Sail forces and sail plan ?

Discussion in 'Sailboats' started by fcfc, May 28, 2009.

  1. fcfc
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    fcfc Senior Member

    Is there books or papers that give practical formulas to get CL CD (or drive/side force coefficient) from a sail plan ?

    I have "Principles of Yacht Design 3rd" , It give some models for sail aerodynamics from G Hazen, but it is around 30 years old.

    For instance, what is not dealt with is fractional rig (3/4 , .. 9/10, masthead), jib overlap (105% , 135%,150%), boom heigh above sheer, presence of a genoa furler etc ...

    Thanks.
     
  2. fcfc
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    fcfc Senior Member

    Have someone read "Sailing yacht design: Theory" from Claughton, 2nd edition 2006 ?

    Are there usable formulaes in this book, for sail plan and rigs ?

    Usable formulae for me are with numerical application and units.

    I have found some papers that speak more or less with what I want, but they just give the general, without giving the value of constants and units. So they are strictly useless from a sail plan design point.

    Thanks.
     
  3. Tad
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    Tad Boat Designer

    fc....

    In Sailing Yacht Design: Theory, Claughton addresses CL and CD in a chapter on Velocity Prediction Programs...but presents the IMS data from Hazen. He does present some basic equations on dealing with sail sets (two or more sails flying together). He also references an equation from Glauert, H., Airfoil and Airscrew Theory, 1959....

    Claughton also references a paper by Larsson, L., Numerical predictions of the flow and resistance components of sailing yachts, 1987.
     
  4. gonzo
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    gonzo Senior Member

    Most formulas for sails are very simplistic. Sails are soft foils which means they are constantly changing. The static models are very rough approximations. Besides the adjustments that can be done to the sail trim and the combinations thereof, the movement of the boat changes the direction of the air flow.
     
  5. PAR
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    PAR Yacht Designer/Builder

    100 sq. ft of sail area is about 2 HP.
     
  6. fcfc
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    fcfc Senior Member

    You can have a very good paper on hull and rig drag there http://heikki.org/publications/ThesisHansen.pdf (equation 3.11 and 3.12)

    My goal was to have a better estimation of drive and side forces.
    For hydrodynamic estimation, with delft papers, you have very good sources, but for aerodynamic estimation, it is much more limited.

    I know a (small) professional builder, who tried to beefed up a known old design (sold in thousands) with marginal performances in stronger winds. The boat is a daggerboard. The idea was to sell an "upgrade" kit. He put kind of bulb, and increased strengh accordingly in centerboard box and rig. Just to find in sea trials that the boat still had weak performances upwind in stronger winds. Acceptable heel and speed, but too much leeway. After investigation, and contacting a naval architect, he just found the keel was stalling. Not enough lateral surface to cope with the sails side force with increased stability.
     
  7. terhohalme
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    terhohalme BEng Boat Technology

  8. fcfc
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    fcfc Senior Member

    I have already read them.

    "The “fine tuning” of
    the aerodynamic coefficients was done analyzing similar figures for the other two boats."

    It explains all, but do not give a single figure for coefficients. So it is not practically usable.

    The result is a mutidimentional surface. Heff is a function of (roach, genoa overlap, fractionality) , Figures 2.11 to 2.13 gives you some "cut" in the surface, but not enough to rebuild the surface. (2.11 2.12 are the same numbers shown differently).
     
  9. jehardiman
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    jehardiman Senior Member

    Have you read Aero-Hydrodynamics of Sailing by Marchaj? It is fairly good discussion on why there is no "F=C*A*V^2" equation for sails and the problems of model to real correlation.
     
  10. terhohalme
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    terhohalme BEng Boat Technology

    What makes you think that there is a simply equation for a complex phenomenom?
     
  11. fcfc
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    fcfc Senior Member

    For Marchaj book, it is not available on amazon. There are only (old) used editions available. Are there differences between 1979 to 2000 editions, or they are just reprints ?

    For sail physics , I agree if equations do exist, they are very complex to solve. Kind of CFD. It is not within my reach.

    What I was looking is more of regression analysis. Which are very basic, even if they have many parameters. For instance, Fossati paper contains an atempt to do a regression analysis of one parameter Effective rig heigh, rather hard to measure, from 3 parameters easy to measure (overlap, roach and fractionnality).

    For Delft series hydrodynamic forces, real physical equations are very complex. But the regression analysis is very simple, with rather accurate predictions in the domain range (at least accurate enough for me) . I was simply hoping something similar for sails.
     
  12. Guest625101138

    Guest625101138 Previous Member

  13. Guest625101138

    Guest625101138 Previous Member

    If you want to make some simple analysis then work on a lift to drag of say 6 for a fractional rig per attached.

    Rick W
     

    Attached Files:

  14. daiquiri
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    daiquiri Engineering and Design

    Beware of using JavaFoil for the analysis of very thin airfoils.

    JavaFoil is based on a vortex panel method with is a part of the so-called "Thin Airfoil Theory".
    Thin Airfoil Theory is known for giving very good results for nearly all types of airfoils (if a correct boundary-layer correction is applied)... except for thin airfoils. :D

    That's because thin airfoils are subject to leading-edge flow separation at any finite angle of attack (AoA). The only case when it doesn't happen is when they are set at the ideal AoA, which is the one you have when fluid streamlines are perfectly parallel to the airfoil at the leading edge.
    Standard boundary-layer corrections (such as those used by JavaFoil) cannot cope with that situation with an acceptable accuracy and therefore are not valid for thin airfoils at a finite AoA.
     

  15. Guest625101138

    Guest625101138 Previous Member

    I have always used tufts on sails to ensure attached flow except when pushing very high trying to make a windward mark. So confident I have attached flow.

    I expect for these conditions the JavaFoil analysis is quite adequate but if you have better analysis please make an offering.

    Rick W
     
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