Cut down sail

Discussion in 'Hydrodynamics and Aerodynamics' started by revintage, Feb 22, 2019.

  1. revintage
    Joined: Nov 2016
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    Location: Sweden

    revintage Senior Member

    As the A-class sail I have will give to high CoE for my trimaran foiler, I am cutting it down and adding a small jib I already have, to compensate for the lost mainsail area.

    The cut can be from the bottom or from the top. I have done course calculations of CoE and aspect ratio. Made the sketch quite simple by drawing the luff vertical.

    My idea is to cut from the bottom as this would give a more effective high speed combination. I am also reluctant to cut from the top, as one might need to try out a new the luff curve. The cloth is also lightly damaged/repaired in the lowest panel.

    Would really appreciate any recommendations and thoughts about the aerodynamics.

    SEGELCOE.png
     

    Attached Files:

    Last edited: Feb 22, 2019
  2. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    Revintage

    Looking at your pic on a non-high resulution screen, it appears to be an applied yarn laminate or 3RD type construction. If so don't cut it down. You will be removing all of its horizontal strength. There must be yarns or tapes running along the load paths from tack to clue. Also, the load paths from the CE to the purpose new tack and clew will not be the same as the ones the sail was originally designed to withstand. I predicte that it will rip vertically from any either altered end in short order. Altering a laminated sail is extremely difficult.

    If I am wrong and it is constructed from woven sail cloth, then alteration is straight forward. Remove and reinstall the corner patches. However without assistance from a sail maker you will lickly lose optimal foot curvature. It will stay together but have slightly less performance.

    Good luck
     
  3. revintage
    Joined: Nov 2016
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    revintage Senior Member

    Hi Blueknarr,

    Thanks for the advices. Decided to go for cutting from the bottom, as I want to keep as high aspect ratio as possible. The top cut also looked so ugly;).

    The sail is sewn with horizontal panels with heavily glued and sewn joints under each batten pocket. Will reuse the old big patches, dump the window and use the same foot curve.

    Can´t see why it would rip as the vertical load lines only differ marginally. Maybe the top cut would be worse in that respect.

    Visited the nearby sail loft before digging into it. Doing all cutting and taping myself before handing over to the sail loft to add seams and tack&clew eyelets.

    SEGEL.png
     
  4. patzefran
    Joined: Feb 2011
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    Location: france

    patzefran patzefran

    I have recut two A class sails for my Strike 15 Tri wich use a shortened A cat carbon mast (1 m removed at the top).
    One is laminate, square top and was recut at the top, the other is dacron very old style an recut at the bottom . Both sail / mast behavior is good, the old style is less powerful
    which is better for the boat .
     
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  5. revintage
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    revintage Senior Member

    Hey Patzefran,
    As I leave the top of the sail uncut, I will also cut the mast from the bottom, to keep the upper parts together.
     
  6. patzefran
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    patzefran patzefran

    I should not do that to the mast !
    IMHO , cut the top of the mast , easier for a sailmaker to recut the luff of the sail.
     
  7. revintage
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    revintage Senior Member

    Hi Patzefran,

    I see it another way. If both the top of the mast cut and the top of of the sail are left uncut no experimenting with recutting the luff will be needed.

    If cutting the bottom end where the mast is stiffer, adjustments to suit the lower part of the sail can be done by adjusting the prebend with the diamonds.

    As I see it the A-cat sail and mast are done with a long top to make the rig self adjusting, don´t want to change that.
     
    Last edited: Feb 24, 2019
  8. patzefran
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    patzefran patzefran

    IMO, If you want the same self adjusting of the rig as A class cat, you must maitain the same ratio between the upper and lower panel. So you have two options :
    cut the mast at the top and the bottom
    or cut at the bottom and put the cap shroud higher.
    Any way, if you cut at the bottom, you have to :
    change the location of the diamond spreaders
    change the location of the lower diamond tangs
    change the location of the gooseneck
    change the location of the groove to insert the sail
    change the intern
     
  9. patzefran
    Joined: Feb 2011
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    patzefran patzefran

    sorry, following :
    internal cunningham and the holes for the cunningham lines.
    All in a carbon mast , I assume
    change the location of the rotation arm.
    I don't think the gain is worth !
     

  10. revintage
    Joined: Nov 2016
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    Location: Sweden

    revintage Senior Member

    No prob, no holes in or fittings on the mast tube yet.
     
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