Foils for sailing boats

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

  1. TANSL
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    TANSL Senior Member

    Thanks for your answers @sigurd.
    Point 1 is clear.
    Regarding point 2, I know what cavitation is and why it occurs. I asked the question because I have heard that the big designers of the foilers are working precisely in this matter, to avoid cavitation so that it does not suppose a limit to the speed of the boat, but I could not imagine how it can be avoided. Now, with your answer I have the matter somewhat clearer. It seems that the studies go there to get the few possible improvements in the foils. That and the influence of turning the foils about the "Z" axis. What do you think?
     
    Last edited: Nov 13, 2017
  2. sigurd
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    sigurd Pompuous Pangolin

    What is the Z again?
     
  3. TANSL
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    TANSL Senior Member

  4. sigurd
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    sigurd Pompuous Pangolin

    Do you mean in order to turn the hull towards the wind and reduce air drag?
     
  5. TANSL
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    TANSL Senior Member

    I'm not sure what the expert meant when I heard his comment. I interpreted that it was about turning the foil very slightly with respect to that vertical axis but keeping the course of the ship with the rudders.
     
  6. sigurd
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    sigurd Pompuous Pangolin

    I'm not talking about the course but the heading - where the bows are pointed. Clearly, moving a boat sideways through air is draggier than moving it forward through air. So when the boat is free of the water it is only sensible to turn the bows into the wind. So then you need to turn the main foil in Z axis as well as the rudder. I imagine some interesting crashes will result however...
     
  7. TANSL
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    TANSL Senior Member

    I see, I had not realized that the ship can take that position with respect to the wind you describe. Surely that is what they are studying as the possibility of winning in speed, although stability is more precarious.
    Thank you, you have been very helpful, I think I now understand something better, the catamarans with .foils.
     
  8. sigurd
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    sigurd Pompuous Pangolin

    I really want to be able to do a 'flying shunt' one day. #flying tack #proa
     
  9. TANSL
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    TANSL Senior Member

    Interesting idea I would like to have information about it as that idea develops, if you continue with it.
     
  10. CT249
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    CT249 Senior Member

    Is it? Time trial bicycles are, I think, lower drag when they are moving at an angle to the airstream (ie "moving sideways") than when going straight into the wind. If a shape as unwieldy as a cyclist and bike can create lower drag at an angle, why not hulls? There is also, of course, the issue of what happens if you touch down with the hull at an angle - in my very limited experience, it hurts.
     
  11. TANSL
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    TANSL Senior Member

    I guess you have to take into account not the direction of the real wind but that of the apparent wind, which varies as the speed increases.
    This also leads us to think about the number of sensors needed, the data collection and data analysis systems that provide the helmsman with instant information for decision making.
     
  12. CT249
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    CT249 Senior Member

    Yes, the time trial bikes also take into account direction of the apparent wind.

    Personally I feel if you're going to give the helmsman too many sensors you may as well just replace them with a computer, and while we're at it why bother to use sails when a simple off-the-shelf outboard motor or two will do the job?
     
  13. TANSL
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    TANSL Senior Member

    Yes, I agree that, in these machines, the art of sailing no longer makes sense, a good navigator is no longer useful, but I suppose that, although there is a computer on board, important decisions remain that only the navigator can adopt.
    In the last America's Cup the crew had more cyclists than sailors. That has nothing to do with ships like the "America". The human factor has lost much of its value but, perhaps, we have to sacrifice everything because "show must go on".
     
  14. sigurd
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    sigurd Pompuous Pangolin

    It would be nice indeed to have computers take control of the boat in commercial sailships, otherwise it's going to be much more expensive to run them.
    I haven't followed Skysails the past few years, but they had a system where you just pushed a button, it was described, and the machines would launch, fly and retrieve the kite with no manual help.

    CT, that's interesting about the biker, but he can hardly pass for a racing foiler even if I squint. I'll give you this, we are very good swimmers for a primate.
    The projected frontal area of a boat going sideways is bigger than on one going forward, and I would bet you a beer the Cd is bigger too. Look at a high speed foiler cat from 20' (?) off the bow, and tell me what you think.
    Then go and look for some colour CFD of fast cats, the ones with the swirls showed, if you still don't agree with me.
    Finally ask yourself, which shape can be optimised best: One that has to be optimised for two angles 40' apart, or one that is optimised for only one angle.
    Are you in for a beer?
     
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  15. HJS
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    HJS Member

    Can a supercavitating foil with an interceptor according to Achkinadze and Fridman be an option for a foil which works closely and in the watersurface?

    Ryssfoil 2.jpg
     
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