Sailboats providing their own wind

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Summary:

If you aimed a fan on a boat at its own sails, could it blow the boat forward?

Main Question or Discussion Point

I happened to be watching this cartoon and this one scene seriously bothered me...


This... just feels like it wouldn't be a workable solution. Am I wrong?

I can't say I perfectly understand how fans or propellers work, but I would imagine that by pushing air or water in one direction, equal force is created in the opposite direction for the base. Thus how a boat moves forward by pushing water back, or a plane moves forward by pushing air back...

In this case, pushing air forward towards a sail should push the boat backward, and the sail catching that created air-current should then counteract that, creating forward force roughly evening things out and keeping the boat stationary.

Am I wrong? When stuff like this is done under the masquerade of an educational series I can't help but wonder if they might be trying to troll adults who watch this or get them pestered by angry kids who can't make it work.
 

Answers and Replies

Bystander
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Your intuition is correct; Wile E. Coyote has done extensive research on "cartoon physics."
 
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No. Your question borders on perpetual motion, but it is not PMM if the fan is powered by batteries.

But again no. The fan's wind on the sails would produce a forward thrust, but the fan blade would produce a reverse thrust. Think of the fan like the propeller on a single engine airplane, but this propeller blows in the wrong direction for forward thrust.

Saying it again with other words, if you hang a parachute behind a propeller airplane, it will not fly backwards.
 
A.T.
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...roughly evening things out and keeping the boat stationary...
The key word here being "roughly". Depending on how the sail is shaped the boat can go forward, but it's not as efficient, as just using the propeller.

 
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how the sail is shaped the boat can go forward, but it's not as efficient, as just using the propeller.
"Balloon" does not equal "propeller."
 
A.T.
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Don't have to; delta P goes from non-zero before to zero after; ergo movement. A fan/propeller starts from delta P equals zero and ends at delta P equals zero; ergo no movement.
 
DaveC426913
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It can be done in theory, using a principle known as entrainment.

Even though the fan is attached to the boat, and pushes back with equal force, the air is viscous, and carries some air next to it along for the ride. You can get a little forward motion (if you angle the sail right), but its very inefficient.

It does not violate conservation of energy, since, after all, you are adding energy into the system, by whatever powers the fan. And it doesn't violate Newton;s Laws of motion because it's not a closed system.

This has been demonstrated.
 
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A fan/propeller starts from delta P equals zero and ends at delta P equals zero; ergo no movement.
Does a jet engine counts as a fan? Than google for thrust reversal. A sail can do this job as well (but it is very ineficcient).
 
A.T.
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It can be done in theory, using a principle known as entrainment.
It's much simpler. Replace the sail with a board, and let the passenger collect balls from the ground and throw them at the board, so they bounce off and leave with backwards momentum. The boat gets the opposite forward momentum.
 
DaveC426913
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The boat gets the opposite forward momentum.
..., after losing momentum to the ground as the passenger scoops them up; net zero.
 
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I think you may be missing the point. It is supposed to be instantly recognizable as impossible. That’s why it’s funny when it works. It’s exactly like a cartoon character picking themselves up by their bootstraps.
 
Ibix
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..., after losing momentum to the ground as the passenger scoops them up; net zero.
Not if the ball is rolling backwards after it lands. The ball has a acquired negative momentum from somewhere.
 
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Ibix
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..., and that would be...?
The only other entity in the experiment is the boat, so the boat's forward momentum must have increased. What else could it be?
 
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so the boat's forward momentum must have increased. What else could it be?
Bootstrap violation of "conservation of momentum?"
 
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Does a jet engine counts as a fan? Than google for thrust reversal. A sail can do this job as well (but it is very ineficcient).
My Daddy taught his kids enough about tacking into the wind -- having the wind blowing full face opposite against your intended direction is very pleasing when it's compared to being becalmed --

"Water, water, every where,
And all the boards did shrink;
Water, water, every where,
Nor any drop to drink."

From The Rime of Ancient Mariner, by Samuel Taylor Coleridge
 
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Here's a video of someone displaying a method by which a fan can be employed to propel a boat, which method is not as ludicrous as trying to impel a sail with a not-especially-dissimilar fan . . .

 
jbriggs444
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Thought this had been discussed previously; guess the provoking cartoon was Finding Nemo.
It's an open system. The air enters, is pushed by fan, is pushed by sail and exits. It need not exit at zero velocity. Hence thrust reversers as have been mentioned already.
 
A.T.
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Last edited:
tech99
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However, if you have a windmill on a boat, driving a water propeller, it can push the boat against the wind.
 

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