upside-down hydrofoils

Discussion in 'Multihulls' started by rlawler, Oct 1, 2010.

  1. rlawler
    Joined: Oct 2010
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    rlawler Junior Member

    Many cats and tris have been made with the hulls placed quite far apart (wide overall beam) in an attempt to keep the sail vertical.
    Does anyone know if someone has tried the concept of using an inverted hydrofoil to windward to generate a DOWNWARD force to counteract heeling?
    Thanks,
    Rick
     
  2. jehardiman
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    jehardiman Senior Member

    Why? all it causes is additional hull drag for a given induced drag. It is better to use the foils to generate a "up" force on the leeward hull, which reduces hull drag for a given induced drag. Go look at photos of hydrofoil multihulls such as Hydroptere.
     
  3. Doug Lord
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    Doug Lord Flight Ready

    ======================
    Both the Rave, Hobie Trifoiler, the 40' SKAT and the new 18' Osprey all use systems that do that. These systems use dual independent altitude control systems. On the windward and leeward side of the Rave there is a "wand" which is basically a stick dragging in the water. The wand is set up so that the flap on the foil is in neutral with the wand angled back about 45 degrees.
    The two forward foils on each side are set to a +2.5 degree angle of incidence relative to the static waterline of the boat-which is parallel to the flight waterline.On some versions the angle of incidence of the windward foil is automatically reduced.
    When the boat is moving slowly the wand is angled back way more than 45 degrees and that causes both flaps to go down creating extra lift to get the
    boat on foils.
    Now this is the key part: After the boat is on foils- when it just starts to heel a little- the windward wand moves forward of 45 degrees and that moves the foil flap up reducing lift on the windward foil. The opposite thing happens on the lee foil. The windward foil will gradually pull down and the lee foil will always lift up. Between them they generate all the righting moment for the boat so the crew can just sit in the middle.
    On the Rave this force is fearsome and can destroy the boat if it is sailed past the recommended speed. When close to that speed(30knots approx.) the sails must be depowered or reduced to eliminate the chance of an accident. There is a speedometer in the cockpit so the crew knows when to be afraid.
    Dr. Bradfield, who designed the Rave thought,at one time, that it might be better to move the crew rather than just have them sit there because that would reduce the work the foils had to do to create righting moment. I think that may be the case on his newsest boat-the Osprey.
    Hope this is helpful....

    Go to the very bottom of this page and view the Rave/Osprey control animation: http://sites.google.com/site/hydrosail/HydroSail-Home

    pictures: Rave under sail and Rave foil system+wand(click on image)
     

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  4. jehardiman
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    jehardiman Senior Member

    Doug, my take on the OP was that he was not talking about fully controlable foilborne operations, but rather using the foil down force as added righting moment when flying a hull.
     
  5. Doug Lord
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    Doug Lord Flight Ready

    ======
    May have been. Gary Baigent uses fixed fully submerged foils to lift to leeward but they're not controllable and I think your post applies well to them.
    In almost every instance I know of, when a foil is used just for stability it does not reduce the wetted surface of the hull it is attached to and is,therefore, of dubious value. Stability is "paid for" more easily,with less drag, by moving the weight that is already there. Bradfield and Speer have both made comments to that effect and Speer references the 40' Aussie foiler Spitfire that instead of varying the angle of incidence on its surface piercing foils carried ballast. Hydroptere carries ballast as well. If the foil(s) lift the boat they, at least, reduce drag enough to increase speed dramatically in most cases. But a foil for just stability seems like a waste.
     
  6. Eralnd44
    Joined: Oct 2006
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    Eralnd44 Wanderer

    foil only at rudder makes pitch control also stable sailing. i read you like that mr doug. here you say is to be waste. i stand confused to your words. how to go from this place.
     
  7. rlawler
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    rlawler Junior Member

    Thanks for the responses.
    Doug your reply shows that, yes, apparently several designers have already thought of that and are using it. The Rave pics show the idea taken to the extreme--windward hull out of the water and it would keep pivoting around to the point of capsize if not for the hydrofoils on that side keeping it held down. I was thinking not so much in terms of minimizing hull drag but purely in terms of--what forces are available, besides gravity (hull weight) to keep that sail vertical?
    Rick
     

  8. Doug Lord
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    Doug Lord Flight Ready

    -------------------------
    Rick, hydrofoils can be wonderfull things-particularly when used to reduce wetted surface. But even then they are not necessarily God's gift to the speed hungry: foils were used on a Little Americas Cup contender catamaran and they lifted the boat out alright-it just wasn't fast. It had four foils. Using foils must be considered very carefully.
     
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