Optimum sail draft for wind speed

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Jul 14, 2021.

  1. dustman
    Joined: Jun 2019
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    Location: Tucson, AZ

    dustman Senior Member

    If I had to choose a specific sail draft for all wind speeds up 20 knots, what should I choose?
     
  2. tlouth7
    Joined: Jun 2013
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    tlouth7 Senior Member

    This depends entirely on the nature of your boat. If you plan to sail at several times the windspeed you would want very little draft, if you need to slowly push a barge then you need lots of lift and so would put up with the drag of a high draft sail.
     
  3. gonzo
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    gonzo Senior Member

    Are you asking about the cut of the sail or the shape created when you trim the sails?
    The first will depend on the boat and rig. The second is adjustable and you change it continuously.
     
  4. dustman
    Joined: Jun 2019
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    dustman Senior Member

    I am pondering a sail design that would be very easy to deal with but it would require choosing a specific draft, as the sail would be contained in shaped battens on both sides, and the only adjustment possible would be vertical tension. I suppose it could be considered a cross between a junk rig and gaff rig. According to wikipedia you want 13-16% percent chord for draft in under 8 knot wind, 11-13% between 8-15 kts, 9-12% for greater than 15 kts. It seems like you would want to optimize for light wind but my concern would be stalling at higher wind speeds.
     
  5. patzefran
    Joined: Feb 2011
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    patzefran patzefran

    Concern with stalling is at lower wind speed, not higher !!!!! with little wind you need less draft to avoid stalling
     
  6. dustman
    Joined: Jun 2019
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    dustman Senior Member

    I was speaking of flow separation with a large amount of camber at high wind speed.
     
  7. CT249
    Joined: May 2003
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    CT249 Senior Member

    For upwind performance, both gaff and junk give away so much performance that worrying about draft efficiency seems a bit irrelevant.
     
  8. dustman
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    dustman Senior Member

    Hence why I'm trying to combine the good aspects of both and have good shape at the same time. As far as I'm aware the primary reason junks don't have good upwind performance IS the lack of of good shape, or draft.
     
  9. gonzo
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    gonzo Senior Member

    The main reason is the amount of drag/turbulence created by all the top hamper , lines, battens, etc.
     
  10. Will Gilmore
    Joined: Aug 2017
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    Location: Littleton, nh

    Will Gilmore Senior Member

    Are you choosing the shaped battens design because that's the sail design idea you have or is your idea to design something that is "very easy to deal with"?
    How and where you want to use this boat will, as tlouth7 wrote,
    Make all the difference to what shape you use the most.

    If I were designing an AC competitor and were restricted to only a halyard for sail control, I'd cut the sail as large as possible with a high aspect ratio and the camber very flat for close reaching on a plane. I'd make her a fractional Bermuda rig so the jib would aid in flattening the sail in higher winds by bending the mast.

    If I were designing a fishing scow that needed to drag nets, she'd have a lot of belly in her large sails with low aspect ratio and full battens.

    If designing a cruising boat that was meant to be comfortable, I'd do a smaller sail, but similar to the AC design.

    Sailing in the trades, lots of belly for DDW sailing.

    Sailing on a lake with squirrelly winds, something in between but short.

    How and where will this boat be used?
     
  11. dustman
    Joined: Jun 2019
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    dustman Senior Member

    I laughed out loud because your questions were so good. It would be for cruising, mainly in the trades. But I want to maintain the ability to go upwind decently if necessary. My extensive reading and watching of sailing content leads me to believe that simplicity and dependability of sail handling is very important for safety and effectiveness when single handed. My goal is to design a sail that will be as efficient as reasonable without sacrificing ease of operation and reliability. Pie in the sky I know, but I believe this sail design would be a good compromise. I want something that will just work in all conditions.
     
  12. dustman
    Joined: Jun 2019
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    dustman Senior Member

    The battens would be carefully designed to be as aerodynamic as possible. They would deviate very little from the horizontal plane so wouldn't have an angle in relation to the airflow like the gaff on many gaff rigs.
     
  13. gonzo
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    gonzo Senior Member

    The airflow on a sail is not parallel to the boom. It does not follow the battens. There are plenty of images online of sails being tested with smoke or telltales to show the direction of flow.
     

  14. AJB
    Joined: Jul 2021
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    Location: 31 42S 152 04 E

    AJB New Member

    Two well proven numbers for a main with a jib in front - half height depths:

    Jib: 14.5 % @ 32 % of chord
    Main: 11 - 12% at 45% of chord
     
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