DDWFTTW - Directly Downwind Faster Than The Wind

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

  1. Guest625101138

    Guest625101138 Previous Member

    There has been suggestions to dedicate a thread to this interesting concept for boat or land vehicle propulsion.

    Jack Goodman created quite a stir with the demonstration of a small cart achieving the objective:

    Goodman published the attached article which explains the vehicles operation quite well along with some actual test data from a controlled test environment.

    Attached Files:

  2. Guest625101138

    Guest625101138 Previous Member

    Spork and JB made significant contribution to the general understanding of the concept by reproducing different versions of the Goodman cart and demonstrate the principle independently. They compiled a video for the Mythbusters challenge:

  3. Guest625101138

    Guest625101138 Previous Member

    Mark Drela has provided the theory of operation of a DDWFTTW boat using an air propeller and a water turbine. In the attached papers he provides the necessary conditions on efficiency and drag to achieve the objective.

    Attached Files:

    • ddw2.pdf
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    • ddwe.pdf
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      29.7 KB
  4. Guest625101138

    Guest625101138 Previous Member

    I have attached 4 slides that provide the basic principle of operation of a DDWFTTW cart for those who struggle with the possibility of it actually being done.

    The numbers provided are consistent with the requirements for a large scale radio controlled vehicle having a total mass of say 20kg. The propeller required to achieve the nominated efficiency is quite large, around 4m diameter for the given windspeed, even on this size vehicle.

    I hope this helps those who struggle with the concept.

    Attached Files:

  5. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member


    Very interesting experiments going on here but I have to be skeptical until one of these things runs into me. It does appear to violate some basic laws.

    A question about the trial video of the cart. When the cart is running at a steady pace in wind, what happens if the wind suddenly drops to zero? The claims here seems to say that the cart would continue to run forward continuously. That can't happen. Sometimes we don't really see what we are looking at. If the total power of driving the car forward is derived from the propeller and the total power on the propeller is derived from the following wind, where does the power come from if the cart is running faster than the following wind?

    If we assume that the cart is running faster than the following wind then the propeller sees a headwind on the propeller. Such a headwind will tend to drive the propeller backwards and slow the cart down.

    We all want to believe in perpetual motion but I just don't see the logic of this argument as yet. We are promised wormholes, warp drive and time travel, but I am certain that I will never see any of them. I can readily believe that there can be momentary speeds faster than the wind downwind but just can't accept the argument of such sustained speed, as yet. I stick in the "as yet" to cover my a** in the case that Newton and other discoverers of basic laws were wrong. In this event, the laws of conservation of energy (energy out cannot exceed energy in) and the law of equal and opposite reaction.
  6. yipster
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    yipster designer

    only see this now and must admit its even harder to grasp than straight upwind
    i'll have it sink in first a bit more before gettin more into blades than foils
  7. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    yipman, Had any more thoughts lately on our old option one?

    Straight upwind driven by propeller in water or wheels on land is not a problem for me because the energy of the wind is never zero in that case. How fast you can go is limited only by the different forms of drag and the efficiency of the power train. Going faster than the windspeed upwind must be difficult but not theoretically impossible.
  8. yipster
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    yipster designer

    Ricks well meant illustarations got me confused and sorry Rick
    and oh god let me learn how to keep my big mouth shot before i know what i'm talking about
    sailing faster tahn wind has been discussed here a couple of times before and has nothing, well nothing, to do with sails vs windmills
    its this simple [​IMG]
  9. TeddyDiver
    Joined: Dec 2007
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    TeddyDiver Gollywobbler

    I believe the main obstruction in our minds against this "phenomen" is what seems to be the mysterious source of the energy. It's a lot easier to comprehend when we'll try to see all movement's of the elements around with the eyes of the passanger, not as a by stander..
    After all, everything needed are the different forces btw 2 elements and the vehicle capable to take some advantage of the force vectors involved...
  10. Jimbo1490
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    Jimbo1490 Senior Member

    The concept that made it clear to me is that the propeller can be a source of 'simple' drag while SIMULTANEOUSLY spinning and producing thrust. This SEEMS impossible, but is what actually happens. So there's no mystery source of additional energy. It's that the cart continues to extract energy from the wind from the drag on the propeller and the clever gearing causes that extracted energy to be converted to thrust, even at speeds greater than the wind. That the cart does not continue to accelerate to light speed shows that an equillibrium is reached (albeit not the one many of us expected), and thus no laws of physics are viloated.

  11. Guest625101138

    Guest625101138 Previous Member

    As soon as the cart starts to move, the wheels turn in forward motion and begin powering the prop. So it is always drawing air backwards through the vehicle. This is akin to a propeller on an aircraft on the tarmac with a tail wind or a boat at a dock with a current from astern, in all these situations the propeller is able to reverse the direction of airflow/waterflow in the local field.

    The propeller will turn in the forward direction because the gearing enables the wheels to overcome the prop torque. If the gear RATIO was reversed then wind blades (turbine) would drive the wheels and the vehicle would move into the wind.

    I am sorry to disappoint you but If the wind stops, the vehicle stops. There must be relative motion between the wind and ground to get the necessary velocity difference that makes up for the losses in the vehicle. No wind, no go.

    I am an engineer, I have no desire to believe in perpetual motion because I know it cannot be done. This vehicle is simply a clever use of propellers.
  12. Guest625101138

    Guest625101138 Previous Member

    You are right about it having nothing to do with sails v windmills. We are not talking about a windmill here (a power extracting device more accurately called a wind turbine). We are talking about a simple propeller driven by the wheels of the cart.

    The difference with this vehicle is that it can exceed windspeed DIRECTLY downwind rather than having to tack. The propeller simply takes a little power from the wheels (at low force and high velocity) to push the veicle a bit faster than the wind (at higher force and lower velocity).

    One of the first steps in understanding is to know we are talking about a propeller not a turbine. If the gear RATIO was reverse then there is a turbine driving the wheels and the vehicle would go directly upwind.
  13. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    All in the venture to learn, not to agitate anyone.
  14. Guest625101138

    Guest625101138 Previous Member

    The energy comes from the air movement relative to the ground. There is nothing fantastic about it. It does not disobey any fundamentals of physics.

    If you understand power is the product of force and velocity you have all you need to know.

    The prop generates a relatively large force at low velocity relative to the air. The thrust it generates is enough to overcome the rolling friction and wind drag on the vehicle plus the force on the wheels to drive the propeller.

    The wheels are collecting power at low force but high velocity because the wind is already providing some of the forward component.

    The vehicle is moving quite fast relative to the ground but only just making forward progress through the air. The best practical system would get about twice wind speed with a cart. This is still well behind what is possible with a land yacht only it must tack.

    I have difficulty how readily people say it is possible to exceed windspeed downwind tacking without breaking fundamental principles but then they see this as perpetual motion. Simply shows they do not take the time to think it through or do not really understand basic physics.

    Not being antagonistic here just that most people jump in without really thinking about it. Go through my 4 slides and understand what they demonstrate. There is no trick.

    For the propeller; Pp = large FORCE x small velocity
    For the wheels; Pw = small force x big VELOCITY

    Pp will be less than Pw because there are transfer losses to overcome.

    The power input to the vehicle is Force on the propeller times the windspeed. (Note that the prop is producing a reverse stream that reacts against the wind) Think of a plane with an airspeed of 100kts in 50kts of wind. The ground speed is 150kts. But the plane has not needed to do any more work to achieve this extra speed. The wind is doing the extra work.

    The power dissipated in the vehicle is the air drag on the vehicle, the rolling drag on the wheels and the conversion losses of power from the wheels transferred to the prop.

    These little demos show how the gearing works with wheels being used to drive against the forcing media. In both cases the vehicle moves faster than the solid media being used to propel them.

    The only difference with the propeller driven vehicle is that the propeller is reacting against a fluid medium not a solid medium. If you deny this possibility then you are denying the possibility of boats and planes ever working with propellers.

  15. Guest625101138

    Guest625101138 Previous Member

    This video was done to show the cart self starts. You will note that the prop turns in the forward direction as soon as the vehicle begins to move:

    The gearing is such that the wheels develop more torque than the prop. Just a matter of having the pitch of the prop, mechanical gear ratio and wheel diameter set appropriately so this can happen. If the ratio was the other way the blades become a turbine and it climbs into the wind.

    The cart goes upwind or downwind simply by changing the gear ratio. Albeit the optimum configuration of vehicle for either circumstances is vastly different.
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