1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Windmill boat on river, no wind

  1. Sep 13, 2005 #1
    A boat with a windmill rotating a water propeller can sail directly upwind. It has been done many times, so take that as a given. (The maximum speed so far is about 0.5 times the speed of the wind. For a land yacht, the maximum speed so far is about 1.0 times the speed of the wind.)

    My question has to do with the correct physics terminology to describe the following situation. A windmill boat on a rapidly flowing river, under windless conditions, sails directly downriver faster than the river by using its relative wind. From the frame of reference of the river, the boat is sailing normally directly upwind.

    However, how does one describe what is happening from the conventional frame of reference of the ground? What is the source of energy? Obviously, the river is the only source of energy. But then how does one describe the windmill's interaction with the relative wind? It appears to be extracting energy from the stationary air, which is not possible. So is the interaction with the air essentially just the equivalent of a fluid transmission to transfer energy from the river to the boat? Or what?
  2. jcsd
  3. Sep 13, 2005 #2


    User Avatar
    Science Advisor

    It seems to me that the source of the energy could be described as a decrease in the amount of energy wasted. A boat flowing downstream through air that is still encounters aerodynamic drag. Without any means of tapping the energy from this friction, that energy is simply radiated away as turbulent diturbances in the air (and some heat and soundwaves), and perhaps more importantly, ripples in the water as the drag causes the boat to resist movement downstream. With the windmill sail, some portion of that energy is captured and put to use, increasing the efficiency of the vessel.
  4. Sep 13, 2005 #3


    User Avatar
    Gold Member

    Not that this is the answer to the question, but don't forget gravity. Gravity is what is powering the river in the first place. In a frictionless environment, the boat will get its movement for free - up to the speed of the water - without extracting any energy from the water.
  5. Sep 13, 2005 #4
    Let us ingore gravity in this case for the sake of simplicity. We could, for example, replicate the same relationships in a test tank with circulating water. There, gravity would not need to be considered as part of the analysis in terms of the energy transfers.
  6. Sep 13, 2005 #5
    If I understand you, and please forgive me if I do not, you are saying that when the boat uses its windmill, it is more efficient than when it does not -- and therefor less inefficient than when it does not.

    But how does that observation differentiate between this situation of the boat on the river and the conventional situation of the boat on a stationary body of water in a true wind? As far as I can tell, your point applies equally to both situations and so does not differentiate between them. Am I missing your point?
  7. Sep 13, 2005 #6
    I like what LURCH has to say. Think of it as two scenarios:

    Scenario A:
    * Stationary water
    * Wind blowing towards boat

    Scenario B:
    * Moving water
    * Stationary air

    Let's say you're on the boat, but you don't know whether the water is moving or stationary. Essentially, you don't know which scenario you're in. You feel a breeze on your face. Can you determine which scenario you're in?

    Either the moving water facilitates the breeze (moving opposite your direction of motion), or the moving air facilitates the breeze (again, moving opposite your direction of motion). Either way the windmill device will use the resulting breeze and create forward motion. I believe LURCH's main point was that if you have a windmill boat in scenario B it will coast downstream faster because the "resistance" of the air that a normal craft would experience would be counteracted by the propeller driven by the windmill. So if a normal craft would experience a drag force of x, a windmill-propeller craft would experience drag of y < x, as it uses some of that air to drive the propeller.
    Last edited: Sep 13, 2005
  8. Sep 13, 2005 #7
    Though it appears to be extracting energy, I believe it's just not losing as much energy from drag. Again, the air may be stationary to an observer on land but it creates a "breeze" for the boat in a direction opposite of its direction of motion. And the windmill-boat can use air traveling like that to create a net positive force in its direction of motion. If it can do that, then it benefits in cases that would normally produce straight-up drag for a normal boat.
  9. Sep 13, 2005 #8
    But then why could we not say that a windmill boat in a true wind on a stationary body of water experiences reduced drag, and is propelled by that reduced drag. That does not sound right at all. In both cases, there is an equivalent conversion of energy by the windmill. We can say that the windmill boat on a river converts drag into power, can we not? But we can say the same about a windmill boat on a stationary body of water in a true wind, can we not? The stationary boat experiences drag, which the windmill converts into propulsion. The windmill boat on the river seems to be doing the same thing. So how does this approach differenctiate between the two situations?
  10. Sep 13, 2005 #9
    I'm not clear on this. "not losing as much energy from drag" as what?

    In both cases -- flowing river or stationary water plus wind -- it is the relative motion between the two sailing media (air and water) that is the source of energy for the windmill boat. If we use the frame of reference of the boat, the two situations are identical (if we assume identical relative velocities in both cases). But the problem is to explain the energy conversion from the frame of reference of the ground.

    On the river, the air produces only drag. The windmill can covert that drag into propulsive energy (power?). Is that the correct way to describe what is happening? But if it is, could we not describe a windmill boat on stationary water in the same way? Do we need to differentiate between the situations in some essential respect?

    I hope I'm not missing your point.
  11. Sep 13, 2005 #10


    User Avatar
    Science Advisor

    There is no effective difference between a "windmill boat" and a sailboat (ignoring friction which I suspect would make the windmill boat a pretty poor sailor!). A sailboat sails with the "apparent" wind- that is the wind relative to the boat, not the wind relative to the land or the water. It doesn't matter to the boat whether the wind is "real" or it is generated by water current- to the sailboat, wind is wind!
  12. Sep 13, 2005 #11
    So all I need to do is describe the windmill boat on the river in terms of the apparent wind, which is the same as for the windmill boat on stationary water in a true wind. Correct?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook