fan powered sails

Discussion in 'Sailboats' started by brendan gardam, Oct 22, 2020.

  1. brendan gardam
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    brendan gardam Senior Member

    i have a really silly question.
    if you put a powered fan on the bow of a sailboat and aimed it across the sails at the correct angle would the boat move forward or remain stationary.
     
  2. Blueknarr
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    Blueknarr Senior Member

    Myth busters managed to achieve forward progress.
     
  3. brendan gardam
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    brendan gardam Senior Member

    Yes
    They put fans behind square sails I am wondering what happens if you blow air across the sail whether it will create the low pressure to propel the boat.
     
  4. Blueknarr
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    Blueknarr Senior Member

    It would still provide forward propulsion. More so than MB achieved. Direct drive from the fan plus lift from the sail.

    Motor sailers get some drive from their sails even in an absolute calm. Any air movement across a sail wether from wind, fan or the drive prop will produce some lift and drag.
     
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  5. kapnD
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    kapnD Senior Member

    Wouldn’t you be just as well to drive the boat with just the fan, and no sails?
     
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  6. brendan gardam
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    brendan gardam Senior Member

    I think they are called airboats.
     
  7. CocoonCruisers
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    CocoonCruisers Junior Member

  8. philSweet
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    philSweet Senior Member

    Nope. There has to be a breeze.

    Don't confuse force multiplication with power multiplication. You can absolutely multiply forces, and you can redirect forces; but you just can't multiply energy. So you can use sails to get the boat to move in seemingly improbable directions given the fan orientation, but the speed is limited by the power of the fan regardless.
     
  9. sharpii2
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    sharpii2 Senior Member

    Another highly counter intuitive example of reactions.

    One would think that the reaction of the forced air leaving the leaf-blower would push him backword at the same rate as the reaction, of the umbrella deflecting this same forced air backword, would pull him forward.

    The reason this is not so is because momentum is conserved, while kinetic energy is not.

    In this example, the reaction from the air coming out of the blower is equal to the the amount of air coming out of it per second. But the kinetic energy in this air is much higher. It is half its weight times its speed squared, where it's reaction is only its weight times its speed. So, the higher the rate of speed that the air comes out at, the lower the efficiency of the reaction.

    If you could somehow lower the rate the speed of the ejected air, while ejecting the same amount per second, you would end up with a more efficient reaction.

    This is where the umbrella comes in. It more or less deflects pretty much the same amount of air per second, but over a much greater surface area.

    Since the amount of air being deflected per second is about the same, it must be deflected at a slower speed--and therefore produces a greater reaction.

    Hence, the cart moves forward.

    This same principle could be used to explain why higher aspect ratio wings produce more lift than lower aspect ratio ones.

    The higher aspect ratio wings (and sails) deflect the same amount of air (for their area) as the lower aspect ratio ones do, but at a much slower speed.[/QUOTE]
     
  10. alan craig
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    alan craig Senior Member

    ....Or he is being pulled along by a piece of string.
    Edit: ( I tried to Quote+ from the post above but it was too complex for me)
     
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  11. CocoonCruisers
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    CocoonCruisers Junior Member

    Or perhaps it's an electric skateboard below. (The bin's wheels when he is watering the flowers are not the same as those shown later, at speed)
    Damn, three days since i posted this and despite sharpii's explanation i'm still scratching my head.
    Could it be that it's entirely possible theoretically and practically, but that it can't be nearly as efficient as suggested by this janitors dubious proof in the pudding ?
     
    Last edited: Oct 26, 2020
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  12. gonzo
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    gonzo Senior Member

    It would depend on many factors. However, assuming the sail area, shape and trim is optimized and there is no wind, the boat will move in the opposite direction from the thrust of the fan.
     
  13. sharpii2
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    sharpii2 Senior Member

    I hate to admit it, but it looks like I've been had.

    Those wheeled buckets get caught on every crack in the sidewalk. I know, because I've used them.

    If my theoretical construct is true, this video is probably not really evidence of it.
     

  14. tlouth7
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    tlouth7 Senior Member

    He definitely has an electric skateboard.

    Reversing thrust in this manner is perfectly possible though - it is what aircraft jet engine thrust reversers do! The trick to getting your head round these things is to ignore internal forces and interactions. Consider the boat/aircraft/mop bucket as a black box, which direction does the air leave the system? The direction of thrust is in the opposite direction to that.

    Blowing a fan over sails will always be less efficient than just pointing the fan out of the back of the boat.
     
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