A Class Cat with automatically canting daggerboards

Discussion in 'Multihulls' started by Doug Lord, Jul 18, 2017.

  1. bjn
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    bjn Senior Member

    I think that if the shape, angle and location of the foils coincides with a circle at the center-line of the boat, it will act like a tube of the same diameter. And a tube should have zero roll stability.

    The PDF you linked to earlier showed a different design of the Nacra 17 foils. Look how close the foils are to being arcs in a circle.
     

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  2. Doug Lord
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    Doug Lord Flight Ready

    Very interesting-thanks....
     
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  3. hump101
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    hump101 Senior Member

    This is not correct. A tube has no roll stability because it's CoG is at the roll centre, and therefore does not change as it rolls, and hence there is no righting moment or overturning moment. When a foiling boat rolls, both the CoG and the lift centre changes, providing roll stability.

    The roll stability when on foils is closely tied to the heave stability of each individual foil, so if a Z and an L have similar heave stability characteristics, they will produce similar roll stability on a catamaran. This is not necessarily the case, however, as the L reduces lift with leeway, whilst the Z increases lift with leeway and relies on surface piercing of the lifting portion to provide heave stability. This is why L foils are inherently easier to make stable, because the change is through angle of attack, not through reduced surface area. Both can be made to have similar heave stability by choosing particular geometries, but they are achieving this through different mechanisms.

    This is why ETNZ decoupled roll from side force in their AC50, using the top of the wing to control roll, but automatically adjusting the lower wing at the same time to keep the side force constant for a given speed, and hence the ride height constant. This would work with both L's or Z's.

    However, most of the comments about stability on foils is related to pitch stability, not roll. Roll instability can be compensated for by steering input, but pitch instability requires feedback between main foil heave, rudder foil pitch, and sail force, which is more difficult to balance.
     
  4. Doug Lord
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    Doug Lord Flight Ready

    Uptip foils are used one at a time while "Z" foils are used two at a time and the windward "Z" foil carries about 25% of the load. There is a post by Tom Speer about how uptip foils work-I'll try to find it.
    Here it is-Tom refers to "uptip" foils as "L" foils. Also, Tom refers to tip breaching as undesirable but there are some designers that incorporate tip breach into their foil design(Martin Fisher-see GC32 below):

    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 flyng 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.

    GC 32(designed by Martin Fischer) setting a speed record-notice foil breach-picture by Sander van der Borch:
    GC 32 speed record 37.9 knots Sander van der Borch.jpg

    From the Morrelli and Melvin paper in post 8, page 1 of this thread.(Note:no load shown for rudder T-foils)
    nacra17 foil loading(from paper).png
     
    Last edited: Jul 27, 2017

  5. Doug Lord
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    Doug Lord Flight Ready

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