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

    Advantages / Disadvantages of Curved foils

    ================
    You're absolutely right. Thats the beauty of curved foils- you can adjust out excess lift if you choose to. These guys were just having fun. The incidence of the lifting portion of the foil is adjustable by moving the head of the foil fore and aft(pivoting the board) and the lift profile of the board can be changed by just retracting it a bit. If they left both foils down AND had rudder t-foils they could probably fly.
     
  2. powerabout
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    powerabout Senior Member

    you can also just lift them and the curve will shorten into the hull
    The foils on a F20C will , if you are too far back lift the whole boat clear (less the rudders) of the water in a puff
     
  3. Doug Lord
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    Doug Lord Flight Ready

    Here are some pictures of Ian Farriers new curved foils, foil mold and the cases for them-tighter curve than most you see-and his comments:

    These particular foils could be used on a number of boats, including the F-82 and on up to the F-33, and as such will retract behind the forward beams for less windage when up, plus they can be left in place when trailering. The tighter curve means more effective vertical lift, while still enhancing windward ability. Construction will be all carbon fiber with a clear finish, and infused together in one piece. Not inexpensive, but they are just about solid carbon through the center to withstand the loads. First foil is being made this week.
    ---
    I emailed Mr. Farrier about adjusting the angle of incidence of the lifting portion of the foil. He said that it can be adjusted 1.5 degrees and gave me permission to post this:

    Doug,

    There is a sliding collar on the float deck within a carbon sleeve, and adjustment is made by fore and aft screw jacks so fine adjustment can be done on the go.

    If looking for the actual earliest foils that were made, then I think it would be hard to beat those that I designed and made for the prototype F-27 in 1984. Can be seen just behind forward beams. Too expensive to use in a production boat however, so we did not persevere, but they worked.

    ------
    1984! 27 years ago-pretty damn cool! see pictures below


    Pix-you can see the tighter curve of the Farrier foils as compared to the Nacra F20 foils, Farriers 1984 curved foils-be sure to click on the image-the curved foils are tucked in right aft of the forward beam :
     

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  4. Doug Lord
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    Doug Lord Flight Ready

    Advantages / Disadvantages of Curved foils

    Thanks to a heads up from Gary Baigent here is the earliest concept sketch by a hydrofoil builder of a curved lifting foil-sometime prior to 1972. In "Hydrofoil Sailing" , by Alexander, Grogono and Nigg, William C. Prior, of Chagrin Falls, Ohio shows a conceptual sketch of a foiling proa using a curved lifting foil on the windward side. The boat foiled but Prior felt that it is "still too impractical and accident prone." Unfortunately, no detail on the curved foil or the other two foils. Since it was a proa, the foils had to develop equal lift in two directions. From previous research, it seems likely that the curved lifting foil had to pull down most of the time or it probably would have created a lot of drag due to leeway.


    picture from the book above:
     

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  5. Doug Lord
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    Doug Lord Flight Ready

    Advantages / Disadvantages of Curved Lifting Foils

    From the new "Sail Racing Magazine", (free http://www.sailracingmagazine.com/back_issues.html )

    " For years French Orma 60 trimarans used curved foils to provide lift for their bows in rough seas and to counteract the power* of their massive sail plans.
    A few years ago, the A Class began curved foil testing with varied results. Until recently, the jury was out on their use. However, as designers began working on angle of attack** their has been a gigantic increase in downwind performance. Groundbreaking multihull designer, Pete Melvin, has won the A Class world championship on two different platforms he designed, first in 1997 on "Waterat" then in 2005 on "A2".
    He explains the design leap of recent history, "Certainly the curved lifting foils
    are a significant improvement. They lift the boat partially out of the water, allowing you to push the boat harder downwind with reduced resistance."
    For best foil efficiency a platform has to be designed around the foils."


    * In my opinion, the foils did not "counteract" the power of the rig so much as they reduced resistance significantly so that more power from the rig gets translated into higher speed. DL

    ** From other articles by A Class sailors it is apparent that adjustment of the angle of incidence of the lifting portion of the foil is critical and can be adjusted by angling the head of the foil fore and aft-like on the DNA. DL
     
  6. Doug Lord
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    Doug Lord Flight Ready

    Advantages / Disadvantages of Curved Lifting Foils

    This is a new cat being designed by Mike Drumond and the Oracle racing Team as a new Olympic Multihull.
    I don't get how the foil trunks would work since the foils have both straight and curved sections.
    Anyone have any insight?

    click on image:
     

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  7. Doug Lord
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    Doug Lord Flight Ready

    Curved Lifting Foils / Farrier

    ================
    Just discovered this interesting tidbit about these new Farrier foils from the Farrier F32SR site:

    The foil section is also significantly improved from that used on the F-27, this having been developed for the F-32SR by Tom Speer, a Boeing airfoil engineer, who also developed the wing mast section for the America's Cup trimaran BMW ORACLE.

    for more go here and scroll down: http://www.f-boat.com/pages/trimarans/F-32SR.html

    click on image:
     

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  8. Doug Lord
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    Doug Lord Flight Ready

    Wing Fox Nano boards-very,very unique

    This is the rationale given by the Company for the relatively bizarre shape of these boards for an A Class cat:

    Nano centerboards. Technology.

    New centerboards WingFoX Nano are made entirely of carbon prepreg. We do not cover them with a layer of topcoat so the run-off edge is very thin though also very tough. The entire element is hardened and annealed in an autoclave.

    Due to specially designed internal structure it was possible to give up the use of filling foam and glue to fix the two parts together.

    The centerboards are made in a single process technology and thus are monolithic, which ensures their high rigidity at minimum weight.

    Special array of carbon fibres, accompanied by appropriate design, yield a significant increase of rigidity. We have put much effort to the selection of centerboard geometry and its rigidity so as they would not cause vibrations. We have paid particular attention to the forming of “higher harmonic vibrations” which are periodically appearing oscillations whose frequency is a multitude of their free vibration frequency. What results is an uncontrolled amplification of the amplitude. In extreme cases it results in breaking the stream flowing round the centerboard and a sudden drop of the lifting power. Windsurfers are well aware of such situations when suddenly the centerboard looses its capacity of generating side resistance force and a spinout occurs. Apart from lower efficiency, an oscillating centerboard causes diffusion of energy and thus generates increased resistance.

    Centerboards require high precision of workmanship. That is why we have applied the CNC technology to make centerboard forms. Specialist software allowed us to optimize the lifting power in relation to the resistance.


    --------------------------------------------------------------------------------



    Nano centerboards. Shape.



    WingFoX Nano has been equipped with centerboards of variable curvature and variable profile.

    Since class regulations restrict the area we can use to generate the lifting force to 40 cm, the shape of our centerboards was designed to take maximum advantage of the area available.

    The geometry of the centerboard was designed so as to allow the lifting part to generate lifting force even when the centerboard is partially drawn out.

    The asymmetric profile used in the lower section of the centerboard generates lifting force already at zero trim. The centerboard box and the elements contained therein permit centerboard trim within +/- 5 degrees of the angle of incidence of the lower section of the centerboard.

    The centerboards feature increased length to allow them to work deeper in the events of more stabilized conditions. In the surface layer we encounter rotating currents, a rotation of water masses, which impairs the efficiency of a centerboard.

    Assuming appropriate lifting force to be generated by the centerboard we took every effort so that a lifted up centerboard could still generate adequate side resistance necessary for the boat not loose its capacity of sailing upwind.

    What happens in the case of centerboards having constant radius of curvature is a considerable loss of lateral surface of resistance, which causes increased leeway and makes upwind sailing impossible. In extreme cases the loss of upwind sailing potential may not be compensated by higher speed.

    The special design of the centerboard box allows its easy raising while maintaining tight fit in every position.


    When designing our new centerboards we focused our attention on selecting an appropriate profile thereof.

    Considering the fact that in majority of circumstances centerboards operate under transient flow, we decided to apply well proven profiles which at low resistance throughout the entire range of velocity provide high efficiency and are highly resistant to breaking of stream.


    In the ‘vertical’ section of the centerboard we have applied the NACA 63-012 symmetric profile which ensures generation of appropriate side force both at small and large speeds while at the same maintains appropriate rigidity.


    In the carrier section this profile turns smoothly into a non-symmetric NACA 63-412, passing through all the intermittent profiles: NACA 63-112; NACA 63-212; NACA 63-312. It must be noted here that this profile is capable of generating adequate lifting force as early as at 0 degree trim.

    Apart from high efficiency proven by computer simulations, the NACA 63-412 profile performs perfectly well also both in the large and ultra light units equipped with hydrowings. The choice of the NACA 63-412 profile constitutes and ideal consensus of low resistance at high speeds and generation of high lifting force in the entire velocity range.
    ..


    -----------------------------------------------------

    Nano centerboards. Profile.



    When designing our new centerboards we focused our attention on selecting an appropriate profile thereof.

    Considering the fact that in majority of circumstances centerboards operate under transient flow, we decided to apply well proven profiles which at low resistance throughout the entire range of velocity provide high efficiency and are highly resistant to breaking of stream.


    In the ‘vertical’ section of the centerboard we have applied the NACA 63-012 symmetric profile which ensures generation of appropriate side force both at small and large speeds while at the same maintains appropriate rigidity.


    In the carrier section this profile turns smoothly into a non-symmetric NACA 63-412, passing through all the intermittent profiles: NACA 63-112; NACA 63-212; NACA 63-312. It must be noted here that this profile is capable of generating adequate lifting force as early as at 0 degree trim.

    Apart from high efficiency proven by computer simulations, the NACA 63-412 profile performs perfectly well also both in the large and ultra light units equipped with hydrowings. The choice of the NACA 63-412 profile constitutes and ideal consensus of low resistance at high speeds and generation of high lifting force in the entire velocity range.


    click on image for larger version:
     

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  9. themanshed
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    themanshed Senior Member

    I've been thinking about curved foils for my tri. Has anybody given any thought as to the building process for curved foils, if so I'd like to hear it.
     
  10. Doug Lord
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    Doug Lord Flight Ready

    curved foils

    ===============
    I'm going to use the same technique on the test model I'm building as I will on the full size. I haven't built curved foils yet but I have designed and built tooling for 40 years and know it well. I have built numerous examples of straight foil tooling including foils with matching trunks for models and full size boats:
    1) build a curved platform with the surface equal to the centerline thru the foil,
    2) build one half the foil on the surface,
    3)* add sheet wax equivalent to the allowance for foil clearance in one half the trunk. I haven't decided on this yet but I notice that Farrier includes some kind of material like outdoor carpet in the trunk for their cassette rudder to allow the thing to slide while still being held relatively firmly.Very important.
    4) use wax and/or clay to make a pivot point(s) for the one half of the trunk you are about to lay up. The pivot point allows the head of the foil to be able to be moved fore and aft to tune the angle of incidence. A molded "round" should be included at the bottom of the trunk in front and in back( or just above any molded insert-see below). The clearance should be a maximum of 1/16" and the round can be designed to have a small thin piece of delron glued to it. Note that the foil angle of incidence will not be adjusted under load.This method doesn't allow for adjustment of the toe-in of the foil except by moving the trunk. Think about this. Also ,it doesn't allow any 'cant' but I don't think that is necessary on your or my boat. Be sure that the top of the trunk allows for the foil to pivot fore and aft. I'd consider allowing -3 degrees(forward) to + 6 degrees(aft) or slightly more. See the pictures here and in the other thread showing how Bimare and DNA adjust the top of their foils.
    5) wax the edges and possibly spray with partall ,
    6) lay up one half the trunk----before pulling attach a light, removable framework to prevent the trunk half from warping.
    7) pull trunk half #1,
    8) IMP: if you design the foil so it is constant chord then now you could go ahead and lay up the second trunk half but make sure to move the pivot point! Constant chord reduces the things you have to build to a minimum.
    9) remove the sheet wax and clay from the foil half plug, clean up and rewax,
    10) lay up the mold for this half of the foil,
    11) pull the mold and build a one off foil inside the mold using the exact same patterns you used to build the first half,
    12) Repeat 3,4,5,6, 7 and 8 for this second half foil mold and two trunk halves.
    -------
    Hope this helps you to think it out-I don't think I've missed anything but study the process carefully so that you're 100% positive about how much clearance you want and how to introduce it (sheet wax etc). In my opinion, the key to a good curved foil installation is proper clearance and the facility to adjust the angle of incidence by moving the head of the finished foil fore and aft in its trunk.
    Make sure you have a system for exactly duplicating the second foil half that you will build in the first mold half! This will allow the parts to match perfectly. You can use an asymetrical section like a 63412 but it will be slightly harder to make.(flater side outboard)
    --I've done this numerous times for straight foils and with great care it should work well.

    -------------------
    Design of the foil is, of course, critical: how much load do you want it to take? At what speed? On ORMA tris they allowed the ama foil to support up to 60-70% of total displacement and they had instances where the foil would lift the whole boat resulting in a crash. I would tend to go with 60% on a small boat like yours-you can always partially retract the foil to reduce its ability to lift until you get comfortable with it. On ORMAS 60-70% of the boats weight was supported by the single curved foil with the balance supported by the ama-which was all the pitch control the boat had. This means flying the main hull which is something you have to look at carefully: You need to find the sailing angle of the boat when the main hull is just clear and the ama supports ,say, 40% of the weight. If the resulting sailing angle is much greater than 15 degrees I'd consider not doing it. If it is greater than 20 degrees I wouldn't do it at all. The single most important determinant of sailing angle is beam, the second is main hull rocker, the third is the designed immersion of the ama with the foil supporting X amoint of weight. An important consideration in the foil design is the curve radius: this is primarly determined by whether or not you also have a daggerboard for lateral resistance. If the daggerboard will develop most of the lateral resistance then the curve can be "tighter" resulting in more vertical lift. Compare the Farrier ama foil with the NACRA 20 curved foils-the Farrier is much tighter because he is using the curved foil ,primarily, for vertical lift whereas on the NACRA it is being used for both with maybe a bit more lateral resistance than lift. Remember, that a curved foil in an ama moves the effective center of lift(buoyancy + foil lift) INBOARD reducing maximum RM. That's why ORMAS were nearly square. Consider this very carefully-I don't remember your designed-for beam so you'll have to check it out. It's unlikely that this will be too much of a problem but it has to be considered.
    You could get a copy of Ray Vellinga's "Hydrofoils: Design, Build Fly" that would help you tremendously.(Amazon)(nothing on curved foils but lots of understandable info on foils, calculating foil lift etc.)
    Depending on how much you know about hydrofoils you might consider getting someone else to do the foil design.

    Good Luck!

    http://www.boatdesign.net/forums/mu...-self-righting-trimaran-test-model-36058.html

    * There are all kinds of ways to make a system that allows the board to slide easily-I haven't decided which way to go yet but here are a couple of ideas:
    1) talk to someone that has built curved foils like Ian Farrier or Steve Clark -both are members here at boatdesign.
    2) you can make a molded insert that is removable for the top and bottom of the trunk and put some kind of material like Farrier uses in his rudder trunk on(in) those pieces-should the stuff wear down the pieces could be replaced. You will have to plan on this early when you lay up the first half trunk. Again, remember that the top of the trunk will be longer than the bottom of the trunk to allow room for the foil to move fore and aft.

    Rough Sketches, L to R-1) trunk illustration showing pivot points and removable insert to facillitate sliding board, 2) Tentative final design of curved foil on my 18' LOA X 22' trimaran. Foil length-8', chord .85' . At max deployment with ama just touching water foil draws 2.25'. Designed for max lift and minimum lateral resistance. 63412 asymetrical section
    Note: if you're willing to allow a foot or less foil to stick out below trunk when foil is retracted you can increase the vertical lift area a bit. My foil will tend to support 75-100% of the boats weight and behave more like a surface piercing foil than an ama foil on an ORMA.
    3) Farrier F32 curved foil + trunk- note room at the top of the trunk for the head of the foil to move fore and aft:

    click on image for better detail:
     

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    Last edited: Aug 29, 2011
  11. bscally
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    bscally Junior Member

    A couple of things I would like to add.

    For practical sailing of a hydrofoil (Windrider Rave)

    1) Foil drag is a huge issue.

    2) As the foil is pitched fore and aft both the center of lateral resistance and the center of lift move. With long foils this can be significant, and is very noticeable.

    3) The Rave foils benefited hugely from having a fence added to the lateral tip. I assume that it reduced the tip vortexes. They are simple T foils.

    4) Form my ex-A-class with curved foils - A nose up attitude will generate significant lift. Without an any lift at the stern this promotes skying of the bow. Generally this just causes the foil to vent and a loss of lift - and a crash back to water.
    However the forward speed of the boat is fast enough when the lift action occurs enough energy can be imparted that a back flip can be performed. I am sure that there was some wind action involved but foil lift has a distinct feel.
    Back flips seem to be highly structural damaging resulting in ex-A-class

    IMO this is hugely undesirable - forces should not be applied unevenly to the structure. It generates badly behaved boats. Lifting foils - Lifting rudders. Or keep the lift so small that it is not a significant portion of the load.

    5) These foils get noticeable hot and cold spots when pushed hard. In cold lake water I always wondered about the possibility of icing.

    Brian
     
  12. Doug Lord
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    Doug Lord Flight Ready

    ---------------------------
    Brian , thanks for your post! This gives me an opportunity to post a report by Martin Fischer on his new Mayfly A Class Cat design. It did not perform too well in its first race exposure at the recent worlds but Fischer believes he knows why and his reasoning is very interesting. His boat utilizes sort of "S" curved foils and rudder "J"( he calls them "L") foils. Take a look at it. I've posted the PDF below and there is more in the boatdesign "Multihulls" thread on the Mayfly.
    Do you have a picture of the fence on the Rave foil? I've sailed the boat and been around a lot of them and never saw the fence on any boats around here. Bradfield just launched his new Osprey foiler that has much better numbers than the Rave. There is a thread in "Multihulls about it , as well.

    Fischer Mayfly report:
     

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  13. tspeer
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    tspeer Senior Member

    This comes from unstable pitch-heave coupling. The stern is comparatively "stiff" in heave - a small change in immersion results in a large change in vertical force. But the stern is quite "soft" in pitch - a change in pitch trim doesn't change the buoyancy of the stern very much. The foil is soft in heave, because there isn't much change in vertical lift until the horizontal part of the foil gets close to the surface, but the lift on the foil is sensitive to pitch trim. This is exactly the opposite of what you need to have stable coupling between pitch and heave motions.

    To be stable, the forward foil needs to be stiff in heave, and the aft foil needs to be stiff in pitch. In other words, when the craft rises the lift needs to drop off more on the forward foil than the aft foil, such that there is a bow-down pitching moment created. And when the craft pitches bow up at constant flying height, there also needs to be a bow-down pitching moment created. These three trends ensure that the boat will tend to come back to its equilibrium flying height. The easiest way to ensure this is to use a T foil at the stern and either a surface piercing foil or height feedback for the forward foil.

    It may sound strange, but if you wanted to stably lift the hull part way out of the water to reduce wetted area, the way to do it would be to raise the stern, not the bow. As the boat trimmed more bow-down, the change in pitch would reduce the lift on the stern. Obviously, this approach would require considerable sheer in the bow to prevent pitch-pole. But in principle, it might be a workable solution.
     
  14. Doug Lord
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    Doug Lord Flight Ready

    ===============
    Tom, if you get a chance read Fischers report-it touches on this subject.
     

  15. bscally
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    bscally Junior Member

    Rave Foil Fence

    Attached is a picture of my fences. There are others that have only a vertical section.

    On Mayfly I had heard a comment that the helm was observed as generally being further aft that most of the other competitors.

    This is anecdotal, but in reviewing photos of the event it appears to be true.
    Not sure if this has any relevance to the issue of boat speed, but as a personal thing I like the boat to set up so that I am about in the middle of the tramp on a reach. Slightly forward upwind, slightly aft downwind.

    And thanks for sharing Martin's report. I will check out the Mayfly since I am just stating my new A-Class design.

    Tspeer - I totally agree.. But not an easy problem.
     

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    Last edited: Sep 30, 2011
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