Drivers for the "sqare top" width?

Discussion in 'Hydrodynamics and Aerodynamics' started by jmf11, Dec 23, 2021.

  1. jmf11
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    jmf11 Junior Member

    Catamarans, trimarans, landyacht, windsurf have square top sails. I wonder how can be determined the optimum sail square head width. Especially in a simple case like A-Cat, with only a mainsail, no backstay. Some have quite large square head. Some have much smaller

    Is the driver for determining the optimum width? What are the "inputs" to determine this ? And the consequences of setting value out of the optimized domain?

    Are the key drivers the overall aerodynamics of the sail (sail plan like in a wing design)?

    Or is it for the dynamic behaviour of the rig, to depower in gusts?

    I'm really curious about what drives that specific parameter in rigs design.
     
  2. wet feet
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    wet feet Senior Member

    In my limited experience its a combination of what the relevant rules dictate as the mast maximum height and the amount of sail it is possible to support by clever design and placement of battens.
     
  3. Erwan
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    Erwan Senior Member

    In CFD Handbooks the optimum taper ratio (tipchord/footchord) which minimize induced drag is around 40% depends on the AR.
    For an A-Cat, the sail area at the top helps to lift the windward hull downwind, when sailing the "Wild Thing"
    But if you prefer to gain windward you will make the top more pointy
    Merry Xmas
     
  4. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    I suggest reading CA Marchaj's Aerohydrodynamics of Sailing for a comprehensive description of the fluid dynamic forces that govern the shape of a wing/sail/foil. Coles Notes: lift distribution on a body will take an elliptical distribution. If the body is too pointy, lift becomes too high and the tip stalls. If the end is too square, it still only makes eliptical lift, so then the end is extremely underloaded, and makes too much drag for the lift it provides. A trapezoid form where the tip is 40-60% of the root cord is close enough to the ideal elliptical form to work just fine.
     
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  5. jmf11
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    jmf11 Junior Member

    Thanks Erwan and Dogcavalry four your answers. So this is about limiting the drag and lift distribution. I have looked at Tom Speers documents on the optimal planform, which is elliptical.

    I definitely need to put Marchaj's book on my read list. I have already read the French reference one from Bertrand Cherret, but have to go to the more complete one.

    Erwan, when you state the "CFD handbooks" and the reference to the AR of the rig, do you have a reference in mind? Some academic papers on the topic?

    Happy Christmas,

    JMF
     
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  6. Erwan
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    Erwan Senior Member

    Yes , Elliptical lift distribution is a special case of the optimum lift distribution with constant (hence linear downwash)
    the general case is linear downwash but not always constant, just linear.
    To investigate further have a look at T.Jones' workpaper.

    Cheers
     

    Attached Files:

  7. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Well, the general case is only linear downwash, if you consider foils of infinite length to be the general case.
     
  8. DCockey
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    DCockey Senior Member

    "Optimum" depends on the metrics and constraints. Elliptical lift distribution results in minimum induced drag for a given span with no constraint on root bending moment / heeling moment. If the root bending moment / heeling moment is constrained but span is not then then ellipetical lift distribution does not result in minimum induced drag.
     
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  9. jmf11
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    jmf11 Junior Member

    Thanks all for the provided information,

    So the Square Top results from:
    - the optimization of the sail planform for some criteria,
    - the trapeze being an acceptable approximation of the above planform.

    For a landyacht/sailboat, the eliptical load (to minimize drag) is not adequate. Optimization criteria is more about maximizing the lift for a constrainded heeling moment. This seems to be what is developped in Minimum Induced Drag of Sail Rigs and Hydrofoils http://www.tspeer.com/Planforms/Planar.htm. I now have to understand those concepts and conclusions, and play with the associated Excel sheet.

    Marchaj books does not look easily available anymore. Do you know whar are the respective scopes of "Aerohydrodynamics of Sailing" and "Sail Performance: Techniques to Maximise Sail Power" and which one would be the more usefull for a (hobbyist) sail designer (sail design oriented) ?

    Edit: by the way, any advice about "Aero-hydrodynamics and the Performance of Sailing Yachts: The Science Behind Sailing Yachts and..." from F. Fossati (and interest compared to Marchaj books)?

    Best regards,

    JMF
     
  10. Erwan
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    Erwan Senior Member

    JMF11,
    I think you make it, the seminal workpaper you mentionned+VORTEX Excel downwash calculation speadsheet is imho a good step forward as you will "touch" the downwash concept , and you will have very good ideas of the magnitude of Induced Drag which is not obvious for rookies.
    The next step could be VLM and its froggy* application XFLR5 which is not DOS environment, but Windows, so a bit more user friendly for rookies, (*Handbook exists in French) all that for free on sourcesforges.org.
    Welcome to the CFD rookies' Club.
    Bonne continuation
    EK
     
  11. jmf11
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    jmf11 Junior Member

    One sentence I picked about the "Square top" in Tom Speer text (from Minimum Induced Drag of Sail Rigs and Hydrofoils http://www.tspeer.com/Planforms/Planar.htm):

    "By definition, the basic and additional lift distributions assume that the twist is not a function of angle of attack. However, if the twist is varied with the angle of attack, as shown in equation X, then the center of effort and the aerodynamic center will coincide once again, and the drag reductions shown in Figure 20 can be realized even when the gust response is considered. This is basically what is happening with modern fat-headed sails in which the head of the sail is allowed to twist off under increased loading."

    But no hints about how to adjust this twisting in gusts to the sail static design. This is another driver for the square top.

    JMF
     
  12. Erwan
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    Erwan Senior Member

    My first square top main sail from Goodall bought in 1994 was a great one like all stuff from AHPC.
    I remember, rigging the boat, putting tension in the main pulley block with the boat horizontal on the beach, and it was fun to see how much I could twist the whole sail, just with 1 finger pushing just a little bit on the corner of the square top.

    JMF11 ,Tu peux me joindre par MP si tu veux des photocopies du Marchaj.

    Fair wind

    EK
     
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  13. Erwan
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    Erwan Senior Member

    Hi , Please find below a workpaper link which address more or less similar issue.


    eshttps://www.ijert.org/cfd-simulation-of-different-taper-ratio-wings-performing-trade-off-assessment-and-development-of-a-new-methodology-to-plot-lift-distribution-curve-and-3d-local-coefficient-of-lift-distribution-graph https://www.ijert.org/research/cfd-...f-lift-distribution-graph-IJERTV9IS040471.pdf Tapered wing plan form dominates the wing design mainly because of its manufacturability, low induced drag, fewer tip vortices and an increase in the lift which accounts in the increase in the overall efficiency of the Wing. In this paper Selig's S1223 aerofoil was used and wings with various different Taper ratios have been designed and developed in CATIA V5 and were analysed in the ANSYS FLUENT 16.0 and MATLAB software. The new methodology to plot the lift distribution curve and the Local coefficient of Lift distribution curve was developed and tested and a database of Lift Distribution graph and local coefficient of lift distribution graph was generated. A detailed trade study was performed on the basis of lift distribution, the local coefficient of lift distribution and structural strength..
     
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  14. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    There is an important distinction to be made between maximum force, and minimum induced drag.
    Maximum force is fine for a wallowing manatee of a boat, like a classic chinese junk. The l/d of the hull is low enough that it will never move fast enough to shift the apparent wind much at all.
    At the opposite end of that scale are ice boats and land yachts, where lift to drag ratio is the paramount attribute of the rig, beyond all others. They move so quickly that apparent wind is many times greater than actual wind, so l/d ratio imposes a very harsh limit on speed.
     
    Last edited: Feb 16, 2022

  15. Steve Clark
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    Steve Clark Charged Particle

    All this about plan form misses the single most important thing about roach on full battened sails.
    The load path is from the head to the clew. All sail area aft of that line is supported by the battens working as cantilevers.
    The more area, the more the sail will twist under load. In the bad old days when sailcloth was stretchy, you could only get away with x amount of roach. And you tuned the gust response by design of the platform, the stretch of the sail cloth and the stiffness of the mast and battens. As sailcloth got better, it was possible, and necessary to increase the area behind the load path to maintain proper gust response.
    So I see the evolution of square heads as a tuning and handling refinement of well understood mainsail mechanics and not as an aerodynamic innovation. There is an advantage to getting more area higher in the boundary layer, but all that is buried by the de powering dynamics.
    SHC
     
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