Why no negative lift foils for greater righting moment?

Discussion in 'Multihulls' started by Jetboy, Jul 19, 2012.

  1. Jetboy
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    Jetboy Senior Member

    I have not seen foils used yet to create greater righting moment. Has this been done? It would seem that potentially ideal foiling sailboat would use the windward foil pitch control to create negative lift or a downward force to create righting moment. It might eliminate the need for ballast movement.

    Obviously it would be difficult to design and produce a system that would work smoothly to adjust for the necessary amount of righting moment at any given time, and possibly that would require some type of sensor to analyze the forces on the boat either the angle of heel or possibly something like the force applied to the shrouds? Not sure how exactly it would work because it would need constant adjustment.

    Anyway, does something like this exist? Any examples?
  2. Doug Lord
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    Doug Lord Flight Ready

    JB, you've missed a few boats. The Rave, Osprey, Skat, Hobie Trifoiler and others all use independent altitude control systems that not only lift the boat but they also create all the righting moment(RM) for the boat. They do this because the surface sensor on the windward side changes the flap(or foil) angle so that it is pulling down while the surface sensor on the leeward side increases lift as necessary. They do this automatically ,though on the Rave, Osprey, and Skat the response of the surface sensor can be tweaked by adjusting tension holding the surface sensor against the water.
    These boats all have fully submerged foils and that means they need to have an altitude control system, like a wand(Rave, Osprey, Skat) or forward "feelers" like the Hobie trifoiler. Foilers like Hydroptere use surface piercing foils that vary their altitude as a result of speed but generally contribute nothing to RM. For that reason , Hydroptere carries waterballast.
    The RM for the fully submerged foil boats is incredible-up to the structural strength of the boat!

    Wand/foil animation from Bradfiled: http://www.youtube.com/watch?v=yuFwDm8t3IM

    Pictures, L to R: 1) 18' Osprey by Dr. Sam Bradfield, 2) 16' Rave, by Dr. Sam, 3) 40' Skat by Dr. Bradfield, 4) Hobie trifoiler by Greg Ketterman, 5) 5' F3 by me(first production RC sailing foiler),5) Rave wand up close: Bradfields wand was copied and slightly modified by John Illett to create the fantastic Moth monofoiler. The Moth system doesn't create RM in and of itself but it allows the technique of "Veel Heel" to increase RM by up to 40% or so.(picture by Dan Dickison)


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  3. Jetboy
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    Jetboy Senior Member

    I wasn't aware of those systems. Thanks. Query whether they might work well on a monohull - especially a planing one like a dingy. Might provide the sailing performance of a trimaran without the complexity cost and weight of amas. Just a couple foils on fold up beams. Basically take a set from a hobie trifoiler and make them larger to go on a bigger planing dingy.
  4. Doug Lord
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    Doug Lord Flight Ready

    A trimaran seems to be the best platform because it can easily(and lightly) be made square or oversquare: the Osprey is 18' long and 22' wide. Greater beam allows the loading of the foils to be reduced improving speed and allowing a lighter wind take off. A mono would have to be quite wide to use this kind of system effectively. "veel heel" on a monohull foiler is a painless and effective way to increase RM on a boat-dinghy or keelboat-that flies.
    PS- check out the DSS threads in "Sailboats"-the Quant 28,etc. The DSS foil allows a mono to have a substantial increase in RM and reduction in wetted surface by using a foil sticking out to leeward.....
  5. PAR
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    PAR Yacht Designer/Builder

    Bolger had an inverted foil like appendage worked into t a bulb on one of his designs, though I could tell you which or how effective it might be.
  6. tspeer
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    tspeer Senior Member

    The negative lift on a hydrofoil creates drag. And it has to be countered elsewhere with positive lift - more drag. Drag due to lift is inversely proportional to speed squared, which means for the same change in righting moment the drag becomes horrendous at low speeds. So you have to be going fast enough for the added drag to be low enough that the extra power from the rig is worth it. It's not a good fit for monohulls that are sailing at hull speed or below.

    Downward lift from a windward foil is also unstable, in that as the boat heels, the foil comes closer to the surface and the lift decreases. If it broaches or ventilates, you can suddenly lose all the lift, and now the boat suddenly has far more heeling moment than it can stand.

    Foil downforce has been investigated for monohulls in the form of a hapa.
  7. Gary Baigent
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    Gary Baigent Senior Member

    You reading the above, Doug?
    What is the point of lifting the same weight twice?
  8. Doug Lord
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    Doug Lord Flight Ready

    RM from foils

    Tom Speer mentioned that when discussing the surface piercing foils on Spitfire(pix below) and the fact that they carried 1000lb ballast. Point being that maybe it is "cheaper" from a drag standpoint to use movable ballast instead of foil generated force for RM....
    Me, I think a tri using two fully submerged foils on the main hull to lift off early and a single surface piercing foil on the ama(windward ama foil clear of the water) to gradually take the whole load is the way to go. The wand controlled main foil would begin to unload just after takeoff and would control pitch, primarily, after takeoff.

    Greg Kettermans comments on the subject:

    "Hydrofoil boats can be categorized into two categories; 1) Incidence controlled hydrofoils* and 2) surface piercing hydrofoils. The difference lies in the way the boat maintains the proper altitude above the water surface. A surface piercing hydrofoil boat maintains proper height by varying the amount of foil submerged. The boat raises up as the speed increases and reduces the amount of foil submerged and therefore the lift. The boat finds equilibrium at the proper altitude. An incidence controlled hydrofoil sailboat has a mechanism that controls the angle of attack of the foil to maintain the proper altitude. It is generally believed that surface piercing is simpler, but incidence control is more efficient. In reality, it is the method that works with fewer problems that is simpler.
    From the beginning it was felt that incidence control was better suited for a sailboat even though most of the existing hydrofoil sailboats were of the surface piercing type. There are many advantages of the incidence controlled foils; however, the most important is what I call the DLA (dynamic leveling affect). This is the increase in righting moment or stability due to the ability of the windward foil to pull down. The DLA has little affect on the low wind performance, but it essentially makes the top speed of the boat limited to the strength of the boat. Conventional boats with a finite amount of righting moment can only extract so much power from the wind, but with the DLA, the righting moment is virtually unlimited.
    Intuitively many people think that the added drag of the windward foil plus the increased induced drag of the leeward foil would offset the gain in righting moment, but calculations show and practice proves otherwise. The dynamic leveling affect not only produces a dramatic increase in top speed, but is also responsible for all the other key features that this stability provides.
    The other major advantage of the incidence controlled foils is they are less affected by the waves and other surface affects. Drag and losses associated with the surface are the major reason incidence controlled foils are more efficient.
    All hydrofoil sailboats have problems with ventilation; however, surface piercing foils have larger problems, because the foils are piercing the surface at a smaller dihedral angle which makes it easier to ventilate."

    * On the Trifoiler the entire foil was moved to control RM, lift and negative lift hence the term "incidence controlled foils". On the Rave the incidence was generally fixed at +2.5 degrees for the main foils though some owners found a way to decrease the incidence on the windward foil. Lift and negative lift on a Rave foiler is generated by the wand (designed by Dr. Sam Bradfield), a surface sensor(dragging in the water) and attached directly via linkage to a flap on each main foil. The wands are independent just like the trifoiler "incidence controlled" foil sensors.

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  9. Gary Baigent
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    Gary Baigent Senior Member

    No argument in higher winds the positive/negative foil situation works BUT the Ketterman boats are absolute dogs in light winds, even moderate breezes when they can't lift free and plough along, near to the point of useless; they start their outboards and go home. And that is simply because of high drag, high wetted surface area.
    Imo there is a crossover point where the high lift minimum drag works at its best, a nice balancing act, enough lift to fly, or near fly and no drag up to windward; that I believe is the fastest the boat will go. Anyway, that is what I'm attempting to achieve.
    Agreed your direction with three foils is a good philosophy ... but two foils, is an even better one. I would like to see Hydroptere lift her windward foil sometimes, maybe not in stronger winds (which gives them some better roll stability?) but in moderate to light conditions - believe the boat would be faster, less drag etc.
  10. Doug Lord
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    Doug Lord Flight Ready

    The thing about my "three foils" is that at moderate and high speeds a single foil has all the load-the other two just control pitch.
  11. mtroyka
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    mtroyka Junior Member

    Well, I hate necromancy but this is an interesting discussion. :)

    This is oversimplified. The sail is going to roll the leeward foil into the water no matter what, so that drag/lift requirement is already a given. The sail will also use the leeward foil as a leverage point to yank the windward foil out of the water. Depending on how much lift the windward foil would normally produce, negative lift may or may not increase drag.

    This only applies to aircraft! An aircraft will reduce its alpha in flight as speed increases so that the lift (and induced drag) remains relatively constant (while parasitic drag increases by speed squared). If the angle of attack were fixed (as in most hydrofoils) then the lift and induced drag would be directly proportional to the speed squared, ie fully submerged hydrofoils produce excessive lift at high speeds. The extra drag has less affect on a boat since apparent wind also increases, but the ketterman boats max out at higher speeds since changing the AoA of the foil can reduce lift more than a trailing flap can (and probably with less drag).

    It's more that available righting force is reduced at low boat speeds, which would limit how much rig power the foils could hold at those speeds. Since apparent wind is also less at lower boat speeds, that may not be a major issue.

    That's an interesting concern I hadn't thought of. The interaction is probably different with negative lift, since the leading edge of the foil would be pointing away from the surface in that case. The ketterman foils didn't seem to have broaching problems, and the longshot was stable all the way up to the point where the foils were entirely consumed in cavitation (which was confirmed with tank testing // transcript). It seems to me to be one of those things where "it depends", and the broaching issue is more "compared to what?".

    I'd be willing to bet that the poor performance of the ketterman boats in light wind probably has more to do with their overall design than the foils themselves.
  12. CutOnce

    CutOnce Previous Member

    Might be worthwhile to google Dr. Speer. Here's a link worth looking at.

    I doubt there are many people more qualified to discuss these topics out there.

  13. El_Guero

    El_Guero Previous Member

    Lift and drag within fluids is a fairly straightforward problem. Both forces are always there.

    In a boat, you are trying to move your resistance from the water, high drag fluid, into the air, lower drag fluid.

    But, negative lift solutions have many problems.

    One that I did not see TSpeer mention was underwater collisions. As happened this year in the AC Cup race. If you lose a negative lift foil to an underwater collision, you do not settle abruptly and with difficulty, instead, you would fail to leeward (roll over).

    Immediately. Violently in most cases.

    Boat designers should follow the aeronautical mindset, never use a dangerous solution. And, always choose the safer solution.

  14. upchurchmr
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    upchurchmr Senior Member

    El Guero,

    You must be talking about a Commercial aeronautical mindset.
    Military fighters are deliberately designed as an unstable configuration, requiring continuous control via a computer controlled feed back system. The goal is performance, not safety (within limits).

    Not to suggest we start deliberately make foilers that are unstable but actively controlled. Nobody but Ellison and his buds could afford it. And there would be a steep and expensive learning curve!

    If you don't want a dangerous solution just don't sail fast or don't sail at all.

  15. El_Guero

    El_Guero Previous Member

    Yes, GA and Commercial Aviation versus Military Aviation is always an interesting dynamic.

    But, even military aircraft are grounded when they fail. And most of the actively controlled surfaces are only used because they cannot find a safer substitute .... IMHO.

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