Advanced questions regarding CoE - CLR balance of small boats

Discussion in 'Hydrodynamics and Aerodynamics' started by laukejas, Sep 6, 2017.

  1. Angélique
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    Angélique aka Angel (only by name)

    It sails over my head right now, but maybe it's useful to you . . .

    PDF - The Hydrodynamics of Hull, Keel and Rudder - Chapter 5

    the below quote comes from PDF page 40 (of 44)

    ‘‘ . . . . . . . . The influence of the free surface

    Just as is the case with the induced resistance of a wing there is also an influence of the free surface on the lift generating properties ofa wing. This is represented inthe 'equivalent keel' approach by mirroring the extended keel around the waterline. By doing so the effective aspect ratio of the equivalent keel is more than doubled when compared to the geometrical aspect ratio ofthe actual keel. As soon as the yacht gets heeled however the pressure distribution over the keel is influenced by the presence of the free surface.

    This effect is also influenced by the characteristics of the hull above the keel, i.e. a wide and shallow hull will tend to move the keel closer to the free surface when heeled than a narrow and deep hull. The latter however will also have less of an 'end plate' effect on the keel. . . . . . . . . . . ’’

    P.S. - the above comes from, and there's more of it at . . . .

    Delft University of Technology (TU Delft)

    Last edited: Sep 12, 2017
  2. patzefran
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    patzefran patzefran

    The approximate effect of the water surface on the flow can be approximated using the mirror analogy.
    The flow on the underwater foil is the same as the flow on the real foil associated with a foil wich is its symmetrical
    relating to the water plane. So if the depth of the foil is d, its aspect ratio A corresponds to 2d.
  3. DCockey
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    DCockey Senior Member

    Remember that the simple CLE/CLE balance formulas do not properly model the basic balance of forces and moments of a boat sailing. The sails are not on centerline, and if the boat is heeled both the sails, and the keel/centerboard and rudder move off of centerline. Trying to improve the formulas by improving the physics of individual components of the boat may not improve the overall results without moving to a 3D model of the forces and moments.

  4. patzefran
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    patzefran patzefran

    DCokey , I agree but I answered only to the question about CLR location of 2 combined foils. Heeling on a monohull gives weather helm, due to
    the lateral offset of aero thrust force and hydro drag, on a catamaran lifting a hull it is just the opposite at low heeling angle !
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