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Can a sailing boat go faster than the wind when going in the downwind direction?

  1. Oct 1, 2016 #1

    tech99

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    If a sailing boat is running before the wind, i.e. with the wind exactly behind, is it possible for it to go faster than the wind? If one thinks of a wind generator on the boat it could perhaps operate an electric propulsion motor to give any desired speed? But what about the apparent wind when the vehicle goes at the same speed as the real wind? This question is the subject of many arguments.
     
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  3. Oct 1, 2016 #2

    Simon Bridge

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    Start by seeing how it is possible for the yacht speed to be faster than the wind speed at all, then extrapolate for the situation you want to investigate. You also need to clearly say what you mean - naturally, if you add some other means of propulsion, it is possible to achieve faster speeds than by the wind alone.

    Rather than think of (a) the boat-mounted windmill generating electricity to run a motor which turns some wheels - imagine
    b. the windmill turning the wheels directly via a belt and gears
    c. the windmill runs a generator that runs an electric propeller the pushes the yacht,
    d. the windmill just turns a propeller directly
    ... see the problem? b and d must be more efficient than the equivalent via generating electricity to run a motor, and they both have to be less efficient than getting the wind to, you know, push the yacht.

    Most suggestions like this are attempts to hide a "closed mill" mechanism. ie. they violate conservation of energy.

    Not usually the subject of many sensible arguments though... and a youtube video is not evidence of anything.
    Certainly, the faster the yacht goes, there is less wind to generate electricity from ... but notice I did not have to use that objection above?

    You may want to look at the physics of ice yachts to see how very high speeds can come from wind power alone.

    These forums are not usually tolerant of threads involving possible violations of physical law... I think your question is really borderline but expect the thread to be deleted.
     
  4. Oct 2, 2016 #3

    DrClaude

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    I would bet that if you removed the propeller in that device in the video, it would still go up the treadmill.

    Anyway, as Simon said, we don't debunk pseudoscience on PF, we stick to science.

    Thread closed.

    It appears that I misjudged this one. Thread reopened.
     
    Last edited: Oct 2, 2016
  5. Oct 2, 2016 #4
    No Laws of Physics are broken.
    The wheels power the propellor which pushes the vehicle forward faster than the wind. The thrust of the propellor is greater than the drag of the wheels, so momentum is conserved. In the frame of the vehicle (and air stream at exactly the speed of the wind) the ground supplies the source of energy. (or from the treadmill as per the video). Energy is conserved since the air loses speed relative to the ground.

    It will not work at zero wind speed relative to the ground. That would break physical laws. But it will work at an apparent wind speed of zero, where the vehicle matches the wind speed, and it will work to go faster than the wind.
     
  6. Oct 2, 2016 #5

    rcgldr

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    The boat would need to operate on the same principle as the ddwfttw (directly downwind faster than the wind) model you see in the video. Some people also made a full scale version, described in the linked articles (wiki article includes a link to youtube video).

    http://en.wikipedia.org/wiki/Blackbird_(land_yacht)

    http://www.lhup.edu/~dsimanek/museum/DDWFTTW.htm

    When moving at the same speed or faster than the tail wind, from the perspective of the ddwfttw vehicle, the ground is moving faster than the apparent wind (for example, apparent wind speed is zero if the vehicle is moving at the same speed as the tail wind). This allows an effective reduction in gearing that allows the propeller to generate higher force but at lower speed than the opposing lower force and higher speed at the ground, minus losses in the system (power out is less than power in). The efficiency and overall losses determines the maximum speed.

    Getting back to the boat, the boat would need some efficient way to extract power from the water and use it to drive a propeller (not a sail). To reduce drag, a catamaran setup should be better (less displaced water). The issue is how to efficiently extract power from water when the speed of the boat relative to the water varies, perhaps some type of turbine or paddle wheel(s).
     
    Last edited: Oct 3, 2016
  7. Oct 2, 2016 #6

    Simon Bridge

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    ... thanks, I've needed a decent lay reference.
    There is usually a problem with the way the setup is phrased so it is unclear what is being contemplated.
    Note: in the zero-drag case, a flat plane will be pushed by the wind at the same speed as the wind ... yet the wind is still relative to the plane.
    Nobody says this is a paradox or anything. Post #1 raised the spectre of using the prop to generate electricity...
     
  8. Oct 3, 2016 #7

    rcgldr

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    Another link. Early in the article there's a comparison of a ddwfttw vehicle to a tacking sailboat (due to the spinning propeller, but a propeller is chosen because it is an efficient way to produce thrust, not because it's similar to tacking), but later in the article it provides a better explanation using a lever analogy for the reduction gearing from the force from the ground driving the wheels at some speed that in turn drive the propeller at a greater force, but at a lower speed.

    http://www.wired.com/2010/08/ddwfttw

    Note that ddwfttw is the same as a concept used on a device that dates back to the 1870's, the Brennan torpedo. It had wires wrapped around two contra-rotating spools to drive a pair of contra-rotating propellers, again with the key effective gearing reduction that the wire speed was greater than the propeller thrust speed. The wires were pulled backwards onto ground based spools driven by engines, while the Brennan torpedo would move forwards in the water. To make a comparison with a ddwfttw vehicle, the wires could have been attached to fixed post, with the water moving (with respect to the earth). The Brennan torpedo would end up moving downstream faster than the stream. Wiki article:

    http://en.wikipedia.org/wiki/Brennan_torpedo
     
    Last edited: Oct 3, 2016
  9. Oct 3, 2016 #8

    berkeman

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  10. Oct 3, 2016 #9

    A.T.

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    The speed of this vehicle is limited by it's efficiency, which will be reduced by introducing the conversion into electricity.

    It was, 8 years ago.
     
  11. Oct 3, 2016 #10

    rcgldr

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    Going back to the original post.

    For a boat running directly down wind, what is needed is a "water" generator, not a wind generator. From the boat's frame of reference, some type of device is needed to extract energy from the water by slowing down the water (relative to the boat), and using that energy to drive a propeller that provides the forwards thrust, and for the overall efficiency to be good enough to allow the boat to achieve faster than wind speed. As mentioned in a prior post, a turbine or small paddle wheel(s) on a catamaran could possibly work. The unknown factor here is how much drag is involved versus power produced by the turbine or paddle wheel(s), which would limit the maximum speed of the boat.
     
  12. Oct 4, 2016 #11
    You know, that water creates a lot of drag force on the boat, regardless of the wind speed and/or other means of propulsion. You still have to overcome drag.
     
  13. Oct 4, 2016 #12
    http://sailing.jpg [Broken]

    We're taking a gander at the vessel from the top. We have our pontoon in blue, heading off to one side. The wind is originating from the top. The sail is calculated 45 degrees. In case you're remaining on the watercraft, the wind is "abeam"; that is, straightforwardly from the side.

    At the point when the wind hits the sail, you'd anticipate that the vessel will move down the page, in the same heading as the wind. Indeed, the bottom (which isn't delineated) makes that unimaginable. The bottom is straight far from us, into the page, running the length of the pontoon. To push the bottom down the page, with the wind, you need to move a colossal measure of water off the beaten path.

    Rather, the wind is redirected, somewhat like a mirror: edge of frequency = edge of reflection. That is, the wind goes straight back, with respect to the pontoon. Furthermore, by protection of energy, if the wind is moving in reverse, the pontoon is moving advances. You are cruising on a "pillar reach".

    When you're cruising with the wind ("on a run"), you won't have the capacity to go any speedier than an unfaltering wind, since you will be steady with the wind. In any case, in case you're cruising on a bar achieve, you'll never be steady with the wind: you'll move into new twist blowing from the same course, prepared to be occupied. In principle, there's no restriction to your pace.

    That is distorted, obviously, since there are numerous different strengths included, and genuine mariners spend a lifetime taking in the points of interest. In any case, you can see that you can extend this to move even up to 45 degrees from specifically into the twist ("close pulled"): the bottom and the wind will at present create some forward push. Present day sailboats can cruise significantly nearer to the wind than that, utilizing streamlined sails. In any case, even a fundamental square-fixed vessel can move speedier than the wind.
     
    Last edited by a moderator: May 8, 2017
  14. Oct 4, 2016 #13

    A.T.

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    Yes, the propeller thrust would have to balance the drag of the water-turbine and the hull drag (in water and air).

    Mark Drela from MIT did analyse the required efficiencies:
    http://www.boatdesign.net/forums/attachments/propulsion/28167d1231128492-ddwfttw-directly-downwind-faster-than-wind-ddw2.pdf [Broken]

    For a boat he concludes:
    For a land yacht he concludes:
    And as far as I know, it has been achieved only on land so far.
     
    Last edited by a moderator: May 8, 2017
  15. Aug 12, 2017 #14
    This looks like pseudoscience to me also. I don't see what the treadmill is supposed to be proving.
     
  16. Aug 12, 2017 #15

    berkeman

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    Last edited: Aug 18, 2017
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