Roll Control using foils

Discussion in 'Sailboats' started by Wingz, Oct 19, 2010.

  1. P Flados
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    P Flados Senior Member

    Using horizontal foils extending from the sides of a monhull beneath the water line would probably be more usefull in reducing roll motion, than trying to fully couneract the roll moment from the sail.

    Upward forces to the leeward side can be is most usefull in that it helps ballance the roll moment created by sails and helps lift some of the main hull out of the water (reduced wetted surface). However, since the moment arm for the sails will generally be longer than the moment arm for the foil, a fusl balance of forces would required much higher forces at the foil. Using a foil to produce large forces results in a lot of what is called induced drag. On the windward horizantal foil, you would generally want to avoid any down force as this not only results in induced drag for the foil, but it pushes the boat lower in the water and is a double penalty. For most boats it would be better adjusted to give a very small amount of lift from this foil given that you get less hull in the water for very little drag (foils are generally very efficient a small angles of attack).

    Even though using these foils for large forces may not be a great idea, they could be used in conjunction with ballast to improve performance. Standard ballast forces are the primary forces that balance the "time averaged" roll moment from the sail. As a boat moves through the water, you try to keep the sail at an optimum trim. With large amounts of angular motion (poor pitch stability or roll stability) portions of the sail are constantly at a bad angle of attack resulting in increased drag and reduced lift. Reducing and/or slowing the angular motion of the boat can thust improve sail efficiency (more forward thrust and less drag). The same principles would be at work below the water on the keel and rudder.

    To get a real improvement would probably require fancy controls with a computer that can optimize things based on ever changing conditions. Also I would only expect an improvement in rough conditions. In smoother conditions, the extra drag for having these foils in the water would be just a plain loss with no benefit.

    With minimal overall pefromance advantage, I see this technology as a real plus more for situations where smoothing the ride for the occupants is the goal. For this use, both forward and aft foils would be used to help with pitching and heeling.

    All of this leads one to feel that the most likely use for this technology is for boats where sailing in rough conditions is to be a large part of the boat mission and where crew comfort is the goal. Although most distance sailing routes are planned to avoid rough conditions as much as possible, in some parts of the world this is more challanging than typical.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    P Flados, check out the DSS website posted earlier. These retractable foils
    are currently being used on a number of boats and have been extensively tested.
     
  3. Wingz
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    Wingz Junior Member

    I think putting in that vector would help clarify that diagram. However that diagram is only intended as show in simplest form why a sailboat can sail into the wind.

    What I am doing is working with two other vectors. One is the torque vector which is the heeling force on the mast, the other is the torque vector which is the counter heeling force of the underwater wing. With proper extension of the wings, the two would cancel out and the boat could sail at 0 degrees heel - completely level, while maintaining directional stability.
     
  4. Wingz
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    Wingz Junior Member

    One of the future embodiments of this technology is for cruise ships. By extending and retracting several pairs of the wings in rhythm with the wave action that causes the constant rolling of the ship, and the resulting seasickness, that action could be minimized. One excellent method of commercialization.
     
  5. Wingz
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    Wingz Junior Member

    Thank you!
     
  6. Doug Lord
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    Wingz see post # 30 at the bottom of page two. If the boat is not to drift sideways the force from the keel must oppose the force from the sail-otherwise there would be zero directional stability with the boat moving fast sideways. Righting moment can be created in a number of ways but this way(see your illustration below) eliminates directional stability upwind.(the boat in that illustration would move sideways) The image on the right is the way the forces MUST be aligned to go upwind. Righting moment can be achieved with an angle of heel on a keelboat or with crew hiking out or on trapezes or with DSS foils in combination with either or with a canting keel etc. but not with a foil whose force is roughly parallel and in the same direction as the force on the sail as shown in your illustration.

    click on image:
     

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  7. Cheesy
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    Cheesy Senior Member

    No you arent both of your forces/vectors are already there, the torque is created by the 'pull' vector acting at a distance up the mast from the centre of bouancy, there is a second 'keelpull' lets say acting at about 90 deg to the keel at some depth to try and tip the boat about a longitudinal axis (roughly), this is because as the boat moves through the water the keel produces lift (or now 'keelpull') even though it is a symetric section (guessing it will have maybe a 1 to 10 degree AOA). Then there is the keel mass (if it is ballasted) which will try and bring the boat upright again.

    edit: Dougs post explains it much better
     
    Last edited: Oct 20, 2010
  8. Wingz
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    Wingz Junior Member

    Please check posts # 20 and #11- - - - -and #9 just to keep things in prospective.
     
  9. Cheesy
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    Cheesy Senior Member

    Ok here is an experiment for you, it will help if you have a GPS and a compass, try sailing along with the boat as you have built it, note the velocity and both the GPS and magnetic compass bearings, then sailing on the same tack swap to the centre board that is not meant to be down and measure the same variables and bring them back here.

    The aircraft analogy is not correct for a sail boat
     
  10. Doug Lord
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    Doug Lord Flight Ready

    ---------------------------
    I think I've found something that may help you. You can't have a "righting force" acting parallel and in the same direction as a "heeling force".
    If you analyze the way righting moment is created and how the force acts that results from it you will see that any force acting to right a sailboat MUST act more or less perpendicular to the waters surface(veiwed from aft)-whether it is the weight in a keel, the lift from a foil or a crew on the rail. And any force causing a boat to heel is acting more or less parallel to the waters surface(veiwed from aft)-like the force on a sail or the counter force on a keel.


    In the real world, even though your vertical fins are asymetrical with the curved side to leeward(the wrong way) the boat will still try to use that board
    to prevent leeway-it will just do so poorly. If you were sailing on port tack with your lee fin down(with the curved part of the foil on the wrong side) you would see a startling difference and significant improvement in your upwind ability if you retracted that foil and put the other foil down (with the curved side to windward).
    Have you sailed your skiff yet? I'm willing to bet that the crew will still have to provide the righting moment with the lee board down-even though it is the wrong shape.


    Notice in the illustrations below the directions the forces are acting: buoyancy vertically up, ballast vertically down. This is the crux of your misunderstanding! The forces of righting MUST act approximately 90 degrees to the forces of heeling when sailing upwind.
    Heeling Force=parallel to the waters surface upwind
    Righting Force=90 degrees to the surface of the water(vertical)

    Wingz, this is the general way it MUST work in every case. If you can integrate these facts with the previous illustrations you'll get it.
     

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  11. daiquiri
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    daiquiri Engineering and Design

    Doug, compliments for the patience. I have given up on this one.
     
  12. Wingz
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    Wingz Junior Member

    If I did that, I would capsize almost immediately on the second pass.
     
  13. apex1

    apex1 Guest

    That will definetively not happen!

    Apart from being useless on sailing boats, your system is not a improvement for motor vessels either. Quite the opposite.
    The systems installed today are proven, reliable and sufficient at least.

    But it seems you are not open for critical comments, just searching for confirmation of your preconception.

    Regards
    Richard
     
  14. Wingz
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    Wingz Junior Member

    Does that mean that you are on my side then?:)
     

  15. Wingz
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    Wingz Junior Member

    In fact, I can have a righting force acting in any direction I want, up, down or sideways, as long as it opposes the heeling force.

    Your arguement describes the workings of ballast, which is a gravity based solution, to balance the dynamic forces of the wind on the sail. Ballast, that 30 to 50% of your total displacement, that limits you to 8 knots in a good breeze, only works in one direction - down. When you are sitting level, that ballast has no righting force what so ever, and it's only effect is to pull your boat down in the water so that the various sea creatures have more surface to attach to.

    Ballast only works when your boat heels, and it works because that rotation lifts that huge weight, (that may be half the weight of your boat) from its resting point, directly below the center of buoyancy, to a point to the side of and above its resting point. The height that that weight is lifted provides your righting force, which is always in the down direction because that is the direction gravity pulls it. The maximum righting force is when your boat is on its side. This solution dictates that for your boat to move, it must heel, and to sail your boat effectively, you must have one leg longer than the other.

    This is in contrast to the workings of a foil, whos' lifting force is always at right angles to the top of the foil. I can attach that foil at any angle that I find most advantageous.

    Now if having two opposed wings that can provide roll control and directional stability at the same time, is too much of a challenge, we can always add a dagger board or other external fin(s) to separate the problems and solve that directional problem traditionally. The target here is that the sail is a foil, and the hydrodynamic wing is a foil. They both work for the same reason.

    Balancing similar forces is a simpler and more effective solution than balancing a foil with gravity.
     
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