Foils for sailing boats

Discussion in 'Hydrodynamics and Aerodynamics' started by TANSL, Oct 22, 2017.

  1. TANSL
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    TANSL Senior Member

    I am an admirer of the idea of applying foils to sailboats. I do not think they are the future, I think they are the present and the only thing left to do in them is to slightly improve some aspects. But I'm not an expert on them at all. Although I can think of several questions, now I would just like to raise a couple of them :
    1. why do foils can improve stability? How does the performance of sailboats improve?
    2. when the speed increases, cavitation may occur. How is this phenomenon handled and how can it be used to increase the effectiveness of foils, or at least prevent this phenomenon from limiting the maximum speed that the boat can reach?
    Wing sail + foils boats : why they do not need a spinnaker when they go for a run?
    Thank you for your answers.
     
    Last edited: Oct 23, 2017
  2. Jim Caldwell
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    Jim Caldwell Senior Member

    Foiling boats don't go DW because they are fast enough to bring the AP forward, that is why you see them sheeted in no matter the true wind angle.
     
  3. DCockey
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    DCockey Senior Member

    Also, fast foil sailboats "tack" going downwind rather than going directly downwind. As Jim Caldwell said above that brings the apparent wind forward so that from the boat reference frame it is always going upwind.
     
  4. TANSL
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    TANSL Senior Member

    Jim Caldwell, I'm sorry but I do not understand you, probably because I'm not an expert. I do not understand why foiling boats do not go down wind. Can not they go down wind ?. I believe that has more to do with the type of sails or sails/wings than with the foils, but it is likely that I am wrong. Excuse me if the answer is very obvious and I am not able to deduce it.
    DCockey, I understand how it can increase and vary the direction of the apparent wind but, what does that have to do with foils?
     
  5. DCockey
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    DCockey Senior Member

    TANSL, Jim and I were responding to your question:
    Foil boats could go directly downwind, and could use a spinnaker. But that would be slow because a sailboat going directly downwind will always be going slower than the true wind speed. Very fast sailboats can get downwind faster by "tacking" - heading perhaps 30 degrees to one side of downwind, then 30 degrees to the other side. (The optimum angle to get downwind the fastest depends on the characteristics of the boat, similar to the the optimum angle for going upwind.) The boat speed can then be much faster than the true wind speed, and the resulting apparent wind will be sufficient to keep the boat up on foils going fast.
     
  6. TANSL
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    TANSL Senior Member

    David, first of all, thank you very much for your response and also thanks for Jim Caldwell's.
    I interpret that ships with normal foils and sails do need spinnaker, if they want to take advantage of the force of the wind when they sail down wind. I think the case of sails-wings boats is different. The apparent wind may be greater than the actual wind, I see that clearly, but I do not know how the apparent wind could turn to "so that from the boat reference frame it is always going upwind".
    See my schematic and tell me, please, where is my error, what am I doing wrong and how should I approach the subject.
    Snap32.jpg
     
    Last edited: Oct 23, 2017
  7. David Cooper
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    David Cooper Senior Member

    It does seem extraordinary at first, but a boat can make progress downwind faster than the wind and can beat a balloon in a race by zigzagging instead of going directly downwind. This has been known for a long time with ice yachts where there's very low friction and the speeds are ridiculously high. Now that we have foiling boats, the friction is similarly low and the speeds are sufficiently high for the boat to travel more distance by zigzagging and still beat a balloon that's following a direct path downwind. When sailing at these deep angles, the apparent wind is from ahead because the boat is moving so fast, and the sails have to be pulled close in as a result. The part that's difficult to get your mind round is how the wind can push a boat downwind faster than the wind is itself moving, but all that matters is that the wind is able to keep pressure on the concave side of the sail and generate a vacuum on the convex side, and it can do that even when the boat is overtaking it.

    Another way to think about it is to consider a wedge. If you take a wedge and squeeze it between two blocks, the wedge will shoot out from between them in the direction of the thicker end of the wedge. The wind pressing against the sail is one of those blocks while the sail is one side of the wedge. The other side of the wedge is the vertical part of the foil in the water, and the water serves as the other block. The boat (sail in the air and vertical foil in the water) is the wedge being squeezed between the wind and the water, and the boat is pushed in the direction of the wide end of the wedge.
     
  8. TANSL
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    TANSL Senior Member

    Thank you very much David Cooper, Iit's not easy to imagine but I think I'm beginning to understand what is happening. The simile of a wedge is illustrative. I seem to understand that the triangle of velocities I have drawn can not be applied in these cases because it is rather how and where areas of overpressure and depression are generated in the sails.
    On the other hand, what I think I understand is that the foiling boats do not need to go completely down wind because, although they travel more distance doing zig-zags, the final speed (distance from the origin to the end point divided by time) would be much greater than that of a boat with a balloon going directly downwind.
     
  9. HJS
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    HJS Member

    Snappy.jpg Apparent wind as I experience it when it goes fast.;)

    JS
     
  10. TANSL
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    TANSL Senior Member

    @HJS, yes, you are right, my sketch was incorrect. I have changed it for a more correct one.
    @CT249, I don't know that foils are the present, it's just one opinion. Well, I do not want this opinion of mine that, like all, is subjective, can spoil the good direction of this thread. Two days ago I attended a conference of the naval architect Manuel Ruiz de Elvira and I have deduced that it is only very specific aspects that remain to be studied in the foils. That's why I said foils are the present, not the future. I think that can be said without being an expert in foils.
    I do not want the thread to drift in that direction, but of course we can comment on anything you want.
     
    Last edited: Oct 24, 2017
  11. CT249
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    CT249 Senior Member

    I pulled the post to avoid drift. Thank you for being polite.
     
  12. TANSL
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    TANSL Senior Member

    I suppose they can control and regulate the height on water at which the hulls "fly". If so, how is it achieved?. Thanks in advance for your answers.
     
  13. David Cooper
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    David Cooper Senior Member

    There are two ways of controlling flight height, one active (with many variations) and the other passive. The passive approach is simple - have a tilted foil so that the deeper it goes in the water, the more lift it generates, and the higher it goes, the less lift it creates, so it keeps returning to the altitude where the amount of lift is equal to the mass of the boat. The angle of attack of the foils is usually adjustable so that the flight height can be set to the same level in strong winds as lighter ones.

    Active foils usually have an adjustable flap or allow rapid changes in angle of attack so that the amount of lift generated can be adjusted moment by moment with the whole foil remaining underwater. A "wand" is often used to read the height of the boat above the water's surface so that the foil can be made to generate more lift when the wand is high and less lift when it's low. It's also possible to control the flap angle manually with some boats, but it's hard for human reactions to keep up with events and it's normally essential to have some degree of automation.

    To see passive foils in action, look up videos of the Flying Phantom (L foils used for the most part as if they are V foils) and A Class cats (with "Z" foils) - they use sloping foils to regulate altitude, although some of the altitude control also comes from the foil being more efficient at depth rather than near the surface. For Active foils, look at videos of the Moth, UFO and Whisper (which all use T foils). The America's Cup boats have L foils, but they have a degree of automation in controlling the angle of attack which allows them to use the main lifting part of the foil horizontally without the altitude of the boat rapidly going out of control - without rapid changes in angle of attack of a horizontal foil, the boat will likely porpoise, jumping up suddenly and crashing down violently as the amount of lift changes.
     
  14. TANSL
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    TANSL Senior Member

    Thanks David, I have to read carefully and think about your answers. Probably more questions will come up. Thanks again.
     

  15. sigurd
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    sigurd Pompuous Pangolin

    1. If you put them far out to the sides, they can work like an extra hull when at speed. They then counteract the heeling moment of the sail.
    If you put them forward, they can lift the bow, which is important to many fast boats, so they don't fall 'head over heel' or 'over the handle bar', to use a bicycle analogy.

    2. Cavitation occurs when: The water is moving so fast over the top of the foil, that the pressure is reduced so much, that the water boils. The steam bubbles increase the drag and decrease the lift for normal foils dramatiacally, and they can collapse (implode?) so violently that they ruin the foil surface. If air is available to the low pressure side of the foil, then that will equalise the pressure and replace cavitation with ventilation. Ventilation can happen at much lower speed but only near the surface.

    Ventilation and cavitation is handled the same way, to reduce the negative pressure peak on the low pressure side of the foil. This can be done with particular shaped sections, and with sweep forwards or backwards.
    Some foils, like Jon Howes surface foil, are made to ventilate, so that they will never cavitate. His foil also runs on the surface as soon as it goes fast enough so it doesn't need active height control.
    howesfoil2.jpg
    howesfoil4.jpg
    The speed limit for a subcavitating foil is around 60 knots I read (more with sweep?). Incidently Jon's foil was originally designed for a 100 kt+ boat called 'monofoil'.
    monofoil_anatomy.png


    If you want to know more about cavitation then search in tspeer's posts. He's a jedi at explaining and I remember he's had a fair bit to say about both stability and cavitation.
    This is his cavitation graph.
    Cavitation.gif
     
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