Windsurf raked sail

Discussion in 'Hydrodynamics and Aerodynamics' started by Grunf, Nov 22, 2020.

  1. Grunf
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    Grunf Junior Member

    Why windsurf use raked sail,if upright sail has higher lift coefficient and it is more efficient?

    So much rake basicly kill sail power..

    Why they dont design mast track more back and increased sail foot to close the gap and use upright sails to gain more efficency and speed?
    Now sail will be upright and sailor will still be back at the tail as much as possible to reduce watted area..
    Did anybody try that yet?


    upload_2020-11-22_7-4-11.png
     
  2. patzefran
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    patzefran patzefran

    Mast rake is necessary when sailing fast, planing on the aft of the board, where is the under wing, to have the
    center of presure of the sail above the center of lateral pressure of the appendice ! this close the gap also !
     
  3. Grunf
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    Grunf Junior Member

    You also can do this with upright sail if you design mast track more back and increase sail foot ..
    So I still dont understend why they use this raked concept
    Do you agree?
     
  4. Tiny Turnip
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    Tiny Turnip Senior Member

    Windsurfers are a dynamic system, channelling the lift from the sail against the lift from the foil, through the riders body. This means they can react to different combinations of conditions very dynamically and quickly. The rider has to move the sail, their own body weight, the angle of the board, the fore and aft trim, depending on the point of sail, water conditons, variation in the wind direction etc. etc, altering the relationship between the various forces. Their simplicity, and the almost instantaneous control possible makes them a very robust system, which contributes to them being able to consistently go very fast.

    If you design a vessel to go absolutely as fast as possible, you end up with Vestas Sailrocket, which is fine, but it doesn't function in any conditions other than the very specific set of circumstances it was designed for.
     
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  5. Grunf
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    Grunf Junior Member

    But this not answer my question
     
  6. Tiny Turnip
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    Tiny Turnip Senior Member

    Yes it does. In order to get to the position where a windsurfer is in its fastest condition it has to negotiate a number of states and conditions for which a rig as you describe is not optimal. If the rider is prepared to be towed into positon perhaps, as Sailrocket was, then fine. The flexible, curved shape of the rig helps it perform in these different conditions.
     
  7. Grunf
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    Grunf Junior Member

    Why my sail orientation is not good? Rider is not moving much ,his postitin is fixed in footstraps
     
  8. patzefran
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    patzefran patzefran

    For me Tiny Turnip answer is right ! prior to reach the high speed equilbrium trim, windsurfer need to start from a stable, controlable configuration. I suspect current configuration results from a long selection process toward an optimum all round performance.
    If you don't understand his answer, you should test your own concept !
     
  9. Grunf
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    Grunf Junior Member

    To test my concept I must make new mast track 25,30cm back at board deck and increase sail foot area to close gap with new sail position...
    Then some profesional sailor must go to luderitz channel to see if he can gain more speed due to better sail efficiency...

    You see how foil sail stay upright.
    If rake sail, you are slow.

     
    Last edited: Nov 22, 2020
  10. Eric Lundy
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    Eric Lundy Junior Member

    The only amount of RM the windsurfer has is body weight. If you are suggesting creating more power from the sail how do you suggest to counteract that? Fatter windsurfers?
     
  11. Grunf
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    Grunf Junior Member

    with upright sail you can have smaller sail with same lift/"power".so you reduce aero drag and go faster..

    Also smaller sail has lower center of pressure so you can hold even more power,because lever arm is shorter

    key is that upright sail has better L/D ratio then raked sail
     
  12. Eric Lundy
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    Eric Lundy Junior Member

    You have it backwards. a skinnier more upright sail would move the center of pressure up. not down. if you make the sail area smaller to compensate and reduce drag then by all means create a smaller and more efficient windsurfing sail. Mast rake is needed and occurs like the above posters mentioned I would add that mast rake occurs more with heavier wind conditions and smaller sails to find equilibrium.
     
  13. Grunf
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    Grunf Junior Member

    So concluson is that upright concept wouldnt work at wsurf,so people from windsurfong industry know baiscs aerodynamics?
     
  14. messabout
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    messabout Senior Member

    Grunf. when the sail is raked in a athwartships position other than perpendicular to the water surface, the sail generates some vertical lift. Thus the board behaves as if it had less weight applied to its bottom. I have seen extreme tilted sails in heavy air that lifted the entire boat out of the water. Consider the sails (wings) of an ultralight aircraft. Obviously they generate lift.

    The difference in sail efficiency, to instantaneous displacement may be more important than the gain in sail efficiency offered by closing the end gap. You can be sure that the serious board sailors have this stuff figured out with a great deal of precision. Even if not attacked from a technical viewpoint, the board sailors definitely know what is fastest.

    The last argument is that the foot of the board sail must have some space at the foot in order to tilt the sail fore or aft so as to steer the board.
     

  15. Grunf
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    Grunf Junior Member

    Isnt sail raked to windward produce some vertical lift,not raked back?

    Why do you mean that raked back produce vertical lift?
     
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