Fan moving a Sailboat

[mooeypoo]

Hi guys,

 

This is probably a question that has been asked before, but I just thought about it a few days ago and couldn't quite figure out if I have the right answer.

 

If we look at a sailboat, it's working in a quite "simple" way: The wind pushes the sail and gives the boat momentum. The wind, however, is not something that we can say originates from a specific POINT. It's the movement of air molecules.

 

What would happen if I stick a very powerful fan on the rear of the sailboat, facing the sail? Would my sailboat be faster?

 

When I was thinking about it, the first thing that came to my mind is a comparison to a situation where I'd be pushing the boat from within the boat itself. Which would've given me nothing, obviously. But here, we are talking about wind. Wind is not "stuck to the ground" like me, when trying to push the boat.

 

On the other hand, the fan is still moving WITH the movement of the boat itself.. so.. I kinda got stuck there.

 

So. Would a sailboat move faster with a fan creating wind blows on the Sail?

 

~moo

[Sisyphus]

No, it would move quite a bit slower, or even backwards. The fan is pushing air forwards, which propels you backwards. At best, some of this air is caught by the sail and thus neutralized, but it can't possibly neutralize completely since the velocity of the air will be greatest as it first leaves the fan, and thus the thrust of the fan pushing your backwards is necessarily more than the sails pushing you forwards.

[mooeypoo]

So (and excuse me for "lowering" the level.. hehe) what you're saying (if I got you right) is that the fan acts like the "Propeller" in a boat, only it's in the air instead of in the water?

 

Err.. I am not quite familiar with the forces that apply, so sorry if this is a stupid question.

 

~moo

[[Tycho?]]

So (and excuse me for "lowering" the level.. hehe) what you're saying (if I got you right) is that the fan acts like the "Propeller" in a boat' date=' only it's in the air instead of in the water?

 

Err.. I am not quite familiar with the forces that apply, so sorry if this is a stupid question.

 

~moo[/quote']

 

Well... this is an excellent example of Newtons 3rd law of motion. Every action has an equal and opposite reaction, I'm sure you've heard that said somewhere.

 

So first consider a fan alone, no boat. Our fan is quite powerful, but not too heavy. It is sitting on slippery ice. If you plug the fan in, the air shoots out the front, with a force we'll call A. Its equal reaction would cause the fan to move backwards across the ice with a force B, like a gun pushes back when you shoot it. Newtons 3rd law tells us that forces A and B are equal and opposite.

 

Now lets get to the boat. We have out fan, and its bolted down to a boat, aimed so it will shoot air into the sail with a force we'll call C. But we already determined above that when you turn on the fan it will move backwards with some force. Since the fan is bolted down it doesn't move, so it does push the boat back. It pushes in both directions an equal and opposite amount, and so you dont move.

[Deified]

If you turned the fan around, so it pointed backwards, you would make the boat faster. As you said, it would be a propeller. (boats like this are often used where minimum draft is essential)

[bascule]

http://en.wikipedia.org/wiki/Air_boat

[mooeypoo]

Aahh.. bascule, isn't that fan directed in the opposite direction? it's positioned to blow air "backwards", no?

 

And, yes, I'm familiar with Newton's 3rd Law, I just don't know which of the actions is "bigger" (or.. stronger?) to create a more massive reaction: The air "pushing" the sail, or the air pushing the boat backwards...

 

Now, another simliar question (or an "expansion" of this one):

What if I put the Fan OUTSIDE the boat, but travelling with it - say, with another boat behind it, giving the sail constant wind. Then, the sailboat would move by the power of the wind itself, right? So.. could I create a situation where I position the fan ON the boat, but raise it's "windpower" high enough to overcome the "backward" force and hence get my boat moving forward still?

[Sisyphus]

And' date=' yes, I'm familiar with Newton's 3rd Law, I just don't know which of the actions is "bigger" (or.. stronger?) to create a more massive reaction: The air "pushing" the sail, or the air pushing the boat backwards...[/quote']

 

Well, since one (the fan) is the source of the other (the sail), the second can't possibly be bigger than the first.

 

Now, another simliar question (or an "expansion" of this one):

What if I put the Fan OUTSIDE the boat, but travelling with it - say, with another boat behind it, giving the sail constant wind. Then, the sailboat would move by the power of the wind itself, right?

 

Yes. (But the fan would still be pushing THAT boat backwards.)

 

So.. could I create a situation where I position the fan ON the boat, but raise it's "windpower" high enough to overcome the "backward" force and hence get my boat moving forward still?

 

No, since the "windpower" IS the backward force. The fan is pushing air forwards, which means, by Newton's third law, that it is pushing the boat backwards. When the air hits the sail, it then pushes the boat forwards again, but it can't possibly push the boat forwards more than it pushed it backwards, since it's the same air! At best you just won't move, no matter how powerful the fan is. In reality, though, you'd still move backwards, because the air would diffuse and be moving slower (and thus exert less force) between when it leaves the fan and when it hits the sail. Thus backwards > forwards.

[mooeypoo]

No, since the "windpower" IS the backward force.

 

I thought about that, but then, doesn't the force on the sail is connected also to the distance of the fan from the sail itself?

 

If I imagine it correctly, then if I put the fan very far away, it will barely affect the sail - so what if I put it very very close to the sail? will I never be able to create a "bigger" effect?

 

~moo

[padren]

I thought about that' date=' but then, doesn't the force on the sail is connected also to the distance of the fan from the sail itself?

 

If I imagine it correctly, then if I put the fan very far away, it will barely affect the sail - so what if I put it very very close to the sail? will I never be able to create a "bigger" effect?

 

~moo[/quote']

 

The size of the effect approaches but does not reach 100% effeciency, which still doesn't help.

 

The fan is pulling air from behind moving the ship backward, and the sail is utilizing some percentage of that to move forward. Even if you took every single speedy air particle and converted the entire forward momentum gained by the fan into the momentum of the ship via the sail, you would still at best hit a point where it was cancelling out the backwards force exactly.

 

Its like when you shoot a gun, you get knocked back somewhat, and the target gets knocked forward somewhat. You couldn't glue a gun to the deck and have it shoot the mast to make it go forward.

 

With the sail and fan, instead of shooting one lead bullet you are shooting many particles of air that are floating around the fan at the sail.

 

 

Also with your two ship system:

 

A fan on the aft ship would help the first one go faster, but it would at best be a speed transference system - it could be useful if you have a fast ship behind a slow heavily loaded ship and wanted the faster one to help the slower one so they hit an average middle speed...but it would still be slower than the exact average of the two speeds.

[YT2095]

tie a rocket down to a steel plate with the exhaust aimed at it.

that`s exactly what you have in effect.

it would not take off.

[Edtharan]

years ago I used to sail boats and even raced them (as a deck hand not the skipper). I do know that modern sails do not jsut simply catch the wind to push the boat a long. This only occures when you are "runnning with the wind" (the wind is behind you).

 

It is posible to actually move across the wind (reaching) and even move at an angle towards the wind (you need to constantly tack accross it though and can't head directly into it).

 

So the scenario with the fan at the back of the boat and it blowing the air towards the front is only one way it could be set up.

 

Another could be like reaching. You position the fan on one side of the boat and the sail then directs the air towards the back. This would give the boat a vector more forwards (but still to one side a bit due to the reaction of the fan and the air.

 

Anothyer aspect of modern sails is that they act more like a wing than a parachute. It is the way the air moves past them that gives the thrust to the boat. As the air moves past them it makes the sail pull (due to the bernouli principle) to one direction. It is the centreboard using the resistance of the water and the angle of the rudder that then redirects that vector into a forward motion (rather than dragin the boat sideways).

[gcol]

It is posible to actually move across the wind (reaching) and even move at an angle towards the wind (you need to constantly tack accross it though and can't head directly into it).

 

Interesting' date=' but it seems to me the fan just alters rhe direction of wind flow relative to the boat by [i']pushing[/i] on the sails/rigging, an effect achieved in practice and far more efficiently by the time honoured method of pulling on the rigging with ropes on the opposite side to the fan. I.E, instead of having a fan, you might as well have the crew pushing on the booms and yardarms with long poles from the windward side......

[YT2095]

true.

because in the Original setup all you`de suceed in doing is creating a high pressure zone between the fan output and the sail, this pressure excapes out Sideways.

so in effect the stresses are trying to pull the boat appart Bow/Stern and also trying to squash it in Port/Starboard.

 

much better if you used a Water Proof fan though, you could put it in the water, and Then you`de move along quite nicely

[J.C.MacSwell]

As long as the sail/fan system can direct more air momentum backward than forward it can propel the boat forward.

 

Also a fan can be set up to catch the wind and turn a water propellor which propels the boat straight upwind. No energy source other than the wind required.

[gcol]

As long as the sail/fan system can direct more air momentum backward than forward it can propel the boat forward.

 

Also a fan can be set up to catch the wind and turn a water propellor which propels the boat straight upwind. No energy source other than the wind required.

 

Indeed, a given, but I was stubbornly attempting to stick loosely to what I understood to be the original question.

 

However, a google for "rotary sail" shows what has already been achieved commercially with wind-powered fan type sails driving a water-propeller.

[J.C.MacSwell]

Indeed' date=' a given, but I was stubbornly attempting to stick loosely to what I understood to be the original question.[/b']

 

However, a google for "rotary sail" shows what has already been achieved commercially with wind-powered fan type sails driving a water-propeller.

 

My point is that it is possible to do this. Direct more momentum backwards in spite of the fan facing forward.

[Sisyphus]

My point is that it is possible to do this. Direct more momentum backwards in spite of the fan facing forward.

 

No, it isn't. You would have to be pushing more air backwards than forwards in order to move forwards. Since the only air you're pushing backwards is in fact the same air you moved forwards, that can't possibly be the case.

[Edtharan]

No, it isn't. You would have to be pushing more air backwards than forwards in order to move forwards.

Or push it faster backwards (which without any extra energy it is imposible).

 

If you put the fan at 90 degrees to the direction of travel and the sail at 45 degrees to the fan, then you might get a net push forwards (at 45 degrees to the direction of travel) and then use the rudder to correct the course as needed (not 100% this would work, but it looked ok on a quick paper sketch - no maths). This would moslt likely work better with a flat sail (like a plank of wood). However you would get much better efficiency if you put the fan facing backwards .

 

However, a google for "rotary sail" shows what has already been achieved commercially with wind-powered fan type sails driving a water-propeller.

This is not so much a fan as a windmill. A fan uses energy to push the air where as the "rotary sail" uses the air to turn a propeller underwater. It is not useing a "Fan" to push the boat forwards, but useing the wind to drive a propeller.

[YT2095]

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

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

you can`t have any net GAIN by sailing into the wind using this surely!?

[Skye]

That movement would drive the windmill, but because the system isn't 100% efficient, the drag of the windmill would be a greater force than the thrust of the propeller that results. So you'd come to halt. It only works if there is the wind to overcome the inefficiencies of the system.

[YT2095]

IIRC, there IS a design whereby a force of moving air will bring an object towards that source as opposed to blowing it away from.

I can`t quite rem it 100% but I think its some sort of curved `V` shape, the wind hitting the Base of the V creating a sort of Hi/Low pressure area, the High pressure is in the "Body" of the V shape, and the differential pulls it forwards.

it`s Primary school stuff, you can make them youself that float in your bath and test them, like rubbing soap on the back of cardboard and the surface tension pulls the boat along etc...

 

it`s been a while, but I`m sure there IS a way!

[gcol]

And also remember the two types of sailboat wind turbines, the horizontal and the vertical axis designs. The horizontal being pivotted into the wind, and the vertical being fixed. It seems likely that the 'into wind' performance of each will be different. I seem to remember that a moderate size tanker had two verticals, but it almost certainly had conventional standby engines too.

[YT2095]

you`de have to be a Nut Case if you had a tanker and no back-up!

[J.C.MacSwell]

No, it isn't. You would have to be pushing more air backwards than forwards in order to move forwards. Since the only air you're pushing backwards is in fact the same air you moved forwards, that can't possibly be the case.

 

Same amount at a faster speed. Net momentum transfer backwards of the air.

 

It actually mixes with other air so another way of looking at it is that it is not the "only" air displaced backwards.