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If I stood on a sailing ship and had a very strong pair of lungs (possibly motorized) then if I blew the sail would I be able to move the ship forwards?

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    $\begingroup$ en.m.wikipedia.org/wiki/Airboat $\endgroup$ – agentp Jan 11 '18 at 12:29
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    $\begingroup$ "for every action there is an equal and opposite reaction" so NO, the force you apply to the air leaving your lungs is opposed by the force by your feet on the deck... $\endgroup$ – Solar Mike Jan 11 '18 at 12:56
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    $\begingroup$ @Solar Mike you are sooooo wrong here. Take a look, for example, at the famous "reverse flow in lawn sprinkler" problem. $\endgroup$ – Carl Witthoft Jan 11 '18 at 15:31
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    $\begingroup$ @SolarMike This is a complicated question to handle directly, but consider we can sail downwind several times faster than the true wind speed, and in fact, can now sail upwind faster than the wind speed, so yes, you can do this productively even in a steady state situation. You can blow in any direction you like and construct a sail that will propel the boat. Consider a pelton bucket that reverses the flow. The bucket's reaction force is twice the source's reaction force. $\endgroup$ – Phil Sweet Jan 11 '18 at 19:41
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    $\begingroup$ @SolarMike Of course you can. $\endgroup$ – Phil Sweet Jan 11 '18 at 20:09
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The answer, despite what several responders think, is unequivocally YES . However, you will get more efficient propulsion just by blowing directly backwards. All a sail does is deflect wind, thus changing the direction of momentum (with the help of your keel or centerboard).

The reason this works is that when you blow, you are expelling air more or less purely in the stern direction, and equal/opposite reaction rules apply. However, when you inhale, you draw air in from more or less all directions, and there is next to no effect on the boat's momentum.

Anyone who continues to doubt this is referred to swamp air-propellor-powered boats, as in agentp's comment. airboat

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    $\begingroup$ Prove your statement by giving us a video of you standing on the deck and blowing on the sail. $\endgroup$ – Solar Mike Jan 11 '18 at 15:38
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    $\begingroup$ @SolarMike give us a break, 'mkay? Just draw the force diagram. $\endgroup$ – Carl Witthoft Jan 11 '18 at 18:18
  • $\begingroup$ In a more artistic view, you should jump (diagonally forward) first then blow at the sail. In that sense, you are removing the action-reaction from blowing. LOL. In a more engineering viewpoint, putting an electric fan over a boat will significantly move it, doesn't it? $\endgroup$ – Jem Eripol Jan 13 '18 at 1:14
  • $\begingroup$ This woild be quite easy to test with a scale model. I agree it will work but it will be extremely ineficient, so much so that it probably is not worth the effort $\endgroup$ – joojaa Jan 13 '18 at 12:03
  • $\begingroup$ The reference to an airboat really isn’t relevant, because the fan is blowing backwards and there is no sail. But @Owensteam posted an answer below that shows a proper experiment by Mythbusters that shows it working. $\endgroup$ – Mark Jan 13 '18 at 16:46
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I've done a lot of sailing (won a couple national championships) and the answer is yes. But you wouldn't want the fan blowing forward from the stern. Instead you would want it blowing towards the sails from the side. Even though the fan would want to pull the boat sideways, sailboats are equipped with centerboards or keels which function like a wing underwater. Since water is much more dense then air, you'll have a hell of a time pulling the boat sideways. Instead, the air will wrap around the sail thus creating a high pressure zone on the "windward" side and a low pressure zone on the "leeward" side and propelling the boat forwards. You'll also get a bit of drive from "deflecting" air backwards (equal and opposite reaction).

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Blowing you push the boat backwards. If the sail caught all your wind then it would net to zero.

You would be better off just drop the sail. Face forward to suck and face rear to blow.

Or leave the sail up and rock the boat side to side. On a small boat you can move slow but definitely move.

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  • $\begingroup$ yes, the rocking thing is a good point. $\endgroup$ – Carl Witthoft Jan 11 '18 at 15:35
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Bit of lateral (ha!) thinking - set the sail to an angle, stand to one side of it and blow across the beam of the boat - perpendicular to the axis and intended direction of motion.

That way, the reaction of the person blowing is perpendicular to the desired direction of travel and is counteracted by the keel. Forward motion is provided by the forward component of the reaction of the sail deflecting a crosswind, with the keel counteracting other component forces.

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Well After some googling I found this!

https://www.youtube.com/watch?v=uKXMTzMQWjo

It does indeed work possibly because the sail deflects the wind backward meaning that you go forward with conservation of momentum.

I thought it wouldn't work but hadn't figured that the wind would bounce off the sail

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From my interpretation of the OP the airboats fan is round the wrong way. It's blowing the air out the back which moves the boat forward. You would need to turn the fan around 180 degrees and attach a sail in the middle of the boat for it to be an accurate representation of the OP. Or make a frame and attach a sail behind the air boat.

If you did turn the fan around and attach a sail in the middle from a basic force point of view the air would hit the fan and be transferred down the mast, back through the hull and back up through the fans frame which would cancel itself out.

But when the sails introduced the force dynamics are changed as pointed out by Phil. And if the sail does create twice the reaction force then the boat would move forward.

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If you would be in the water or standing ashore (and would have VERY powerful lungs) then yes. But like Solar Mike said: "for every action there is an equal and opposite reaction". So the ship won't move if you would be standing on the ship itself.

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    $\begingroup$ You could also just stand on the boat and blow air 'overboard' using your extremely powerful lungs - like a jet engine :) $\endgroup$ – Jonathan R Swift Jan 11 '18 at 13:30
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    $\begingroup$ typo s/shop/ship/? $\endgroup$ – ratchet freak Jan 11 '18 at 14:14
  • $\begingroup$ It would move backwards as the sail would not catch all the air. $\endgroup$ – paparazzo Jan 11 '18 at 14:34
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    $\begingroup$ This is just plain wrong. $\endgroup$ – Carl Witthoft Jan 11 '18 at 15:31
  • $\begingroup$ -1. You haven't considered what the sail can produce in terms of reaction force. It you blow forwards, it can reverse the flow, and produce twice the reaction force. $\endgroup$ – Phil Sweet Jan 11 '18 at 19:51

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