Drag Coefficient versus Size

Discussion in 'Hydrodynamics and Aerodynamics' started by Inquisitor, Sep 15, 2020.

  1. khaos
    Joined: Feb 2013
    Posts: 74
    Likes: 2, Points: 8, Legacy Rep: 26
    Location: Virginia, USA

    khaos Infusion Padawan

    So I am barely even qualified to have an opinion and hesitate to open my mouth. But, I might learn something. So, in this academic callisthenic and in the context of flat vs cupped wouldn't the cupped create more speed through push and pull? Based on the fact that the air would be moving faster and cleaner across the leeward side of the sail with less buffeting and possibly a bit of forward lift pulling or at least mitigating some drag? :) This could all be basic sailing knowledge that I don't have. When I mated on 12 meter catamarans we would always pull the battens as tight as we could to create the curved foil.

    Forgive my ignorance if its too much. :)
     
  2. Tedd McHenry
    Joined: Feb 2020
    Posts: 12
    Likes: 1, Points: 3
    Location: Surrey, BC, Canada

    Tedd McHenry Junior Member

    @khaos :

    I'm not sure exactly what you're describing, but it's important to distinguish between sail lift and sail drag. Lift is force generated perpendicular to the bulk wind direction and drag is force parallel to it (i.e., force pointing downwind). When close hauled or in beam reach the driving force is lift and the sail is acting like an airplane wing. In those situations, it's important to keep the flow smooth over the lee side of the sail because if the flow separates (becomes turbulent) you lose lift and gain drag--the exact opposite of what you want. But when running downwind you're driven purely by drag. You don't want to smooth out the flow because smooth flow creates less drag, and aerodynamic drag is what's moving you through the water.
    So, on a downwind run, if tightening the jib or genoa increases area and creates more turbulence that's a double win.
     

  3. Will Gilmore
    Joined: Aug 2017
    Posts: 529
    Likes: 224, Points: 43
    Location: Littleton, nh

    Will Gilmore Senior Member

    The shape of the curve is more important than the depth of the camber. If the curve of the sail foil is too deep, the shape moves too far aft. This means the lift-force increases, but adds more to the heeling force than to the driving force. In that case, flattening the sail will allow for a better angle of attack that gives some forward drive. Of course, that's when on a reach. If sailing DDW, a nice big fat belly is best.
     
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