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Boat question

  1. Jan 11, 2014 #1
    Here's a random question from the depths of my brain. :)

    Say that there's a sailboat on a flat, empty lake. Now imagine that on the boat, behind the sail there's a big electric fan.
    If the fan were to be switched on(blowing towards the sail), would the boat move forwards, backwards, or remain stationary?
    Thanks in advance :)
  2. jcsd
  3. Jan 11, 2014 #2


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    This has been tested with models (even on myth busters). If the sail is large enough and able to capture and deflect / reflect the flow so that the net flow from the sail boat is backwards, then the sail boat will travel forwards. The sail needs to be large enough so that not too much of the deflected flow is drawn back into the intake flow going into the fan. Given this, then the intake flow accelerates from zero speed, but the deflected flow changes directions, so there's more net momentum exerted on the sail reversing the flow than the momentum exerted by the fan that intakes the flow. If the sail is too small, the boat moves backwards, and somewhere between too small and large enough to move forward, the boat doesn't move.
    Last edited: Jan 11, 2014
  4. Jan 11, 2014 #3

    Meir Achuz

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    Draw a big circle around the boat, sail, and fan.
    With a big enough sail that reflects most of the air stream, more air will be flowing back out of the circle than forward. Therefore, the boat will move forward.
  5. Jan 11, 2014 #4


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    Model at 2:15 into this video:


    Full scale at 0:50 into this video:

  6. Jan 11, 2014 #5
    Thanks very much, especially for the videos.
    I can sleep easy now :)
  7. Jan 11, 2014 #6


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    The Mythbusters video reminded me of how thrust reversers are used on aircraft. Although the jet engines can only produce thrust in one direction, that thrust can be redirected forward and at least some aircraft such as the C17 can actually be pushed backwards by the jet engines such as in this video: (starts about 1:00 in)

    Last edited by a moderator: Sep 25, 2014
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