sail propulsion

Discussion in 'Sailboats' started by JimHog, Jun 30, 2007.

  1. JimHog
    Joined: Oct 2006
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    JimHog New Member

    I understand the "new" theory of how the sails work has to do with lift in the same way an airplane wing works:
    1. how did the early sailors think the boat moved up wind ?
    2. when did the newer idea about "lift" come into acceptance ?
     
  2. PI Design
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    PI Design Senior Member

    Well, in the very old days, boats didn't sail up wind. A beam reach was abour as good as it got. Manfred Curry wrote a book in circa 1913 detailing sail aerodynamics, which is probably the earlist publication. I think as soon as aeorplanes were invented, the similarities to sails would have been spotted. Leonardo da Vinci may well have had an idea about lift, but not the physics behind it.
     
  3. jehardiman
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    jehardiman Senior Member

    All sails work by drag. The wind pushes them, and that is what sailors believed, and should continue to believe. "Lift" just happens to be drag in the direction you want to go. See Marchaj.

    A Hellic/Roman/Viking era square sail could work ~ 7 points off the true wind. A lateen ~5-6. In the last days of working sail, a good ship could lay 5, a schooner 4 points. A modern knockabout sloop can lay about 3.

    "Lift" is a modern concept. Generally the modern mathamatical analysis can be said to come from Lanchester in 1907 and Prandtl in 1911. However, this is work based upon Circulation Theory which is derived from work by Michell in 1898 and earlier potential flow work by Rankine 1864 and Cauchy 1815. It must be remembered that circulation theory is nothing more than a mathamatical contrivance and is not based upon the physics of the matter, but the apparent flow field (i.e. it solves the problem backwards from knowing the answer). It was not until the 1920's that aerodynamics begins to show up in experimental design, and the 1960 before its widespread use in preliminary design.
     
  4. national
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    national Junior Member

    Jim Hog. If a sail works by drag how can you explain a boat travelling to windward?
     
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  5. jehardiman
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    jehardiman Senior Member

    Ever squeeze a wet melon seed?
     
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