Blowing your own sail

Nomisf said:
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You might want to work on that drawing somewhat.. ;)

There is not 80Nm of force acting on the front of the sail / in that direction

Also in the picture you have 90Nm acting in one direction and the 100Nm fan which leaves 10Nm, however your legend shows a 20Nm difference.
 
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Energize said:
If the sail was removed the boat would have gone in the opposite direction, the thrust from the fan is going towards the sail which propels the boat in the opposite direction, like an aircraft as acidhell says.
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I agree with AH, it's just that Sanexe seems to think the sail is exerting some sort of force on the boat that is somehow moving the boat forward, when it isn't, it's just vectored thrust from the fan.

Just like a jet engine with reverse thrust.
The jet engine is designed to propel the aircraft forward with thrust from the rear of the jet going backwards however if you close off that path to the rear with the reverse thruster plates, all the air is now being forced to thrust forwards causing the aircraft to reverse.
It's not the jet blowing on the Reverse thruster plates making it go backwards.
 
Scubascorpion said:
You might want to work on that drawing somewhat.. ;)

There is not 80Nm of force acting on the front of the sail / in that direction

Also in the picture you have 90Nm acting in one direction and the 100Nm fan which leaves 10Nm, however your legend shows a 20Nm difference.
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Now you mention it :p

The 80 was supposed to signify the amount of force the sail was catching, just midnight fail drawing !

Edit: Removed the failmath!
 
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Just to add a little bit of controvesy... If you push the boom out over the port side of the boat and then mount your fan on the starboard side so that it blows across the sail would you be able to artificially create a pressure imbalance that would then suck the boat forward? The keel would potentially help counter act one set of forces...

Also if you are using energy to power the fan then surely you are supplying energy to the system. The direction of travel of the boat is then purely a matter of thrust vector?
 
if you stood on the boat and pushed the sail with your hands the boat wouldnt move, but if you stood off the boat and pushed, it would
 
AcidHell2 said:
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.
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But the plane does not fly.
 
Liquidfox said:
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?



The fan in this case is internal as it's attached to the boat.



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?
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Ok, my physics may not be great, but surely this is more to do with how sails work than newton's law, or the fact that the fan is attached to the boat? You're creating an external force by applying power to the fan the same way as if it were a propeller in the water.
 
yashiro said:
But the plane does not fly.
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Groan, of course it flies.
Wheels on a plane are unpowered, therefore no force is applied to the aircraft from the treadmill (well a bit of force from friction but not a lot). Thus the plane takes off.
Cars are different as the wheels are powered and in contact with the treadmill.
 
MattU said:
Ok, my physics may not be great, but surely this is more to do with how sails work than newton's law, or the fact that the fan is attached to the boat? You're creating an external force by applying power to the fan the same way as if it were a propeller in the water.
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Your example is flawed, it would only be like a propella if you put a bug plank behind the propella. As such all the force hits the board and is transferred back to the boat and hence no movement. Just like the helicopter video posted on the first page.
 
Superewza said:
And yet the air you're blowing at the sail isn't coming from inside the craft, it's coming from outside. The fan creates a difference in pressure so the air moves in a general 'towards the sail' direction. Which is essentially exactly the same effect as wind.
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of course its not, Wind coming from outside of the boat pushing a sail, yes wind being created BY a fan on the boat no of course not, compressed air from a cylinder of course that would work but a fan is creating the same thurst backwards by displacing the air as it is pushing into the sail, so you aint going anywhere, certainly not fast!
 
yashiro said:
But the plane does not fly.
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again yes it would, unlike a car which would just stay still as it uses the wheels for foward movement the plane soley uses the engines and as said above the wheels are totally unpowered, initially it would start moving backwards then reach the point at which the wheels spin freely then take off, the fact the wheels are spinning has no relation to the foward thrust being applied, yes it will slow the take off but it wouldnt stop it, (unless the plane flipped or something)
 
I don't get why this is confusing.

Does anyone disagree that pointing the fan directly out of the back of the boat, would move the boat forwards, a-la hovercraft?!

Now turn the fan around and get the sail to act like a 'shield'. The air filling the sail, is then directed backwards and if enough of the deflected air acts on the water in the same way as when the fan was pointed out of the back of the boat, then you will get the same effect, ie the boat moves forward.

It's just that the sail is an incredibly inefficient way of getting the airflow to be in the direction you want. You'd be better just pointing the fan out the back!
 
div0 said:
I don't get why this is confusing.

Does anyone disagree that pointing the fan directly out of the back of the boat, would move the boat forwards, a-la hovercraft?!

Now turn the fan around and get the sail to act like a 'shield'. The air filling the sail, is then directed backwards and if enough of the deflected air acts on the water in the same way as when the fan was pointed out of the back of the boat, then you will get the same effect, ie the boat moves forward.

It's just that the sail is an incredibly inefficient way of getting the airflow to be in the direction you want. You'd be better just pointing the fan out the back!
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I heartily approve of all of the above.

(Except the water bit ;) )
 
div0 said:
I don't get why this is confusing.

Does anyone disagree that pointing the fan directly out of the back of the boat, would move the boat forwards, a-la hovercraft?!

Now turn the fan around and get the sail to act like a 'shield'. The air filling the sail, is then directed backwards and if enough of the deflected air acts on the water in the same way as when the fan was pointed out of the back of the boat, then you will get the same effect, ie the boat moves forward.

It's just that the sail is an incredibly inefficient way of getting the airflow to be in the direction you want. You'd be better just pointing the fan out the back!
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I think it's confusing people because they believe the thrust on the sail would cause the boat to move forwards, when it's the drive from the pressure returning off the sail that's actually supplying the net force. Obviously the boat isn't propelled by the same arrangement of forces as it would be were the wind blowing.
 
I suspect if they used a rigid board then they could prove the reflection as a higher % of energy should be reflected compared to the relatively flexible sail.

| <-- vs { <--

:)
 
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