We are Sailing (again)

The fan is rigidly mounted to the boat and not dangling from some other structure!

The boat moves in a downward direction!! The hull of the boat is not designed to cope with the two opposing forces and as a result it breaks in two and sinks.

Although my original answer is technically correct, I have to admit I especially like this answer!

Hilbillies in Florida! :)

http://www.airboatrides.com/

No, but it will go backward! Not all of the thrust produced by the fan will be stopped by the sail, so you basically have an "air-boat" with the prop turned around.

If the sail were to capture 100% of the fan's thrust, then it would be the same as someone throwing bricks inside a submarine. There would be an initial thrust from throwing the brick, but the moment that brick hit the floor or a wall an equal, but opposite, force will be created on the submarine. Basically, a closed system and no net change in direction!

What you described with the sail boat is similar, except a portion of that energy produced by the fan is not captured by the sail and so it is not a closed system. The net result is a loss in efficency of the fan, but enough thrust is generated to drive the boat backward.

Great question! Although there's a bit of ambiguity. If we assume the spinnaker is extremely small, then clearly the boat would have to move backwards -- much of the the stream from the fan would blow right past: You'd simply have an airboat, with its efficiency degraded to a large or small degree, depending upon the size of the spinnaker, the mast, rigging, etc.

But if the spinnaker is very large, so that the fan's stream is entirely containted within its borders, (and dragging in additional air with it) then the spinnaker would work just like a thrust reverser on a jet (or jet boat) redirecting most of the flow to the rear, so the boat would move forward.

In between, would be the case where the two effects cancel.

Your'e right Ken! Also see my post (#7055) below.

Although I think the first comment is correct, I figured I'd throw in my two cents. As I understand it, a sail creates a pressure differential. The wind piles up in the sale creating a higher pressure zone as compared to the air in front of the sale. The pressure difference produces the force that moves the sailboat.

A fan works by pulling air from one direction and pushing air in another. Behind the fan there is lower pressure and in front is higher pressure air. So a fan blowing into a sail creates a low pressure zone behind the fan and in front of the boat and a high pressure zone inbetween the fan and the sale. The high pressure air will produce forces in the direction of the low pressure zones resulting in equal and opposite forces, so the boat doesn't move.

However, if the sail doesn't catch all the air from the fan then the low pressure in front of the boat will be slightly larger than the low pressure behing the fan, producing a very small net force backwards. Just like the man said.

A follow up question, what would happen if the fan was blowing away from the sail?

The boat might rotate a bit due to gyroscopic action of the fan!

I think you would have an airboat that has drag! Look at the link I supplied. Instead of a sail you have passengers. Passengers are just sails, albeit physically deformed.

I keep going back to a book my father had while I was just starting high school that described physics. I remember a detailed discussion about thrust. The thought experiment consisted of a flat rail road car with a pile of bricks. A man would pick up a brick and throw it off the back of the car, thus providing thrust forward.

Now imagine the same experiment, but two men are on the car at either end. One man throws the bricks over the head or the other. The other man tries to grab the bricks as they go by, but only averages catching two bricks for every three bricks thrown and puts the caught bricks back into the pile.

For each brick that is caught, the thrust produced by the toss is totally negated when the second man catches it (both men are standing on the rail road car). The bricks that don't get caught still produce thrust via Newtonian physics.

The analogy above is exactly what happens with the original sail and fan question. Your question where the fan is turned around is really the same question as the first! It's like putting the aircraft in front of the propeller instead of the propeller first (there are real life aircraft that do this). So, both scenarios behave the same and both behave just like the rail road car with two men throwing and catching bricks.

Actually, if the fan were blowing away from the sail, you'd get a fair amount of forward movement - less than if the sail weren't there, of course. A fan sucks air IN from all directions, but blows it OUT in one direction. The sail wouldn't block the fan's intake enough to reduce the fan's thrust appreciably. It would, of course, act as a partial brake once the boat got moving.

Take down the spinnaker & go. The various combinations of air blown forward or backward (by using the sail as a diverter) don't do anyone much good.
This is equivalent to taking the sail down, switching off the fan and pushing the mast.

Blink s post #7043 on Thursday August 10 nailed the thing square on the head!
If one assumes a normal spinnaker and a powerful fan, arranged so that more than 50% (say 75%) of the air molecules will be caught by the sail and then more or less reversed in direction, the boat will sail forward!

If the momentum change per second of the air molecules at the fan is +mv, then the momentum change/s at the sail is ~ –0.75 x 2 x mv = -1.5 x mv, giving a net –0.5 x mv for the air and thus +0.5 x mv monentum change/s as 'thrust' for the boat. Not efficient at all, but cute!

How about ducting the fan so that it sucks air from the low pressure side of the spinaker?

thermo (1638) wrote: "How about ducting the fan so that it sucks air from the low pressure side of the spinaker?"

Hmmm... It might help, but expensive! How about putting a reduction pulley on the fan axis and a belt to drive a propeller - in the water!

Posted by JohnDG - Quote: "This is equivalent to taking the sail down, switching off the fan and pushing the mast."

Do you still think so after reading post #7055 and others (e.g. Blink)?

I agree the boat may move - but whether it's forward or back depends on the geometry of the sail.
Having the sail raised (when there's no wind) is a waste of energy.

But JohnDG, we are talking about a spinnaker, not a sail at an angle to the wind. And a fan blowing forward, directly into the spinnaker - no other wind.

As stated before, if significantly more than 50% of the fan's air stream gets caught and reversed by the spinnaker, the boat goes only one way - forward. Yea, its impractical – but challenge questions are seldom practical!

Ok, but I try to cut the c**p and look for a better way to do things. The spinnaker is just acting as a diverter - take it down & turn the fan round : )

Agreed, but one practical use of this technique is the reverse thrust braking on jet aircraft. There they can't just turn the fan around, so they "stick up a sail", so to speak.

As long as the amount of the air diverted backward by sail is not caught again by the fan.

I like the submarine example. When you throw the brick backwards the boat moves forward and then stops when the brick hits the bulkhead. However the boat has been moved forward.

Howetwo, you wrote: "As long as the amount of the air diverted backward by sail is not caught again by the fan."
I agree, but with a spinnaker, that is rather unlikely. To compare it with your submarine-brick example, we have to consider not a submarine, but an open decked boat. Suppose forward-thrown bricks hit some or other front 'bulkhead', bounce back somewhat at random angles and most fall off the boat. This boat will have a net forward movement – into the same direction that we throw the bricks! The argument is simple: we expel matter in a backward direction, so Newton said that we would go forward.

This reminds me of the reverse thrust on 727's. The turbine exhaust blows into spinnacker like thingies that reverse the direction of flow towards the front, thus creating reverse thrust. Eg. if the spinnacker were designed to not stop the flow, but reverse it, the boat could indeed move forward.

With the thrust reversers on jet aircraft you are forgetting one thing. Once the pressures equalize the volume of the gas that is ejected from the exhaust of a jet engine is greater than the volume in due to the massive heating of the air through the burning of the kerosene in the combustion chamber, therefore the exhaust velocity is considerably greater than the intake velocity resulting in a net reverse in thrust. This is not the case with a simple fan.

masu (1337) (post #7083) wrote: "...therefore the exhaust velocity is considerably greater than the intake velocity resulting in a net reverse in thrust. This is not the case with a simple fan."

Are you sure? The exit velocity of air molecules from a fan is also considerably greater than the intake velocity! So what is the difference?

If you look at it as a ducted fan the average velocity of the exhaust molecules is identical to the intake velocity. This is not the case with a gas turbine remember the equation (P1 V1)/T1=(P2 V2)/T2.Given that the two pressures are the same and the exhaust temperature is considerably grater than the intake temperature there must be an increase in volume and hence an increase in the average velocity of the exhaust gas compared with the average velocity of the intake gas. Note that today however pure gas turbine engines are rare and most commercial aircraft use fan jets which are really a combination of a gas turbine that drives a coaxial ducted fan and the fan provides the majority of the thrust. Apologies for the terrible way the equation shows up I need to find some way to enter them properly.

OK, I agree with the case of a ducted fan, but not quite for an 'open' fan. In any case, I think that one can take a simple, linearly ducted fan and direct the exit at the forward spinnaker. Provided that enough of the air is caught and deflected somewhat backwards, the boat can still go forward. It will happen if the net momentum change of the air is backwards.

I also do not know how to enter equations on this forum (no Latex functionality, I think).

The boat will move backward.
Remember the bernoulli's principle lab. demo.
with the spool, pin, & paper??.

Pragmatist wrote: "Remember the bernoulli's principle lab. demo. with the spool, pin, & paper??."

How does this demo relate to the sailboat challenge, i.e., in the decision as to which way the boat will move?

The fan & sail together will tend to direct
air from the back of the system to its sides.
The air escaping at the sides of the sail will
entrain air at the front of the sail also moving
it outward. this will cause air moving in to 'fill the gap'
to exert increased pressure on the front of the
sail, (held in place by fan pressure)..
(Here lieth the similarity with the Bernouli
experement with the spoon paper & Pin.)
As pointed out by another, the low pressure
area behind the fan also draws the boat backwards.
As I see it air drawn from behind and directed to
both sides will result in a net rearward impulse.

It looks like we will have to agree to differ! As a closing remark, consider this from Wikipedia:

"A spinnaker is a type of airfoil and does generate lift, contrary to popular belief. Since the lift and drag generated by the spinnaker both act to move the boat forward, the lift to drag ratio is unimportant. The goal then is to generate the maximum amount of lift possible with no consideration of drag. Because of this, spinnakers have extreme amounts of camber, making them nearly hemispherical in form. The large camber maximizes the low pressure on the downwind side of the sail, generating the lift."

I agree that a fan is not the same as natural wind, but the crucial factor is that air is escaping in a backward direction, hence a net forward "impulse", as you called it.

as per the basic laws of centre of gravity .. the internal energy of the system cant change its center of mass ... so there is no chance for the moment

Forwards because the energy from the battery/dynamo-engine/source of power is coupled to the fan which accelerates the air which is then directed by the spinnaker astern thus moving the craft in the opposite direction (ahead, in this case).