Fan moving a Sailboat

[J.C.MacSwell]

I can`t see how That would work either tho Ed?

in theory it`s a perpetual motion machine' date=' you need only blow on the windmill once get moving forwards and that movement even without wind will drive the windmill.

y[b']ou can`t have any net GAIN by sailing into the wind using this surely[/b]!?

 

You need wind to sustain it, but it will go upwind. It's not a PMM.

[Sisyphus]

Same amount at a faster speed.

 

Which is accomplished by... magic? Seriously, though, that violates Newton's third law. You'd be having the air exert more force on the sail than it has to begin with. Nothing rebounds at a greater speed than it impacts.

[J.C.MacSwell]

Which is accomplished by... magic? Seriously, though, that violates Newton's third law[/b']. You'd be having the air exert more force on the sail than it has to begin with. Nothing rebounds at a greater speed than it impacts.

 

I assure you it doesn't!

 

Try blowing through a straw if you want a simple example of the speed exceeding any "impacts". (and it's all impacts, though you can argue the molecule speeds are higher it is the same thing, the fluid can be accelerated, it must if it is funneled)

[MattC]

Speed is not the variable to use, J.C.Macswell.

When you throw something away from you, you exert a certain force on the object you throw. An equal and opposite-direction force is exerted on you.

 

 

A fan pushes a certain volume of air per second. If you attach a straw or similar device to a fan to constrict the volume of air, it must travel faster to keep the same amount of volume/second - it doesn't magically give the air more power, it just focuses it onto a smaller area of the sail.

 

That said, I can't imagine (and have yet to see) a design for a fan-equiped boat that worked best with the fan pointed the wrong way (at a sail, rather than straight behind you ... preferably an underwater fan, ie a propeller). If you actually do use a fan, you would want to reduce the drag on the front of the boat as much as possible, and make the whole thing very bouyant.

[J.C.MacSwell]

Speed is not the variable to use' date=' J.C.Macswell.

When you throw something away from you, you exert a certain force on the object you throw. An equal and opposite-direction force is exerted on you.

 

 

A fan pushes a certain volume of air per second. If you attach a straw or similar device to a fan to constrict the volume of air, it must travel faster to keep the same amount of volume/second - it doesn't magically give the air more power, it just focuses it onto a smaller area of the sail.

 

That said,[b'] I can't imagine (and have yet to see) a design for a fan-equiped boat that worked best [/b]with the fan pointed the wrong way (at a sail, rather than straight behind you ... preferably an underwater fan, ie a propeller). If you actually do use a fan, you would want to reduce the drag on the front of the boat as much as possible, and make the whole thing very bouyant.

 

"Worked best" is not the issue. I'm not advocating it, I just claim it can be done.

[Avner]

| f=mv --> f=mv --> |
____|_______ _______|_____

f=MV <--- \___________ ____________/ ---> f=MV

Consider two separate parts of a boat.

Left half of the boat: Fan pushes an air particle to the right.

the paricle carries a force f.

 

As a reaction, the Left half of the boat is pushed to the left
with force f.

 

The particle, originally pushed to the right by the fan, now
arrives at the vicinity of the Right half of the boat, hits the sail with force
f pushing the Right half of the boat to the right.

 

The combined boat is thus subjected to two equal and
opposite forces and therefore will not move horizontally.

 

Of course, if f is large enough, the ship would be torn apart
by the stress caused by the opposing forces and as a consequence move downward

[J.C.MacSwell]

 

 

 

| f=mv --> f=mv --> |

____|_______ _______|_____

f=MV <--- \___________ ____________/ ---> f=MV

 

 

Consider two separate parts of a boat.

Left half of the boat: Fan pushes an air particle to the right.

the paricle carries a force f.

 

As a reaction, the Left half of the boat is pushed to the left

with force f.

 

The particle, originally pushed to the right by the fan, now

arrives at the vicinity of the Right half of the boat, hits the sail with force

f pushing the Right half of the boat to the right.

 

The combined boat is thus subjected to two equal and

opposite forces and therefore will not move horizontally.

 

Of course, if f is large enough, the ship would be torn apart

by the stress caused by the opposing forces and as a consequence move downward

 

 

Hi Avner...this is an old thread!

 

Particles carry momentum not force.

 

For a boat and air particle initially at rest:

 

If the force accelerating the particle is the same but opposite in each case, and for the same duration, the particle (and boat) will again be stationary...but the boat will be displaced to the left (not as much as the presumably lighter particle is displaced to the right) You may of course consider this displacement insignificant and ignore it

If the force accelerating the particle is the same but opposite, but for more or less duration in each case, the boat will be in motion at some velocity, with momentum equal to but opposite that of the particle.

[MigL]

You are correct in that momentum is always conserved J.C., and that is exactly why a fan located on the boat will not generate any net thrust onto the sail at all.

 

As for tacking, it will not work with a flat bottom boat, since even if the sail is used as an airfoil, it is impossible to generate a net thrust at greater than a perpendicular angle. Tacking only works when you have a 'sail' on the bottom of the boat also. Without this keel/sail interaction tacking is impossible.

[J.C.MacSwell]

You are correct in that momentum is always conserved J.C., and that is exactly why a fan located on the boat will not generate any net thrust onto the sail at all.

 

As for tacking, it will not work with a flat bottom boat, since even if the sail is used as an airfoil, it is impossible to generate a net thrust at greater than a perpendicular angle. Tacking only works when you have a 'sail' on the bottom of the boat also. Without this keel/sail interaction tacking is impossible.

A fan on a boat can generate thrust. A sail or sails can be used to redirect the thrust, including reversing it 180 degrees. The fan without the sail has net thrust, and the fan/sail system can have net thrust. Momentum and energy are of course conserved. Think of a pump (fan) and a hose (arguably a "sail") you can get net thrust in any direction you wish...it is just a little trickier setting up actual sails in the right way but it can be done.

[Delta1212]

A fan on a boat can generate thrust. A sail or sails can be used to redirect the thrust, including reversing it 180 degrees. The fan without the sail has net thrust, and the fan/sail system can have net thrust. Momentum and energy are of course conserved. Think of a pump (fan) and a hose (arguably a "sail") you can get net thrust in any direction you wish...it is just a little trickier setting up actual sails in the right way but it can be done.

It would be far more efficient to use a fan without a sail to supply thrust than to use a fan blowing into a sail, though. And very easy to set up the sail such that it almost entirely negates any thrust supplied by the fan, which would, in any case, be in the opposite direction that the fan was blowing.

 

The situation as conceived in the original question, for instance, would result in a net thrust in the opposite direction desired, but a very minuscule one as the sail actually acts to blunt the already tiny thrust supply by the fan.

[J.C.MacSwell]

It would be far more efficient to use a fan without a sail to supply thrust than to use a fan blowing into a sail, though. And very easy to set up the sail such that it almost entirely negates any thrust supplied by the fan, which would, in any case, be in the opposite direction that the fan was blowing.

 

The situation as conceived in the original question, for instance, would result in a net thrust in the opposite direction desired, but a very minuscule one as the sail actually acts to blunt the already tiny thrust supply by the fan.

 

 

 

"Worked best" is not the issue. I'm not advocating it, I just claim it can be done.

As per 7 years ago, my point is that there is nothing in principle that prevents it being done...

[MigL]

Sorry J.C., my original ( too quick ) analysis considered a very large sail only 'catching' the fan's flow. If it redirects it in the opposite direction, and we consider final momentums, then yes there is a net thrust and I'm wrong. Not the first time and I'm sure it won't be the last.

 

Incidentally, this is the way thrust vectoring and reversers work on aircraft.

Edited May 12, 2013 by MigL

[N3r0]

Can someone please explain this ?

and this

How is this possible, with newtons 3rd law? Edited October 14, 2015 by N3r0

[overtone]

s this possible, with newtons 3rd law

 

 

I can guess. They're tacking, essentially. Any sailboat with a keel or something to resist sideways motion can sail upwind, against the wind, and the same thing is happening here. They are directing the fan blow at an angle to the centerline of the boat, so the force from the fan is not directly opposed to the net force on the sail. All of the force from the fan is directed at the angle of blow, and a lot of it is trying to push the boat sideways. Less of the bounce force off the sail is directed sideways, more of it down the centerline, and the difference propels the boat.

Edited October 17, 2015 by overtone

[J.C.MacSwell]

Can someone please explain this ?

and this

How is this possible, with newtons 3rd law?

I can't believe Myth Busters couldn't figure it out. If they had used a spinnaker (perimeter of sail angled back) instead of a square sail (perimeter of sail directed at 90 degrees) it would have been much more effective.

[Manticore]

Downwind faster than the wind.

https://phys.org/news/2010-06-wind-powered-car-faster.html

I believe the same team later managed to travel upwind faster than the wind also.

[Gerardo Bridges]

Hello sir,

 

I've been reading your question. Maybe this can help you... If you add a fan to your sailboat you SHOULDN'T PLACE IT AT THE BACK OF THE BOAT (blowing into the sails from behind). Newton’s action-reaction law will slow the boat down. You should, however, place the fan IN FRONT of the boat (blowing backwards, sideways into the sails). This sounds counterintuitively, I know. But this will not only propel the boat forwards, the faster moving air will also create a low-pressure-area which will provide lift for your sails (principle of sailing into the wind). This is an interesting movie about the physics of sailing which explains the principle:

Of course, the most energy-efficient way of moving your boat in a windless environment is having a propeller that thrusts air backwards (without sails). But if you absolutely want to use sails the method mentioned here above is the best way to do it, I think. That, and having a truly gigantic fan.

 

I hope this was helpful.

Edited April 30, 2017 by Gerardo Bridges