New Catamaran Foilers

Discussion in 'Multihulls' started by Doug Lord, Nov 18, 2014.

  1. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    No, it is not wrong!
     
  2. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Well, I'd like you to explain why it is not wrong, if you're so kind.
    To be consistent with myself, I will explain why I believe that it is wrong. The first thing that should be explained is with respect to which point or axis we must take the moments of the forces. Without that, to speak of righting or heeling moments does not make any sense and, therefore, the "moment arm" that you point out does not mean anything.
    Thank you, once again, for the patience in answering my questions.
     
  3. sigurd
    Joined: Jun 2004
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    sigurd Pompuous Pangolin

    Maybe it can. I believe the main idea with the L foil is that when the vertical part comes more out of the water, the boat skids more to leeward. That leeway reduces the angle of attack of the bottom of the L, causing the boat to fall back in. Call it mechanism 1). I'm not sure it works all that well, because you often see these boats running the foil very close to the surface, which would be mechanism 2) - lift decreases when close to the surface, but drag increases. Infact the wingtips are very often breaching the surface, which plain reduces the projected horizontal foil area - mechanism 3) is also draggy.
    I don't see why you couldn't do the same with a T, except that the lee side of the T could not use all the three mechanisms to the same extent as the ww side of it could.
    Here is a quick CFD to show the basic premise.
    L foil.jpg
     
  4. Doug Lord
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    Doug Lord Flight Ready

    =============================
    See this(post 2657)-I hope it will help: High Performance MPX Foil/Self-righting Trimaran-The Test Model https://www.boatdesign.net/threads/high-performance-mpx-foil-self-righting-trimaran-the-test-model.36058/page-178#post-818508
     
  5. Doug Lord
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    Doug Lord Flight Ready

    =============================
    T-foils that are canted are less draggy than those that are vertical underway. Vertical t-foils used as mainfoils tend to have high and low pressure on the same (lee side) side of the foil (daggerboard + lifting foil). If the foil is canted that problem is largely eliminated. A good example of this is the Vampire cat which flys with a single canted, wand controlled t-foil and has proven at least as fast and recently faster than foilers using a single uptip foil.
    The principle is that the cant angle unloads the daggerboard in much the same way the Moth daggerboard is unloaded with Veal Heel. The main foil provides both lift and lateral resistance.


    photo by Tim Bees-- note the cant of the lee t-foil:
    Vampire2015b- Tim Bees.jpg

    photo from catsailingnews:
    Vampire-Euro cat 2017-photo yachtclub carnac-catsailing news.jpg
     
  6. Doug Lord
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    Doug Lord Flight Ready

    ==================================================
    Sigurd, if you haven't read this already you might find it interesting-it's Tom Speer explaining how an uptip foil works:

    The curved part of the vertical foil produces essentially the same lift as it rises. This is necessary to counter the side force from the sail rig, which does not change as the height changes.

    Because the horizontal lift is constant but the vertical area is reduced as the boat rises, the leeway angle increases. It is the coupling of leeway with heave that is exploited by the L foil to provide vertical static stability.

    The dihedral angle of the horizontal wing is set so that the angle of attack of the wing is reduced as the leeway angle increases. This satisfies the static stability condition that the vertical lift decrease as the heave increases.

    Because the same horizontal lift is produced over a reduced vertical span, the sideways wash in the wake is also greater and the trailing vortices are more intense. This causes a coupling with the horizontal wing that increases the vertical lift, because the horizontal wing acts as a winglet for the vertical part of the foil (and vice versa). The dihedral angle required for vertical stability is greater than what one might expect by looking at the wing alone because it must overcome this wake-coupled influence. The result is there is a range of dihedral angles that provide positive vertical stability and a range of dihedral angles that are destabilizing in heave because of the coupling with the shed vorticity of the vertical part of the foil.

    Although there are times when the foil tip has broached the surface, this is not the normal mechanism for providing heave stability in L foils. The best performance is obtained with the hull just above the wavetops and the wing submerged well below the surface. The leeway-modulated heave stability is still effective in this condition, and the induced drag is minimized.

    Canting the foil inboard has the effect of increasing the dihedral angle of the wing, which enhances the heave stability. The vertical lift is spread over a greater span because the curved part of the foil is oriented to provide more vertical component of the force. This reduces the induced drag due to the vertical force. However, the induced drag of the horizontal force would be increased, so cant is typically used off the wind when the side force from the rig is less and the side force produced by the foils is correspondingly less. The foils still have to support the weight of the boat, so the vertical force is not lessened, but the relative proportions of vertical and horizontal force are changed, making the canted foil better suited to the operating condition. Cant allows the leeway-modulated heave stability to be increased an an acceptable penalty in the induced drag because of the lower side force and the higher speeds, which also reduce induced drag.

    Upwind, the foils are canted to their vertical position to minimize the induced drag from the high side force and reduced speeds. The reduction in horizontal wing dihedral angle with vertical cant impacts the leeway-modulated heave stability, which is why it is much more difficult to achieve stable flight upwind than downwind. The crew had to be more active in trimming the wing and foil to deal with the reduction in natural heave stability, which was very hard on the grinders when flying upwind.

    Whether canted or upright, the mechanism for providing natural heave stability was still the coupling between heave and leeway, which led to a reduction in vertical lift because of the designed-in coupling between leeway and vertical lift by virtue of the wing dihedral. Reduction in horizontal/vertical-lifting area due to the foil tip broaching the surface was not part of this primary source of heave stability. Allowing the tip to broach the surface had big penalties in terms of induced drag and increased leeway due to insufficient vertical span.
     
  7. UpOnStands
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    UpOnStands Senior Member

    Doug
    "This is necessary to counter the side force from the sail rig, which does not change as the height changes."
    The side force may not change significantly (sail area constant and we can assume that wind load is constant) but the moment certainly will as the CE rises?
     

  8. Doug Lord
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    Doug Lord Flight Ready

    ================
    Heeling moment certainly does change. But the foil is only concerned with side force.......
     
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