Foiler Design

Discussion in 'Sailboats' started by tspeer, Nov 12, 2003.

  1. Penfold
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    Penfold Junior Member

    i've heard from one of the top moth designer builder/sailors that 4 series foils are stalling above 15kts when there is any risk of cav. He has suggested a 63012 for my 14 footer foil - which of course is required to produce +ve lift up wind and -ve lift downwind. Thanks for all your answers - especially the Cl vs a grapf above - i need that or my new foil!
     
  2. tspeer
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    tspeer Senior Member

    Here is the incipient cavitation diagram for a number of different sections, including the NACA 63-412 and NACA 4412.

    At 15 kt, the critical pressure coefficient, Cp, for the start of cavitation is -3.24. The NACA 4412 only reaches this level at high angles of attack, which is unlikely when you're going that fast. The NACA 6-series section would be worse under the same conditions. At low lift coefficients, the NACA 4412 is more susceptible to cavitation, but on the bottom surface and at speeds more like 25 kt than 15 k5

    However, this is for the foil section by itself. At the junction of the foil and strut, the local velocities are much higher and cavitation will start at a lower speed. The NACA 4-digit sections are front-loaded, which means they have higher peak velocities at moderate to low lift coefficients than more modern sections. These will be amplified at the strut/foil junction, especially if both strut and foil get fat at the same place.

    The NACA 63-012's characteristics will be symmetrical about zero lift, of course, so you can visualize what that might be like by sliding the NACA 63-412's curve to the left by 0.4 in Cl. This will require that you have the foil loading quite low to avoid cavitation, say, 200 lb/ft^2 (10,000 N/m^2) or lower. That means a lot of area that will have lots of parasite drag.

    If you really need to include zero lift in the design range, you might consider a NACA 63-212 instead. This will allow you to operate out to a lift coefficient of 0.4 and still be in the low-drag/low-cavitation region, and possibly cut the foil chord in half compared to the 63-012.
     
  3. Penfold
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    Penfold Junior Member

    This is great info Tom... I've got loads of questions and data to compare but I really don't want to hijack the moth design forum that this thread was set up for, so maybe we should continue the development of the 14 rudder foil on the other thread I've started (naca 63a012 in sailboats)? Or are people happy to keep it on this thread and widen the input...?
     
  4. Doug Lord

    Doug Lord Guest

  5. Stephen Ditmore
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    Stephen Ditmore Senior Member

    I've sent the message below to Rich Miller and Eugene Clement, as it's recently struck me that their work has important things in common. I've also forwarded this to Scott Sandell, the leading agitator for "Modern Moths" in the United States.


    ----- Original Message -----
    From: Stephen Ditmore
    To: rich@ski.org ; eclement5@aol.com
    Sent: Wednesday, October 19, 2005 4:40 PM
    Subject: similarities in work of E. Clement / R. Miller

    Dear Eugene and Rich:

    Reading http://www.exigent.info/miller.pdf and http://www.proboat-digital.com/proboat/200510/ (page 164), I'm struck by the similarity of your work. Both of you are using a forward planing surface based on supercavitating foil research forward and a submerged hydrofoil aft: inverted "Y" (R.M.) or "V" (E.C.). While I don't have much to offer either of you, I'm convinced you might have insights to offer each other.

    I have one question for Rich: why not use a forward foil that's curved looking fore & aft (like a portion of a ring) with the center where the pivot point of the "T" foil it replaces would be? Wouldn't this eliminate the need to articulate the "T"?

    Best of luck to you both. I'll be thinking of you while I'm building my Moth.

    Stephen
     
  6. Doug Lord

    Doug Lord Guest

    Moth?

    Stephen, very interesting! Are you building a "skinny" (Australian/World) type Moth? Foiler? Details!
     
  7. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Nope, I'm building the classic type. I think the largest fleet is near you in St. Pete. Right now I'm just looking to get in the game at the dino level of tweaking hull shape.

    The real difference is that racks are allowed on the "Modern Moth," but not on the "Classic", and along with that the Classics have more limited sailplans. We're hoping to have a few of each type (see opening page of www.mothboat.com for further explanation) racing at Sag Harbor, NY in summer 2006. To my knowledge no foilers are yet planned, but they'd be welcome in either category.

    For anyone interested in getting involved in the Moderns the guy to talk to is Scott Sandell (his e-mail is on the www.mothboat.com opening page). And Doug, if you want to be the Scott of the South, I'm sure the Scott of the North would be very supportive.
     
  8. Doug Lord

    Doug Lord Guest

    Graves Little Boatyard

    Stephen, the"Scott of the South" already exists-to some extent at least: Charlie Graves has tooling for a Moth that looks sort of like a small Finn and was/is planning on putting hydrofoils on it. Google his url.
    I think the work of Eugene Clement in his refinement of Plum stepped hulls has lots of possibilities in planing sailboat design and Ian Ward in Sydney has done quite a bit of work with the Miller forward foil concept. I don't know how the two could be combined but there are interesting possibilities all around!
     
  9. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Yes, I've seen photos of Charlie's boat. Rod in New Jersey has designed & built a similar boat called Tweezer D, which was 2nd at the Brigantine NJ regatta in June. Mine will be slightly narrower and have a more slab-sided bow (like a mini Volvo 70).

    So is Charlie planning to compete as a Classic Moth using hydrofoils? I figured we'd see I-14 style rudder foils on Classics, but I think it'd be hard to get one out of the water without a pretty stiff wind.

    It's the "Modern Moth" here in the U.S. that is comparable to the International/Australian Moth (square headded sails & racks). If an American were to order up a hydrofoil Moth from Australia or England and want to race here, they could as a "Modern." The "Classic Moth" class would be the place for Europe sailors to race now that the Europe is no longer an Olympic class. I think there's room for a hydrofoil within the Classic Moth rules, but most of us have assumed we'd see them first among the "Moderns" (assuming there continues to be an active "Modern" class. It's been pretty dead in 2004 & 2005, but it looks like Moderns are prepping to race again in 2006, thanks to Scott).
     
  10. andy_cherub
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    andy_cherub New Member

    Hi, im a student at Plymouth uni and im designing a vessel to beat 48 knots at the Weymouth speed week. Ive been researching different foils shapes etc, as im a cherub sailor and also used to sail a moth, so I fully understand the foiler moths.
    I was sat in a lecture about super cavitating props on surface pericing vessels and asked the senior lecturer about this method on hydrofoils, he didnt know.
    Does anyone have any information I could use on this matter? As i think that super caitating foils will be the way forward to beat the record.
    As for the sail area, im designing it in the class A, 10-13 sqm sail.
    Many thanks for your help,

    Andy Lang
     
  11. Stephen Ditmore
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    Stephen Ditmore Senior Member

    I urge you to read the two articles I linked to in post #455. I think Eugene Clement now lives there in England, and he replied to my e-mail, so I know he's reading it. I'd love to see a younger person take an interest in working with Clement to do a sailboard based on his ideas. Some combination of Clement & Miller ideas could probably be applied to multihull foilers as well.
     
  12. andy_cherub
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    andy_cherub New Member

    Many thanks, do you have Clements email address? Do you know where abouts in England he is? I am also looking for a company in the south West of England to do my industrial placement from Uni. Does anyone know of a foil company?

    Andy
     
  13. tspeer
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    tspeer Senior Member

    In addition to skin friction drag and induced drag, a supercavitating foil has cavity drag. The lift is less because the section is basically stalled, which requires bigger foils. So a big challenge to making a supercavitating hyrofoil sailboat is to overcome the drag of operating in the cavitating regime, which is on the order of 10% to 20% of the lift.

    Hoerner's "Fluid Dynamic Drag" has some approximate formulae for computing the drag of cavitating sections. Basically, the idea is the force on the cavity side is neglible, so it all comes down to the design of the pressure side. For a flat-bottomed section, Hoerner has the lift and drag as:
    CL = 0.5 * pi * alpha (1+sigma)
    CD = CL * tan(alpha) + Cf
    alpha = angle of attack in radians
    sigma = (ambient pressure - vaporization pressure) / dynamic pressure
    Cf = skin friction drag coefficient

    Sigma is known as the cavitation number. For reference, at 50 kt, sigma is around 0.3. The faster you go, the lower sigma is.

    The lift given above is only a quarter of the lift of a subcavitating hydrofoil. So a supercavitating foil needs four times the area and will have roughly twice the skin friction drag (because only one side is wetted) of a comparable subcavitating foil.

    There also seems to be an upper limit to the lift of
    CLmax = 0.5 * pi * alpha + sigma
    and the drag when so limited is
    CD = (2/pi) * CL * (CL - sigma) + Cf
    Hoerner shows data for ogival sections at an angle of attack of 5 degrees that has a section CD up around 0.05 at cavitation numbers of 0.4 to 0.6 and lift coefficients of 0.8 - 1.0. At the 50 kt cavitation number of 0.3, the section drag coefficient is about 0.04 and the lift coefficient is 0.4. Of this, friction drag accounts for about 0.008, or 20%.

    The pressure drag of the supercavitating foil can be reduced by curving the bottom surface. According to Hoerner, the lift equation for a cambered section is approximately

    CL = 0.5 * pi * alpha (1+sigma) + 7/16 * pi * gamma
    gamma = angle at the trailing edge between camber line and the chord line

    The drag that accompanies the lift due to camber is not as great as the drag due to a flat pressure surface because the forward parts of the cambered surface have a normal vector that points forward instead of aft. Hoerner gives the example of a flat plate tested at CL=0.3 and sigma=0, that required an angle of attack of 14 degrees. A cambered plate with gamma = 8 deg (3.5% camber) needed an angle of attack of 6.5 deg (vs 4 deg as predicted above). The presssure drag was cut in half (Dp/L = 0.1 as opposed to 0.2).
     
  14. Stephen Ditmore
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    Stephen Ditmore Senior Member

    What if the upper surface is only underwater, say, 30% of the time. The rest of the time the foil is planing on the surface. Does the above still apply?
     

  15. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Oh, I see.... so the answer is yes. Thanks for the info, bro.
     
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