Foil Assist: small or large monohulls

Discussion in 'Sailboats' started by Doug Lord, Dec 11, 2005.

  1. Doug Lord

    Doug Lord Guest

    I'm thinking of this particularly in reference to a small boat in the Moth range or a bit larger but I imagine it could be used to one degree or another in large mono's as well.
    On a small boat I envision a boat designed to plane but that would use a shorter, longer chord, lower aspect daggerboard foil than normal with a hydrofoil on the bottom -same on the rudder. Both daggerboard and rudder foils would be retractable and the mainfoil would have an easily adjustable(with a knob+screw?) angle of incidence.
    The foils would be short to prevent a crash from too much altitude if the boat got going fast enough for 100% liftoff and to permit the foil to be retractable(foil needs to be on the bottom of the rudder or daggerboard for this to work).
    Area would be based,loosely, on the same foil loading as the Moth so they would have more or less half that area for the same weight or more than half for a heavier boat.Lots to learn- maybe the foils could lift 60% of the weight?(Foil lift would be apportioned at approx. 80% on main foil; 20% on rudder foil)
    There would be no altitude control system since the hull effectively does this when the foil is properly set up.
    ORMA 60 multies use "foil assist" with curved daggerboards that theoretically take around half the weight of the boat reducing the ama loading. I bet this would work and be a good way to break into full flying foiling and maybe be a blast in its own right.
    What do you think?
     
  2. tspeer
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    tspeer Senior Member

    The Catri trimarn uses much the same principle. Foil assist, with the hull partially in the water for stability and trim.

    There's really not much drag you're going to reduce by shortening the board. Any effective extension to the span by a foil at the end costs more wetted area than physically extending the span would.

    As for whether lift on the foil would reduce the drag, a foil's optimum L/D occurs when the induced drag and profile drag is equal. Since the foil is wetted on both sides, it must reduce the wetted area of the hull by roughly four times the planform area of the foil to break even.
     
  3. Doug Lord

    Doug Lord Guest

    foil assist

    Thanks ,Tom. The shortening of the daggerboard/rudder was an idea for a foil trainer that would hopefully prevent crashes from too much altitude if the foils ,for one reason or another, developed too much lift.It's not necessary or desirable on most monohull applications of "foil assist".
    I looked at the foil performance(roughly) for a 785 pound SDB(Sliding Deck Ballast) boat I'm designing with 255 sq.ft.SA,a D/L of 60, SA/D=48 and a SCP/disp.=30%.
    For foils with a total area of 2.68 sq.ft. one side it looks like "break even" according to your definition, would be just under 10 mph with a 63412 foil at a Cl =.6 and at 12mph the reduction in wetted surface would be 7.27 times the foil area . Using a flap @ 20° and a higher AOA to get
    started "break even" could occur at 7mph.
    --
    I didn't factor in the boat most likely starting to plane at around 8mph which will be interesting in combination with the foil assist system.
    Seems to me that the concept has potential at least on this type of boat.
     
  4. Tim B
    Joined: Jan 2003
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    Tim B Senior Member

    If you're using foils, you might as well get the whole hull out of the water (unless it is utterly impractical). Advantageously the wave drag then drops to pretty near zero.

    I would suggest that the strut for the rear foil be fixed, and independant of the rudder. This should reduce excessive sideslip on the rear foil, and thus excessive drag. The biggest problem I foresee is control. the boat will not "fly itself" due simply to the setup needing to be different for different conditions. I think there may be an advantage (for a smal boat) in using a gyro and servo setup to control the angle of attack of the rear foil, leaving the forward foil fixed. If you don't use some form of control you'll likely end up with a new form of porpoising.

    Best of Luck,

    Tim B.
     
  5. Doug Lord

    Doug Lord Guest

    foil assist

    Tim, I see "foil assist" being used on monohulls that would otherwise plane and the "foil assist" just helps them to plane earlier and faster. The idea of partial lift eliminates the need for an altitude control system. The most successfull foilers that fly completely clear of the water use an altitude control system based on a "wand" surface sensor;. The wand is tied into a flap on the main foil not the rear foil(so far). On both the Rave multifoiler and the Moth monofoiler the rear foil flap control is seldom used. A guy built an I14 using manual control of flight altitude using the rear foil flap and nothing on the mainfoil-it worked but is not the ideal setup.It appears that for best control moving the flap on the mainfoil is the way to go(or maybe both simultaneously as Tom Speer has suggested).
    "Foil assist" is used on the Orma 60 trimarans to reduce the leeward ama displacement when the boat is flying the main hull.
    Foil assist can offer very much increased pitch stability as well as wetted surface reduction w/o the attendent problems of a full flying set up requiring altitude control.Since , in my monohull application, the boat will be planing wave drag is already overcome; "foil assist" just reduces the amount of load the hull surface has to support while planing.
     
  6. Tim B
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    Tim B Senior Member

    So you are thinking of using foils to control what in power-boat terms is the running angle. I don't see how you can hope to do it in an anything like optimisable way without some form of control system. Also, there will still be wave-drag, because there's still something sticking through the water.

    Aircraft have been using a single tail-mouted elevator for many years (almost since conception) birds also use there tails for pitch control very sucessfully. You are into a question of aircraft stability and tail moment arms. Shorter moment arm, bigger elevator, but the faster it will respond. opposite for long moment arms. I have been flying model gliders for many years, and have found that given sufficient elevator, almost any glider (particularly true of slope-aerobatic models) is very responsive indeed. Sometimes there is a need to couple flap-mixing in with the elevator. This gives greater Clmax and is particularly useful around tight turns (as in slope pylon racing).

    I think this idea needs some investigation, which should not be difficult for a man of your capabilities.

    Tim B.

    I make no apology for non-marine terms used in this post, they can all be Googled if necessary.
     
  7. Doug Lord

    Doug Lord Guest

    foil assist

    Tim, there are numerous examples of the use of foil assist in multihulls: the Orma 60's, the Catri Tri and even on Steve Clarks "Patient Lady" C-class cat(banana foils + rudder t-foils)-all use(d) lifting foils w/o an altitude control system. I'm not familiar with any examples currently being used on monos-but there is no reason, that I know of , why it wouldn't work.You're 100% right: careful design and testing will be required for it to work well.
    That is not to say that even with the best designed applications there aren't problems: the Orma 60's have been known to exceed the design speed of the foils which causes the ama(with main hull flying) to come clear of the water. When it does the foil is likely to ventilate since it has all of a sudden gone from a fully submerged foil to a surface piercing foil and if it does ventilate a crash can result. If that doesn't ruin the day another problem may: many of the boats used the part of the ama still immersed as the main source of pitch stability; when the ama comes clear of the water the boat is unstable in pitch even on those boats equipped with t-foils on the main hull rudder(diagonal roll).
    On the monohull version the foils won't so much control the running angle as BE CONTROLLED by the running angle.They will ,however, tend to prevent divergence(pitchpole) from the nominal high speed running angle of the boat.Lot's going on but I believe a workable system is possible...
     

  8. Tim B
    Joined: Jan 2003
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    Tim B Senior Member

    You might like to take up the question of fixed control surfaces with some free-flight aeromodellers. They are gods at this type of thing. One thing you notice though, is that there is a lot of cleverness in setting up said models, and they are only set up for one speed. That is not the most efficient solution for a boat, as I think you wil agree.

    Tim B.
     
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