Fan on a sail boat

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cbrunny

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If you have a sailboat with a fan on it and point the fan at the sail and turn on the fan does the boat move forwards?
 
Chaotic42

Chaotic42

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Red Squirrel

Red Squirrel

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If anything the sail will deflect the air and while it may make the ship move, it's basically the same effect as an elevon on a plane and just redirecting air. You are better off pointing the fan AWAY from the boat and not having the sail at all. The sail is just impeding the air flow in this case. Or use the sail as an elevon to control direction if fan can't move. (ex: like a hovercraft)

Now if you had two boats, one with a fan and one with a sail the one with the fan could push the one with the sail, but it would also be pushing itself away, so it would need a motor to keep it going. Opposing forces and stuff. Same reason you don't want to high five in space without being tethered.
 
Ruptga

Ruptga

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Hey guys, if I put a parachute behind the engine of a plane, will the plane fly backwards?
 
IronWing

IronWing

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Red Squirrel said:
If anything the sail will deflect the air and while it may make the ship move, it's basically the same effect as an elevon on a plane and just redirecting air. You are better off pointing the fan AWAY from the boat and not having the sail at all. The sail is just impeding the air flow in this case. Or use the sail as an elevon to control direction if fan can't move. (ex: like a hovercraft)

Now if you had two boats, one with a fan and one with a sail the one with the fan could push the one with the sail, but it would also be pushing itself away, so it would need a motor to keep it going. Opposing forces and stuff. Same reason you don't want to high five in space without being tethered.
Click to expand...
Actually, in theory, with my set up, the boat could move faster than the speed of the airflow coming from the fan. This isn't possible with a simple fan boat.
 
SKORPI0

SKORPI0

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Jan 18, 2000
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qZgB2.jpg


vs5Bf.jpg

Blowing your own sail?

The concept of blowing your own sail really does have to do with conservation of momentum. In that very episode of Mythbusters you speak of, the sail was removed, the fan was spun around and the ship/boat was propelled forward much faster than with the fan facing into the sail (i.e. figure (1) is much faster than figure (2)).

The reason is really quite simple and can be explained with throwing a ball off a boat.
Suppose you are on a boat carrying a ball with total mass mball+mboat" role="presentation">mball+mboat
where mboat" role="presentation">mboat also takes into consideration your mass. Now if you throw the ball off the boat at velocity vball" role="presentation">vball then you and the boat will have momentum mboatvboat=−mballvball" role="presentation">mboatvboat=−mballvball
. This is analogous to the figure (1).

Now consider the case of figure (2). In this case, I throw the ball at the sail, it bounces off the sail and into the water behind me. Because the process is inelastic, the ball now leaves the boat with vball&#x2032;&lt;vball" role="presentation">v′ball<vball

. Therefore my momentum is now mboatvboat=&#x2212;mballvball&#x2032;" role="presentation">mboatvboat=−mballv′ball
.
Now just replace the ball with air molecules and the analogy is complete. Therefore it will always be more efficient to spin the fan around and blow the fan in the opposite direction while forgetting the sail.
Click to expand...

related Mythbusters episode (2nd picture) - https://www.youtube.com/watch?v=uKXMTzMQWjo
 
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monkeydelmagico

monkeydelmagico

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Nov 16, 2011
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cbrunny said:
If you have a sailboat with a fan on it and point the fan at the sail and turn on the fan does the boat move forwards?
Click to expand...

Is the sail perpendicular or parallel to the fan? Is the fan 3 or 4 blade? What is the circumference of the fan? What is the square surface area of the sail? What is the airspeed of an unladen swallow?
 
GagHalfrunt

GagHalfrunt

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Wait! What if we take the Mythbusters experiment and add in an anti-spin gyroscope powered by Adlep's E-Cat doohickey?
 
Ruptga

Ruptga

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IronWing said:
See the Boeing 737 for details.

See 1:50:

Click to expand...
That's not a parachute, that's ducting that redirects practically the entire airflow. A parachute that captures more than half of the air from the engine (unlikely) would just reduce the plane's thrust by about as much due to drag. Ducting that redirects more than half of the flow makes the plane move backward by changing the direction that the engine's net thrust is applied. A sail is far more like a parachute than ducting, so it's obviously a less stupid idea to just remove the sail and turn the engine around to the direction you want to go.
 
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S

SaltyNuts

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May 1, 2001
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LOL bunch of morans in this thread. Force of wind generated by the fan pushing on sail is equal to a force in the exact opposite direction being generated against the fan blades (equal and opposite reaction, remember that anyone?), with that force being transferred to the fan, and then to the boat to which the fan is attached, cancelling out 100% the effect generated by the wind from the fan. Deeeeeeeeeeeeerp.
 
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IronWing

IronWing

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SaltyNuts said:
LOL bunch of morans in this thread. Force of wind generated by the fan pushing on sail is equal to a force in the exact opposite direction being generated against the fan blades (equal and opposite reaction, remember that anyone?), with that force being transferred to the fan, and then to the boat to which the fan is attached, cancelling out 100% the effect generated by the wind from the fan. Deeeeeeeeeeeeerp.
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That's why you mount the fan at 90 degrees to the keel and tack. The forces are no longer aligned and the boat moves forward. Sailors figured this out centuries, maybe even millennia, ago.
 
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lxskllr

lxskllr

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IronWing said:
That's why you mount the fan at 90 degrees to the keel and tack. The forces are no longer aligned and the boat moves forward. Sailors figured this out centuries, maybe even millennia, ago.
Click to expand...
Been thinking about this. A sufficiently powerful fan would increase the draft, giving more resistance. I wonder how that correlates with the energy you get from the wind.
 
Red Squirrel

Red Squirrel

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Actually one thing I always wondered, if you mounted a squirrel cage style windmill on a boat that made it control a propeller (whether it's electric, or directly), you could technically sail directly into the wind. At least in theory I think that would work. Or would the forces cancel out?
 
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