading edge to flatten your sail.

    there are five ways to arc a mast: 1. pull down hard on the mainsheet, lasers and finn sailors do this
    2. the use of a very powerfull vang
    3. force the lower section of the mast forward

    4. fractional rigs tighten the back stay against the tension of the forstay

    5. use swept back spreaders and increase the tension there
     


    Last edited: Jun 22, 2017
  11. shemandr

    shemandr

    Joined Jan 1, 2006
    2,789 posts, 176 likes
    Marblehead Skiff 14'
    US Greenport, NY Greenport, NY
    Over tightening the backstay can bend the boat. Beyond the point where the boat bends there is no more positive effect. I used to have a Mark 25 which is a C&C 25 at heart with a tall spindly fractionally rigged mast with jumper struts. The power to add and subtract power so easily with a adjustment of the backstay was amazing. When I originally got that boat I worried about a rig failure. I learned that the rig was fine but the hull would bend from the rig. Overall, it was a good sailing boat if you didn't mind the oil canning.
     


  12. jon hansen

    jon hansen

    Joined May 25, 2012
    442 posts, 111 likes
    john alden caravelle 42
    us toledo ohio-----sturgeon bay wis sturgeon bay, wis
    on my star and on my A scow the base of the mast at the bottom of the hull is movable to control rake / weather helm and some arc maybe.
     


  13. jon hansen

    jon hansen

    Joined May 25, 2012
    442 posts, 111 likes
    john alden caravelle 42
    us toledo ohio-----sturgeon bay wis sturgeon bay, wis
    Dan, on a masthead rig, tightening the backstay is done to straighten the leading edge of the jib
     


    Last edited: Jun 22, 2017
    Gene Neill likes this.
  14. Don Guillette

    Don Guillette

    Joined May 17, 2004
    1,849 posts, 28 likes
    Other Catalina 30
    US Tucson, AZ
    jackdaw's picture tells the story. Suppose you want to describe it to your dock neighbor or a person at the YC bar and you don't have jackdaw's picture with you. Here's how you do it -- use your index finger to show a straight mast. Now bend it and tell your friend to notice how the middle of your finger (knuckle area) pulls the fullness out of the middle of the sail. If your friend has a "telephone pole" (non bendy mast) tell him to forget the demo because the mast won't bend -- it will rake and the boat will think he picked up the mast and moved it aft.

    Then your friend might ask to illustrate TWIST. Form your right hand as if you were going to salute. Next, twist your only fingers to the right and explain you're reducing pressure on the boom vang. That illustrates how air is spilled out of the top 1/3 of the sail. Spilling air reduces the power of the sail. Then explain your cranking on the boom vang as you turn your fingers into a salute to illustrate you're now inducing power back into the sail so that is drawing from top to bottom. .
     


  15. WhiteNow

    WhiteNow

    Joined Jan 2, 2015
    35 posts, 0 likes
    Beneteau 340 Oceania
    US ventura
    the rigging on my Bene 34 was quite loose. if you shook shrouds the rig really bounced around forestay was quite loose too. We tightened the all shrouds and back stays. Now I'm thinking that mast is too straight as the in mast furling is jamming like ever before at the vert. batons. Should have consulted Bene before adjusting. But I think we took to much of the prebend out
     


  16. NotCook

    NotCook

    Joined Dec 29, 2008
    436 posts, 46 likes
    Treworgy 65' Custom Steel Pilothouse Staysail Ketch
    US St. Croix, Virgin Islands
    There was another thread last week or so that gave me the opposite impression. The discussion was about in-mast furling jamming, and what I took away from it was that the mast might have been bent, causing the furling to bind. The impression I came away with was that bending the mast was, at least to some degree, incompatible with in mast furling. I have a friend whose Jeanneau 37 was having problems furling and unfurling is old and new sails in-mast, and I was planning to site up his mast to see if there was any bend in it. I'd welcome clarification from the experts on here.
     


  17. Davidasailor26

    Davidasailor26

    Joined May 17, 2004
    799 posts, 79 likes
    Beneteau Oceanis 37 LE
    US Havre de Grace
    The manual for our US Spar furling mast says "The mast must be as straight as possible (only 3" of pre-bend)". Sighting up ours I'd guess it's about 5" and we've never had any trouble.
     


  18. DayDreamer41

    DayDreamer41

    Joined Oct 29, 2016
    365 posts, 103 likes
    Hunter 41 DS
    Un Michigan Port Huron
    This is an excerpt from the Selden B&R rig tuning manual
    The amount of a pre-bend depends on the cut of the
    mainsail. A general rule is that the pre-bend should be
    1% of the distance from the masthead to the gooseneck,
    measured at the centre of this distance. The mast must
    have no lateral bend. All pre-bend must be in the fore-
    and-aft direction
     


  19. Parsons

    Parsons

    Joined Jul 12, 2011
    351 posts, 65 likes
    Catalina 36
    US Bay City, MI
    Off topic -- I love reading these sail trim questions with you guys! Thanks to each of the contributors for explaining this in simple, understandable terms. Even though I understand this, it is a pleasure to read each of your explanations.
     



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