Facing out wards foils on a lightweight tri.

Discussion in 'Multihulls' started by waynemarlow, Sep 22, 2014.

  1. Doug Lord
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    Doug Lord Flight Ready

    Outward pointing foils on a small tri

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    Kettermans foils provided both vertical lift and lateral resistance for the Hobie trifoiler. It was critical for best performance that the tip point inboard. He actually tested outboard pointed foils and they were no where near as good as the same exact foil pointed inboard. The way they are oriented is critical to the way they lift. Ketterman shows in his papers the foils(including T-foils) that have positive and negative pressure on the same side of the foil. Not only does that dictate which way the Trifoiler foils point, it is also the reason why the Trifoiler was faster than the Rave, according to Ketterman.
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    The thing Welbourn has done is to make a shallower foil that does not develop lateral resistance-he uses the keel for that. Same with his 14 footer that uses a daggerboard for lateral resistance.(See Aeronamics under "New Monohull Foilers 2016" in the Sailboats forum here: http://www.boatdesign.net/forums/sailboats/new-monohull-foilers-2016-a-54975.html posts 7 & 13 ).
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    The IMOCA foils work on the same basic principle that uptip foils work: the outboard portion that is pointing up is nearly equivalent to the daggerboard portion of an uptip foil-and the part going into the hull is equivalent to the "up-tip" portion.......
     
  2. Doug Lord
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    Doug Lord Flight Ready

    What's a Moebius foil?
     
  3. sigurd
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    sigurd Pompuous Pangolin

    A Moebius strip is a strip of surface that is connected to itself so that if you walk around it, one revolution you will spend on the upside, the next you will be on the underside, and then back again, without actually having crept around to the other side. It is confounding, and I thought, if we have a moebius foil, can we have infinite span while lifting to windward and up at the same time, with no pressure reversal? Just a joke.
    wikipedia
    [​IMG]
     
  4. Doug Lord
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    Doug Lord Flight Ready

    Moebius Foil

    Thanks.....
     
  5. waynemarlow
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    waynemarlow Senior Member

    Doug, I'm not sure about this, if you think about the differences between a dinghy and a Tri lee float when the boat is heeled in light wind, then you can by balancing the dinghy control the amount of foil in the water and angle with human ballast, controlling the amount of lift you need and limit the amount of drag to a minimum. With the Tri, the whole foil is fully submerged all of the time creating a lot of low speed drag, probably enough to prevent foiling in low winds and through the large size needed to lift the entire boat at lighter winds, preventing high speed in higher winds, although I guess it would act as a Vfoil by lifting higher with speed reducing surface area, until an equilibrium was reached.

    My guess is that on the Tri for the outward foil to work, you would need to have a centre T foil as well to lift the main body, as per your model, but then you wouldn't need such large Ama foils, more just enough to act as stabilising foils, which had been my original aim, that of a main hull skimmer with stabilising foils when things go wrong and with enough speed, to lift the main hull with RM and dagger board resistance to sideways load and maintain foiling of the lee ama, thus only needing a very modest foil surface area.
     

  6. Doug Lord
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    Doug Lord Flight Ready

    Outward pointing foils on a small tri

    If the tri was designed with the right amount of crossarm dihedral there is no reason that the immersion of the lee foil could not be 100% controlled. If the boat was designed specifically for light air foiling it wouldn't matter. The windward and leeward ama foil would almost never touch the water at the same time. With the ama foils retracted and the boat balanced neither ama would touch the water.
    You're right that a boat like this would benefit from a foil on the daggerboard(+ a rudder T-foil) to aid light air takeoff.
    The size of the foils would be dictated by the load they carry and by the wind(boat) speed you want the thing to takeoff in*. The Q23 has very low foil loading(lower than a Moth) and can therefore foil in a 5 knot wind or less. Thats a good target to shoot for.

    * The lee ama foil has to be designed to take around 80% of the load even if you have a daggerboard T foil because the faster the boat goes the more the main foil unloads and the more the lee ama foil loads up. So you couldn't use shorter ama foils if you also use a daggerboard foil IF you keep the foil loading low-which you should. And the maximum RM depends on a long lee ama foil....I wouldn't go shorter than the Q23 foils scaled for your tri's length.
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    >Largely a MYTH: this idea that if you have foils that work well in light air they can't deliver top end speed is flat wrong in many cases. The Welbourn, UptiP, surface piercing foils can be designed to have max lift in light air by twisting the foil , then as the boat speeds up that part of the foil comes clear of the water allowing both light air foiling and high top end speed! Fully submerged wand controlled foils can change their coefficient of lift with a movable flap(or by moving the whole foil) to enhance their light air takeoff. They can't change their area as they speed up but on multihulls they can generate loads of additional RM to increase top end speed**. An example is the Whisper that claims to be able to foil in 4kts of wind and yet has high top end speed(over 25 knots so far).
    ** By using dual, independent wand(altitude sensor) controlled main foils.
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    On the Test Model the foils are not retractable because of RC considerations-on the full size boat they would be. With Welbourn foils they could be 100% retractable and the boat could be balanced with neither ama even touching the water:

    [​IMG]
     
    Last edited: Jun 16, 2016
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