Best shape for surface piercing foil

Discussion in 'Sailboats' started by grob, Jun 15, 2004.

  1. grob
    Joined: Oct 2002
    Posts: 215
    Likes: 5, Points: 0, Legacy Rep: 53
    Location: Hove, Sussex, UK

    grob Senior Member

    I have heard lots of people say that traditional foils, NACA and the like, are not good for surface piercing, but have yet to find anyone to say what is good.

    Does anyone have any opinions or better still evidence on what is the best shape for a foil in the region where it pierces the surface of the water. I am most interested in small beach catamarans 16-20ft long, 15-25Knots, but any insights will do.

    Thanks

    Gareth
     
  2. John Perry
    Joined: Nov 2003
    Posts: 241
    Likes: 16, Points: 18, Legacy Rep: 129
    Location: UK

    John Perry Senior Member

    Foil makes lift by influencing the water flow around it, at least thats one way of putting it. In the region very close to the surface you cant get much lift from the top surface because there is not much water on top of the foil to be influenced, so lift comes more from the underside, i.e. it is acting more as a planing surface. Hence section is probably less important than for deeply immersed foils. A slightly concave underside and sharp leading edge may be desireable. Look at surface piercing propellers, eg 'cleaver' propellers, these have such features.

    However, once you are two chords or so down the surface effects are disappearing fast and you need more conventional aerofoil sections. Unless the water is flat and you have very precise control of flying height it is not going to be possible to have a special section just for the surface piercing region, it may well be better just to optimise the whole foil for deep immersion and accept a small penalty for the region within a chord or so of the surface.

    John
     
  3. CDBarry
    Joined: Nov 2002
    Posts: 736
    Likes: 26, Points: 0, Legacy Rep: 354
    Location: Maryland

    CDBarry Senior Member

    You probably want a "barn roof" lift distribution, to minimize the suction peak. This will minimize the depth of venting. This is basically the same problem as minimizing cavitation (not cavitation effects - so a super cav section is not what you want), or stall in aircraft, so there are a lot of applicable sections, one is the GAW-1. There may be some merit in reducing the section drag right at the surface, but this might be tricky as Mr. Parry notes.
     
  4. Kiteship
    Joined: May 2004
    Posts: 143
    Likes: 7, Points: 0, Legacy Rep: 81
    Location: SF Bay area

    Kiteship Senior Member

    The prototype Trifoiler had a good compromise section. Below the topmost fence the foil was asymmetrical, high lift with conventional rounded leading edge. Above the top fence the foil was symmetric, sharp L.E., in other words, lower (effective) lift, less peaky lift, and the sharp entry reduced spray. I asked Greg about it specifically; he liked the solution a lot. I don't recall if this made it into the production boats.

    A lot of older hydrofoils used 7% ogive foils (flat bottom surface, arc-of-circle top surface). Sharp LE, very constant, non-peaky pressure distribution. Moreover, being sectins of circles, the foil was easy to machine in metal or foam, even if tapered. These needed fences if they were inclined (typical surface piercer "V" foil), but not if they were vertical or near-vertical.

    Dave Culp
     
  5. tspeer
    Joined: Feb 2002
    Posts: 2,071
    Likes: 131, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    I don't understand people's insistence on sharp leading edges for surface piercing foils. From what I've read, ventilation of these foils is the biggest problem. Could there be a more ventilation prone shape than a sharp leading edge? I can't think of one.

    A surface piercing foil regulates lift by changing the immersed area of the foil. This is unlike the fully submerged foil which varies its lift coefficient (either through changing angle of attack or use of feedback to a flap). If a fully submerged foil is a constant area/variable incidence device, the surface piercing foil is ideally a variable area/constant incidence device. This may not be perfectly true, but the variation in lift coefficient over the range of operating speeds is going to be a lot less for the surface piercing foil than for the fully submerged foil.

    This allows the surface piercing section design to be more highly tuned than for the fully submerged foil. For example, one may wish to trade off high lift for a higher cavitation speed. Or trade the width of the drag bucket for a lower minimum drag.

    At the same time, any given section on the surface piercing foil will be deeply submerged or operating near the surface, depending on the operating speed. So the section needs to be designed to work well in either condition. That means it has to be robust with regard to separated flow, especially near the leading edge, to minimize ventilation.

    So it seems to me that a surface piercing foil section should be designed using an inverse method like XFOIL to the specific requirements of the application.
     
  6. Kiteship
    Joined: May 2004
    Posts: 143
    Likes: 7, Points: 0, Legacy Rep: 81
    Location: SF Bay area

    Kiteship Senior Member

    I guess it depends (at least partially) on what your "surface piercing" foil is going to do. If it's just a strut, as to carry a fixed depth, angle incidence altering foil through the water's surface, the desire is to minimize spray and wave making (yes, wave making). Making the surface piercing section sharp and with minimum lift suits this very well.

    If it's a lifting, inclined foil, like a ladder or Vee foil, the parameters are different. For one thing, you need the same section (likely, not necessarily) all along the span. Both spray and wavemaking drag (the latter is heavily influenced by the same things that cause ventilation--i.e.: peaky sections) are still important. To my knowledge, all such foilers used some variation of sharp leading edge, flat lift curve foils (hugely slanted towards ogive foils).

    You are likely right, Tom, that a new look ought to be taken towards these types of foils (variable submergence surface piercers), but the evolution of foilers seems to favor fixed submergence, variable incidence foils, vertical surface piercers. I suspect this is because of the relatively heavy drag penalties that inclined variable submergence foils pay, especially at speed.

    I suspect there are unplumbed depths yet to be discovered for true ladder foils (more or less horizontal rungs). The ability to alter not just the section but the chord as well as the span is very attractive. I know of two applications which were very, very effective; the old Baker hydrofoil and MIT's Decavitator human-powered hydrofoil. (not to mention Dave Keiper, whose ladders are legendary, though they were properly a hybrid, of both ladders and surface piercing elements).

    Dave

    Dave
     
  7. tspeer
    Joined: Feb 2002
    Posts: 2,071
    Likes: 131, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    grob's original question was aimed at section design, so that's what I answered. The range of possible surface piercing foil configurations is wide open.

    I've tried to compare a fully submerged T foil and the principal surface piercing foil types, all designed to the same requirements. I've also fitted the same model to a limited set of test data, and it does a good job of predicting the trends.

    At any given design point, you can design a fully submerged foil that will beat any of the surface piercing types. But away from that design point, it's a different story. Depending on how the foil is operated, the surface piercing foil has the potential to have lower drag than the fully submerged foil at off-design conditions. A powered hydrofoil may choose to operate at its most efficient speed, but a sailing hydrofoil must operate over a wide range of speeds. This can make the surface piercing foil competitive with the fully submerged foil for sailing applications.

    Active control and surface piercing foils are not mutually exclusive. The surface piercing foil is much more dependent on proper trim to optimize its performance, and it may benefit from active height control as well.

    I think it's critical to understand the various sources of drag and their importance. From the test data I've been able to find, spray drag is not a big player. The same goes for junction drag, even with ladder foils, as long as the junctions are near 90 degrees or more.

    But the induced drag can be very different from simple handbook estimates. The complex configurations of struts and foils that are used many surface piercing designs can result in very non-intuitive spanwise loading of the foils, and this can significantly increase the induced drag. I've found this to be the case when I've analyzed ladder foil configurations with a panel code.
     
  8. grob
    Joined: Oct 2002
    Posts: 215
    Likes: 5, Points: 0, Legacy Rep: 53
    Location: Hove, Sussex, UK

    grob Senior Member

    Tom,

    My original question was meant to be about the section design of surface piercing rudders. You seem to be talking about foils as in foils for generating lift in the vertical direction. Do your comments still apply?

    Also why does a sharp leading edge minimise wave making and spray generation?

    Gareth
     
  9. markdrela
    Joined: Jun 2004
    Posts: 290
    Likes: 26, Points: 0, Legacy Rep: 324
    Location: MIT Aero & Astro

    markdrela Senior Member

    On the Decavitator we originally tried a surface-piercing V-wing. The idea was to give automatic area reduction as the boat came up to speed and the V-wing lifted up. But it was a flop. The suction side would completely ventilate at around liftoff speed. A possible reason was that the piercing angle was rather shallow -- only 30 degrees from the horizontal. But we decided to abandon the concept, and switched to inverted-T surfaces which worked very well. The sections at the waterline were NACA 0010, which worked very well with no tendency to ventilate, even when going close to 20 knots.
     
  10. Kiteship
    Joined: May 2004
    Posts: 143
    Likes: 7, Points: 0, Legacy Rep: 81
    Location: SF Bay area

    Kiteship Senior Member

    Hey Mark, it's great to see you here! Thanks for the reply. FWIW, I'd love to be a fly on the wall, listening to you and Tom Speer discuss foiler applications.

    Dave Culp
     
  11. tspeer
    Joined: Feb 2002
    Posts: 2,071
    Likes: 131, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Yes. I think the principles for ventilation apply to all foils, especially vertical ones.

    I don't know. I've not found any data that says it does.
     
  12. tspeer
    Joined: Feb 2002
    Posts: 2,071
    Likes: 131, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    That's interesting - 30 degrees doesn't sound all that shallow to me.

    If the foil hadn't ventilated, is there any reason to suspect there could have been flow separation? I suspect you were trying to reduce wetted area to a minimum...

    On a related subject, is Ncrit = 3 the recommended value in XFOIL for designing hydrofoil sections?
     
  13. markdrela
    Joined: Jun 2004
    Posts: 290
    Likes: 26, Points: 0, Legacy Rep: 324
    Location: MIT Aero & Astro

    markdrela Senior Member

    The wing had a strong inverse taper, so the surface chords were largest.
    3D relief should have reduced the local cl. Hard to say for sure, though.
     
  14. tspeer
    Joined: Feb 2002
    Posts: 2,071
    Likes: 131, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Interesting. You must have been going for constant drag at a constant CL as the speed increased.
     

  15. MalSmith
    Joined: May 2004
    Posts: 111
    Likes: 9, Points: 18, Legacy Rep: 116
    Location: Australia

    MalSmith Boat designing looney

    Tom,

    I recently read your hydrofoil trade study and note your observations regading tapered and untapered surface piercing V foils. Assuming that tapering to zero at the vertex is structurally impractical, would the drag value variation between a fully tapered and an untaperd V foil be roughly linear?

    On another subject, what is the truth in the rumour that raking the foil forward will reduce vetilation?

    Mal.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.