Why aren't sailboat hulls designed as airfoils?

Discussion in 'Sailboats' started by kerinin, Jun 12, 2009.

  1. Charles Burgess
    Joined: Jun 2009
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    Charles Burgess Naval Architect

    In answer to your original question...

    First of all my answer are based upon techniques used in wind tunnels and tow tanks.

    Second, given your aeronautical background, the batwing shape of the B-1 bomber can illustrate how a sailboat sails upon the density interface where water and air join.

    Basically, a sailboat flies a lot like an airplane: take a scale plastic model of a B-1 bomber you can get at any hobby store, and dip one wing in water up to the fuselage center-line. The fundamental difference between the wing in the water and that in the air is density of the fluids - otherwise all the factors are nearly the same. It is the interface of the density change that is the most interesting.

    If you had made the model B-1 with some clay in one wing for ballast and glued it so that no water can enter, you could put the model in a pool (or your bathtub) and blow on the wing that is in the air, and the B-1 would sail. If you compare the whole assembly as an airfoil (which the wing and body of the B-1 is), and compare it with the sailboats of the first half of the 20th century you'd find a lot of similarities ;)

    Because the wing of a sailboat that is in the water is in a fluid of greater density than the wing that is in the air, thus the keel needs less area than the sail to equalize the pressures. Thus, the sailboat hull is an airfoil.

    [edited to add: also do the same with a model of the X-15 airplane, and you'll gain some insight into the latest performance sail hull designs, the good and the bad.]
     
    Last edited: Jul 6, 2009
  2. markdrela
    Joined: Jun 2004
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    markdrela Senior Member

    Where did you find this quote? If it's describing Xfoil, then it's 90% wrong.
     
  3. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

  4. Paul B

    Paul B Previous Member

    Except the lift of either the "sail" or the "keel" would be the wrong way around.
     
  5. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    =================
    Wrong-assuming the foil is symetrical the lift in the water and in the air would be identical to the way it is on a "normal" sailboat.
     
  6. Paul B

    Paul B Previous Member

    You continue to be an idiot. Does the B1 have symmetrical lifting surfaces?
     
  7. Doug Lord
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    Doug Lord Flight Ready

    =============
    For the purposes of Mr. Burgess's illustration you are dead wrong.
     
  8. Paul B

    Paul B Previous Member

    You never tire of being the biggest fool on internet sailing forums.

    In fact, if anyone did perform the experiment noted by Mr. Burgess they would find that blowing air on the model from an "upwind" angle the model would simply blow backward due to the Cd/Cl.

    I'm not sure if Mr. Burgess even understands what the B1 is. His reference to "batwing" seems to refer to the B2, not the B1. Note the B1 and B2 illustrations below.


    Since we are talking about models, maybe you would like to share all your breakthrough designs in the One Meter and Marblehead Class R/C racers? Those are the benchmark classes for design in the community.
     

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  9. tippo747
    Joined: Jul 2009
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    Location: perth australia

    tippo747 New Member

    if the airofoil is the perfect shape...........i think they reversed the airofoil......to give room......so you can operate the ropes....and stuff....
     
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