DDWFTTW - Directly Downwind Faster Than The Wind

Discussion in 'Propulsion' started by Guest625101138, Jan 4, 2009.

  1. Will Gilmore
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    Will Gilmore Senior Member

    Not all of us know that wouldn't work. I actually thought the cylindrical Earth metaphor made so much sense that I have to think it would work.

    I'd have to figure out where the lateral resistance is coming from, but as of yet, I don't see why it can't be done.

    Except for the efficiency variance along the rotating sail. Apparent wind would be much different near the hub than at the tips. That might have a big effect on sailing via rotating sails.
     
  2. kerosene
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    kerosene Senior Member

    Will, the cylindrical world analogy would work. For the prop to be sails, and efficient, there would have to be twist, like in a propeller, doable.

    But freely spinning prop (as in not connected to wheels or keels or rails) would just spin and not really generate thrust.
     
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  3. Will Gilmore
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    Will Gilmore Senior Member

    Perhaps that is true. Resistance needs to be introduced, but in the right direction.

    I feel there is a solution, I can't quite see it, though.
     

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  4. tlouth7
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    tlouth7 Senior Member

  5. Robert Biegler
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    Robert Biegler Junior Member

    Apologies if I repeat something already posted; I have just discovered this thread and have not read through all 29 pages of it.

    There is a way of thinking about all this that may help remove one of the misleading intuitions, and that is changing the frame of reference. (I am sure this is not my original idea, I think I read it in AYRS Catalyst, but I can't find the exact source.)

    Do you accept that a windmill vehicle can move directly upwind? If yes, consider that what you need for sailing is two media moving relative to each other. We tend to think of one as stationary and one as moving, but that is exactly one of the things that can mislead your intuition. I hope to outline steps for getting around that.
    1) It is possible to use a wind turbine to drive a propeller in water and move directly upwind, like so:

    The turbine boat has its hull and propeller in the water, and the turbine that extracts energy to drive the propeller is in the moving medium, the air. If you want, you can imagine that the hull is a submarine, and the only thing that sticks into the moving medium is a strut holding the wind turbine. That will help making the mental switch when we get to point 3)
    2) There is nothing special about the moving medium being air. If you built a heavy windmill cart driven by a water turbine and you drove it along an underwater road, that would be analogous to a windmill vehicle on land. With good streamlining and traction, you should be able to move upstream. In fact, you could test the principle by making a model boat with a water turbine geared to wind in a piece of string. That gives you perfect traction. Drop the model from a bridge, let it drift down stream while you pay out the string, then hold it still. With appropriate gearing and a good hull, the model should work its way upstream.
    3) Now we mirror everything in point 1). You are no longer in a submarine in still water, sticking a wind turbine into moving air. Instead, you are in an airship, in absolutely still air, suspended over water flowing South, say at 5 knots. You drop a water turbine into that stream of water to drive a propeller that pushes you North, upstream. That is a mirror image of the wind turbine submarine in point 1) in that the turbine is now in the water and the propeller is in the air. And to help with that shift in the frame of reference, the air is stationary, just like the water was in 1) My claim is that if the wind turbine boat in 1) can go directly upwind, then the water turbine airship can go directly upstream.
    4) You wave to a friend, sailing down below, headed due North, directly into the stream. The air is still relative to the Earth, but the boat below is in a 5 knot current going South. Your friend in the boat experiences that as a 5 knot wind from the South. Let's say that the boat moves under spinnaker, sailing dead downwind at 4 knots. That means speed over ground is still 1 knot to the South. Your friend sees you moving directly North, where the wind is blowing, and says "Bloody hell, you are sailing directly downwind, faster than the wind! That is impossible!" If you treat the air as stationary and the water as moving, you are using water power to drive a propeller to go directly upstream. If instead you treat the water as stationary, that very same thing is seen as sailing directly downwind faster than the wind. The two are the same thing, and what you call it depends only on your frame of reference.
    5) If you think scenario 3) is possible, but you think that I can't claim that the boat under spinnaker is driven by a wind from the South because the air is still relative to the Earth, then please explain what would change if the current gradually slacked and the air gradually started moving, maintaining the same 5 knots difference all the while.
    Edit:
    6) Add to the scenario a windmill boat. Its crew treats the water a stationary, and sees a 5 knot wind from the South, and uses that to go directly into the wind, say 3 knots through the water for 8 knots over ground. From the frame of reference of the airship, the windmill boat is going directly downstream faster than the stream.

    The idea that DDFTTW violates conservation of energy is an illusion caused by treating one frame of reference as privileged and translating that into "water or ground is stationary, air is moving, and energy is extracted from the thing that is moving". That leads directly to the intuition that when apparent wind becomes zero, you can't extract any more energy. If the premises were true, that conclusion would follow, but one of the premises is false.

    In the most general terms, what you need for sailing is two media moving relative to each other, and a means to extract energy from that relative movement. You could sail an airship by dropping a kite from it until the kite reaches a body of air that moves differently, and then using the airship and the kite as two foils that you angle appropriately. Dynamic soaring, as exploited by albatross and some RC model pilots, does the same, only instead of coupling two foils mechanically and exposing them to both streams simultaneously, they dip into the two streams successively, using momentum to go from one to the other.
     
    Last edited: Jul 4, 2021
  6. Landell
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    Landell New Member

    If you apply Newtonian physics (Force = ma), which no one seems to have done: The sail re-directs a mass of air (m) of the apparent headwind backwards, which decelerates (a) when it interacts with the undisturbed atmosphere to produce turbulence. This action creates a backward force (Force = ma). The reactive equal and opposite forward force pushes the sailboat ahead.

    Then boats can sail faster than the wind because wind velocity is only one part of the equation for the force generated. For example, doubling the sail size will double the mass of air re-directed (2 x m), and therefore, double the force generated by the sail (2 x Force = 2ma). Also, the sail is pushing against the wind to generate the force, which is a moving mass and not a fixed or static point. Momentum is transferred from the apparent wind to the boat via the sail by slowing the wind down. See attach summary, which is from a new physics book.
     

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  7. DCockey
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    DCockey Senior Member

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  8. gonzo
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    gonzo Senior Member

    When the cart is moving at the same speed of the wind, the apparent wind is zero. Where is the power input coming from if there is no wind relative to the cart? You miss the basic concept of relativity. Only relative wind speed is relevant for this system.
     
  9. gonzo
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    gonzo Senior Member

    That is correct. However, when the vessel moves at the same speed as the wind, the media has zero relative velocity. As you explained, the available energy is zero.
     
  10. DCockey
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    DCockey Senior Member

    When the cart is moving at the same speed as the wind the apparent wind speed relative to the cart is zero.
    - However -
    The relative wind speed relative to the a location on the blade of the rotating propeller/turbine is non-zero. That is where the energy to move the cart comes from.

    That is why a cart with a fixed sail cannot travel at the same speed as the wind, and a propeller/turbine is required.
     
  11. gonzo
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    gonzo Senior Member

    That would be true if the turbine was not attached to the cart. Otherwise, this is like the type of design where an engine runs a generator that runs an electric motor that then runs the generator. It may be possible to accelerate the cart to wind speed, and then it will slow down as the stored energy is dissipated by friction. However, that would require some kind of energy storage device.
     
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  12. DCockey
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    DCockey Senior Member

    In addition to the rotating propeller/turbine the other necessary part of the system is the speed difference between the wind and the ground.
    Continuous, steady speed operation is possible without energy storage.

    Similar erroneous arguments have been made about why the systems cannot possibly work using "simple" physics, including by the physics professor who lost a bet. The error is the model/analysis is too simple and misses key elements of how the systems work.

    Do you also claim that an iceboat or some sailboats cannot tack downwind with the component of speed in the direction of the wind equal to or greater than the wind speed?
     
  13. kerosene
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    kerosene Senior Member

    and just to clarify it is always a propeller - thrust device. Never a turbine.

    this has been debated so many times over. it is not a perpetual motion.

    My best verbal summary:

    the relative speed differences in the two interfaces allow leveraging from the higher speed difference of [vessel to ground]
    to higher force in the lower speed difference of [vessel to air].

    did you see the videos I posted earlier?
     
    Last edited: Aug 23, 2021
  14. kerosene
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    kerosene Senior Member

    the one from 2010




    and one from 2021



    it is a leverage thing.
     

  15. gonzo
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    gonzo Senior Member

    If a propeller or any other device reaches the speed of the wind, the relative velocity is zero. Where is the energy coming from if there is no wind?
     
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