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Direct Upwind Sailing Boat

  1. Jun 13, 2009 #1
    Can you make a wind turbine powered boat or cart that travels forwards directly into a headwind without storing energy at any time?

    Like this:

    It seems to me that it's a bit like making a cart that's geared in such a way as to go farwards when you push backwards against it - which AFAIK is totally impossible.

    Can anyone give me a worked example to work out the drag from the wind on the propellor verses the energy it can harness to drive forwards?

  2. jcsd
  3. Jun 13, 2009 #2
    For what it's worth:

    A few months back I discovered that a small wooden airplane prop I have screwed neatly and loosely on a common 1/4 -20 carriage bolt. As it turns it advances along the bolt. Holding this up to a fan, I discovered it will indeed screw itself into the wind.

    It was self-starting, so there is no storage required, but it does accelerate, so storage happens when there is no load.

    It works whichever side of the prop is oriented toward the wind, but works much better when the 'back' side of the prop is toward the wind.

    This little experiment convinced me a boat like this is viable, in principle anyway. I'm fairly sure that directly upwind would be its slowest point of sail, though.
  4. Jun 13, 2009 #3
    Short answer: Yes. The physics is valid.

    I'm not sure how practical it would be compared to a regular sailboat's tacking. It's certainly worth investigating, though.
  5. Jun 13, 2009 #4


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    I'd be surprised if it could actually do that, since you have the inefficiencies of the propeller and turbine to overcome.

    And I wouldn't call it a sailboat either - it is a wind turbine powered power boat.
  6. Jun 13, 2009 #5


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    The turbine by itself would get blown backwards. The turbine free spining on a boat or cart would also get driven backwards. In the case of a cart, if the turbine is connected to the wheels via some gearing, the cart could advance into the wind, same as the prop on the screw example mentioned above. In the case of a boat, if the turbine in the air is driving a prop in the water, the losses would be greater and the water is easily pushed back, compared to a wheel on the ground, and I'm not sure if the boat case is possible.
  7. Jun 13, 2009 #6


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    Last edited by a moderator: May 4, 2017
  8. Jun 13, 2009 #7
    A rotating prop will have less drag than when it is not rotating. The angled blades allow it to present less surface area to the oncoming wind when it is rotating. The force that would normally go into pushing the boat backward is instead transferred to the prop in the water. Whether that would be enough to actually move the boat against the wind would depend on efficiencies and the aerodynamics of the boat.
  9. Jun 13, 2009 #8
    Ok, thanks for all replies.

    Everyone agrees that the energy gathered by the Windmill is enough to drive the windmill forward directly against the drag of the wind that is powering it.

    Anyone prepared to show me the maths to prove this? I guess we need to know 1) the drag of the prop, 2) the capactiy of the prop to gather energy and 3) the amount of energy required to drive forwards against the drag.

    Just out of interest is it possible to create a cart with a suitable gearbox to convert a push from the front into "forward" drive. In other words can I create a device that, when pushed from the front, with (say) a 10n force will harness that force and move in the direction of it's energy source. I had thought not but given the windmill cart/boat maybe that's wrong.
  10. Jun 13, 2009 #9
    A number of decades ago I read about a similar design based more upon the design of an anemometer except that instead of hemispherical cups it used vertical curved vanes. It would spin regardless of the direction of the wind and drive a propeller.
  11. Jun 13, 2009 #10
    Not only is this possible, it is feasible.

    A force-balance diagram can be constructed of a sailboat in an upwind tack. A mirror image force-balance diagram, mirrored about the wind vector is superimposed.

    This combined diagram is equivalent to an air screw and water screw mounted on a hull sailing directly upwind--though possessing two small problems. The water screw tends to stop turning, and the air screw tries to spin too faster under the conditions required to close the force diagram.

    (Closing the diagram means all the forces, added up, are equal to zero. In this condition, the boat is in steady state motion; sailing at a constant velocity upwind, neither speeding up, nor slowing down.)

    However, the available power from the air screw exceeds that required to keep the water screw turning, as required to close the force diagram. So, yeah. It'll work.
    Last edited: Jun 13, 2009
  12. Jun 14, 2009 #11
    Anyone prepared to show me the maths for this? I guess we need to know 1) the drag of the prop, 2) the capactiy of the prop to gather energy and 3) the amount of energy required to drive forwards against the drag.
  13. Jun 14, 2009 #12
    If you think in terms of Newton III alone, action/equal and opposite reaction, you would come to the conclusion it wouldn't work.

    But, the actual operative principle behind this device is that of the first class lever: any force applied over sufficient distance can move any mass, but the distance moved will be proportionately less than distance through which the 'long' end of the lever was moved. And the motion will be in the opposite direction of the applied force.

    The propeller is just a more complex version: wind force on the propeller is converted to force at right angles to the original. From there, it can be mechanically converted to any direction. If you understand how easily the first class lever produces force in the opposite direction to the applied force, it de-mystifies that apparent paradox.

    In my propeller on the carriage bolt experiment I observed that for every revolution of the prop its mass was levered forward (into the wind) the distance of one twentieth of an inch (1/4 - 20 carriage bolt: 1 revolution = 1/20th inch = .05 inches). The prop diameter is 9 inches, meaning we have two levers of 4 1/2 inches being pushed by the wind through a distance at the tips of (9 times 3.1416) 28.27 inches for each 1/20th inch advance into the wind. That's force applied over 28.27 inches to get a mere .05 inches of advance. Not so miraculous.

    You can see that, if you wanted to move a one ton stone into the wind you could set up a lever and fulcrum and then unfurl a sail at the end of the lever. The sail might have to be pushed 10 yards to move the stone one inch, but you have moved your mass into the wind by force of the wind.
  14. Jun 14, 2009 #13
    That's a bit of work involving about a dozen vectors and their vector equations. It's really easier as a geometric problem in two dimensions, some fluid dynamics, and some educated assumptions about real world preformance to get the geometry reasonably proportioned. The equations follow from the geometric construction.

    It's understandable there's an error in your assuption about drag on the prop. I assume you mean the water propellel. The forces acting to slow the screw are both lift and drag.

    Are you thinking of building something like this?
  15. Jun 14, 2009 #14
    You're right, and this could be an easier approch than I was taking, though equivalent. Given some typical L/Ds for the blades and typical radii of an airscrew and water screw, then account for the torque on each shaft.
    Last edited: Jun 14, 2009
  16. Jun 14, 2009 #15
    Attached is a force diagram for sailing directly up wind.

    The air screw drives the water screw. Presumably, though I haven't yet drawn the diagram, to sail directly down wind (faster than the wind), the roles would be reversed and the water screw would drive the air screw.

    It all works out very nicely, but any other heading with respect to the wind would require control over such things as propeller pitch (both water and air screws), gear ratio between the two screws, and orientation of the air screw.

    Other than beating the wind downwind, and sailing directly upwind, mechanical advantages over a sailboat are the reduction in parasitic drag from the rigging as well as the hull which produces drag from both air and water. This is because both the air speed and water speed compared to a sail boat are reduced over most directions of tack. Parasitic drag increase as the square of the velocity.

    Of mechanical disadvantage is the frictional losses in the gear train between screws.

    Attached Files:

    Last edited: Jun 14, 2009
  17. Jun 15, 2009 #16
    Been done by a few people. Seen a 30ft boat with a windmill -water screw setup firsthand.
    With a vertical axis , vane setup with cyclic pitch control you don't even need a drivetrain and waterscrew. electric Motor/generator on it and you can motorsail or generate and/or sail.
    Losses are less than 10% drag to lift in good foil designs. Air or water. More winddrag loss from your boat most likely.
  18. Jun 15, 2009 #17
    Coincidentally, I hadn't considered cyclic pitch until driving home, just now. This would seem to serve the same effect as rotating the direction the propeller faces, though I'm not sure yet.

    Utilizing cyclic pitch, how do you figure you don't need a drivetrain and water screw?
  19. Jun 16, 2009 #18
    It gets a bit complicated. For one thing due to real world situations due to wind shear as you go from (near) stationary sea surface where wind velocity is near zero, to full wind velocity at altitude you can make it work without any pitch adjustment. The energy harvested by the turbine 10m above the water in higher velocity wind can drive aerodynamic lift in the opposite direction at say 2m above sea level because the airflow there has a lower relative velocity.
    Aside from that (large differential that can be used for large advantage as it is), lift to drag ratios in energy harvesting parts of the cycle being over 9/10, and simular lift to drag ratios in using that energy to propel in other sectors of the rotation makes it work with cyclic pitch control even if wind shear was absent.
  20. Jun 16, 2009 #19
    I see. I hadn't even considered wing gradient. The two dimensional model is hard enough. I'm going to look at an autogyro arrangement to see how that works--autogyro propeller + fixed keel.

    Sailing directly downwind faster than the wind, by the way, is also feasable with the same wind screw - water screw combination.
  21. Jun 17, 2009 #20
    The problem is a real mess to model for a tacking airscrew. I've got an upwind-moving blade, a downwind-moving blade, and a possible variable cyclic pitch going on. The induced drag varies as the square of the angle of attach...
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