Advantages/ Disadvantages of Curved Lifting Foils on Monohulls/Multihulls

Discussion in 'Hydrodynamics and Aerodynamics' started by Doug Lord, Apr 6, 2011.

  1. Doug Lord
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    Doug Lord Flight Ready

    Can anyone speak to the disadvantages of these types of foils? I can see many advantages-but I'd like to understand any non-obvious ramifications of having a curved(asymmetric or symmetric) foil that provides vertical lift and lateral resistance.
    On multihulls the vertical lift can slightly reduce RM-not neccessarily true on mono applications-any other observations/facts would be very helpful.
    Thanks!

    Pictures: curved lifting foils on the NACRA 20 cat and on the Open 60 V3:

    (click on image)
     

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  2. Michael Y
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    Michael Y Junior Member

    Are they daggerboards that you can raise and lower? Are they intended to be lowered on the flying hull and raised on the other?
     
  3. Doug Lord
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    Doug Lord Flight Ready

    curved lifting foils

    ============
    Curved daggerboards started out on 60' trimarans partially inspired by the maximum beam rule. It then became clear that by varying the radius of the curve the amount of lift, lateral resistance and draft could be changed.
    It took years but the curved boards are now used on almost every racing multihull-even those where there is no motivation fostered by a rule. In the last four years or so curved asymmetrical daggerboards have been used on beach cats , A class cats and on Steve Clarks Aethon C Class. One advantage of these boards is that the lift "profile" of the board can be changed by retracting the board a little or a lot so that it is possible to almost totally eliminate the vertical component of lift leaving just lateral resistance ,if desired. On cats, the windward board is normally raised after tacking though some have discovered(accidentally) that off the wind with both boards down the boat will lift clear of the water-not such a good thing w/o rudder t-foils!
    In 2009 the Verdier/VPLP designed Open 60 "Saffron" was the first monohull racing keelboat that I am familiar with to use curved asymmetrical lifting foils-they got third place. This year in the Barcelona Round the World Race, the Verdier/VPLP designed Virbac-Paprec 3 won after leading almost the whole race and was the only boat using curved foils. Second this year was Mapfre that used angled straight boards-angled OPPOSITE to the way Open 60 boards used to be angled to allow some vertical lift. A curved lifting foil seems to offer a great deal more adjustability as compared to a straight angled foil. On the big monohulls they are changed(one up,one down) every tack/gybe.
    I think it is possible to design a performance dinghy using just one curved asymmetric foil retracting into an athwartship continuous, partially open trunk. I think high performance monohull dinghies are possible using two curved foils as well.
    That's why I'm interested in learning everything I can about these foils.
     
  4. Gary Baigent
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    Gary Baigent Senior Member

    Doug, the first curved daggerboards were drawn three or more decades ago by Derek Kelsall for one of his 28 foot trimaran European lake racers.
     
  5. Doug Lord
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    Doug Lord Flight Ready

    curved lifting foils

    ====
    Thanks for the info ,Gary. Do you, by any chance, have any pictures or a link?
     
  6. Gary Baigent
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    Gary Baigent Senior Member

    No, but here is the original Flash Harry which was displayed with a bunch of other amateur and professional kiwi designers ... in 1983.
     

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  7. tom28571
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    tom28571 Senior Member

    I built a pair of curved daggerboards for a trimaran for the World 100 Race in 1988. Because of no testing time, they were not used in the race and the trunks were sealed. They were used in a later race and were effective. The shape was an ogive with flat bottom. Don't know how effective they were but the upper surface developed compression wrinkles in one day of racing, indicating significant lift. Construction was laminated ash and clearly not strong enough. Later I gave them to a local artist to be used in a sculpture of some kind. Probably effective for that too.

    That was World 1000, as I guess you knew.
     
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  8. Doug Lord
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    Doug Lord Flight Ready

    Thanks Gary and Tom thats really interesting, cool stuff!

    PS- were that foils that you used able to be adjusted for the angle of incidence, toe-in or cant? Thanks for helping me to learn more about these foils and their history!
    -Gary, what is the board in the drawing at the aft end of the ama on the main hull-a daggerboard? What was your impression in sailing with those foils?
     
  9. tom28571
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    tom28571 Senior Member

    If you were referring to me, our foils were fixed and only adjustable in amount of insertion. Even so, they were effective in lift. Whether they offered an more speed is another matter. A tri has lots of power and I am not certain that the greater righting moment would actually pay its way relative to getting the same moment from the leeward ama. Its a multifaceted problem.
     
  10. Doug Lord
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    Doug Lord Flight Ready

    ===
    Thanks, Tom. Most of the curved ama foils I have seen have the center of lift of the foil inboard of the CB of the ama so with the main hull flying they tend to slightly reduce righting moment-same on the Nacra cat. Were the amas you used very small so that the foil actually provided more lift the the immersed ama? Trying to understand the setup....


    Nacra video-you can see the lift from the curved foil: http://www.youtube.com/watch?v=82CDZcpqyNU&feature=player_embedded#
     
  11. philSweet
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    philSweet Senior Member

    Doug, some random thoughts-

    I think adopting curved foils will require the entire boat to be re-optimized. The angle of attack of a vertical foil doesn't depend on pitch or heave motions. When the foil is curved or canted, it does. As the cant angle increases, designers should emphasise decreased pitch and heave excursions in order to preserve a given level of foil performance. This assumes the pitch and heave excursions tend to be greater than the yaw and what-ever-you-call-heave-in-the-transverse-direction. I've seen the term-forgotten it.

    The first thing I would want to do if trying to design curved or canted foils is plot the angle of incidence of the flow on a vertical and horizontal foil with the boat underway at sea. If the horizontal plot is less steady than the vertical, the cant angle should be less than suggested by a steady state model.

    Intutively, The cant angle should be greater on a broad reach and less going to windward. A boat is much steadier in pitch and heave off the wind. Also it has more speed potential, so reduction in wetted suface do to lift is more valuable.

    The only hydrodynamic reasons to adopt a curved foil that I can think of would be to reduce junction drag where the board exits the hull, and to exploit a difference in the flow characterists as draft increases (wave orbitals?). There may be a whole host of mechanical reasons that make them appealling.
     
  12. Doug Lord
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    Doug Lord Flight Ready

    ===========
    Thanks Phil. I appreciate your comments. As best I can tell, so far, if you want "foil assist"(not full flying) as part of your design-that is, you want some vertical lift from the foil the best solution appears to me to be a curved lifting foil vs a straight angled foil or t-foil:
    --1) because it can be fully retracted(compared to a t-foil)
    --2) because the angle of incidence of the lifting portion of the foil can be adjusted by angling the foil(top back for more-pivoted at a point in the trunk)-this is already done on many current applications.
    --3) because the "Lift profile" of the board can be changed at will by simply retracting it a little or a lot. In other words, the foil can be retracted so that it is only producing mostly lateral resistance and virtually no vertical lift. Or ,like in the case of the Nacra cat in the previous video, the board can be slightly retracted to avoid "pitch-ups"-reducing vertical lift a little(which could also be done by changing the angle of incidence as in # 2) or a combination of both.
    --4 because the characteristics of the vertical lift of the foil can be altered by changing the radius of the curve(at the design stage) and /or by canting the board(head moves athwartship-like on Steve Clark's Aethon) in an appropriate trunk(see below).
    --5 because the angle of incidence of the lateral resistance portion of the foil can be adjusted independently of vertical lift by adjusting the toe-in of the board.(done on most current applications or fixed at a constant toe-in)
    ---------
    Phil, I think there is tremendous development room possible to incorporate these foils(for "foil assist") in monohull dinghies, sportsboats etc.
    I appreciate everyone's comments so far-I want to learn as much as I can about these foils-good, bad and ugly........
    ---------
    I agree 100% with this Any boat that will use lifting hydrofoils of any kind for "foil assist" must be designed from the get go for them
    to get the greatest benefit. That's one reason I think that the successful, race-wining use of curved lifting foils by V3 in the BWR round the world race is such a milestone for monohull sailboat design. Both monos and multies need to be designed to take advantage of the specific attributes of this type of foil
    to be really successful. Thanks again for your thoughts......


    Pictures, cant angle adjustment range of Aethon's curved foils:
    (click on image)
     

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  13. Doug Lord
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    Doug Lord Flight Ready

    Advantages/Disadvantages of curved lifting foils

    Here is the angle of incidence adjustment system for the Bimare cat-sliding the head of the board back pivots the board increasing angle of incidence of the lifting portion of the board-does not affect the lateral resistance portion of the board:

    (click on image)
     

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  14. powerabout
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    powerabout Senior Member

    if you follow the A class modded boats its not a no brainer that curved boards are all round better.
    Nacra put a huge amount of testing to get theirs right and I think the A cats guys will certainly agree with that.
    Dont forget Nacra also built a new boat to work with them.
    My experience sailing the 20 is that you need to keep the boat almost flat for max speed or maybe we need a canting mast??
     

  15. daiquiri
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    daiquiri Engineering and Design

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