Blowing your own sail

Liquidfox

Liquidfox

Liquidfox

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Following on from GD's favourite Plane on a conveyor belt:

Can a fan attached to a boat power a sail boat on a windless day?

Newton's First Law: Every body remains in a state of constant velocity unless acted upon by an external unbalanced force.
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The fan in this case is internal as it's attached to the boat.

Newton's Third Law: The mutual forces of action and reaction between two bodies are equal, opposite and collinear. This means that whenever a first body exerts a force F on a second body, the second body exerts a force −F on the first body. F and −F are equal in magnitude and opposite in direction.
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The fan pushing on the sail creates the opposite force that the fan is creating by pulling in the other direction.

But, will it sail?
 
I would say unlike the plane which flys, the boat would stay stationery. As you said equal and opposite reactions.

Just like myth busters did helicopters tied to a base. The force down, just hits the base and as such,no net force.
 
Ofcourse it will sail. The fan is displacing air. Displacing air creates pressure and movement. Just like a propeller on an airplane but pushing it onto a sail.
 
If all the force is going sideways then no, but assuming that the sail is at a bit of an angle to the boat, the air stream would be bent round to go backwards out of the boat, so it would push you forward. You'd probably be better just pointing it straight backwards like a hovercraft though.
 
I think they are right, the fan is providing to much thrust and it's bouncing of, so not working in the way people think. Smaller fan would produce no movement. Just like in this episode.

2:40 in
 
AcidHell2 said:
Nothing like a jet engine in reverse.
The boat is not moving in the typical sense.
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Indeed, everyone knows that boats don't actually move, they just move the universe around themselves. This is how they reach 200% fuel efficiency... oh wait no that's Futurama, sorry.
 
Lightnix said:
Indeed, everyone knows that boats don't actually move, they just move the universe around themselves. This is how they reach 200% fuel efficiency... oh wait no that's Futurama, sorry.
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By not standard way there are different type of sails, the ones people thing of just catch the air, this should not go forawrad, if you blow your own sail, due to newtons laws. On myth busters as they say it looks like the fan is producing so much air that it is being reflected by the sail and giving a net force, so the sail is nit being used in it's typical sense.
You then get aero foil type sails.
 
It's more about efficiency of each as to whether it would work or not.

Take a big enough fan, that just blowing into fresh air exerts a force on the boat via it's mountings as it sucks the air from one side and blows it out of the other.
Stick a sail onto the blow side of the fan and you have a counteracting force.

The sail has to be more efficient at catching that wind than the fan is at blowing it or the boat will not move.
 
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