How can an vehicle move faster than the wind that is powering it?

[spork]

So, for everyone but Topher, I'm still at a loss on what's left to discuss. Is there honestly any real doubt that ice-boats are doing as claimed?

If so, how can you explain the sailboat tacking into the 5 knot relative wind (on a calm day with a 5 knot current) in the middle of the ocean?

 

[ThinAirDesign]

Of course a sailboat can tack upwind. But NO it is not exactly the same thing.

Topher, does this brainteaser sound familiar?

You're in a sailboat on a day with NO wind. However, there's a 10 knot current taking you directly toward your intended destination. What's the fastest way to get to your destination? Should you drop your sails to reduce drag, or use your sails to get there more quickly? You can only use the energy from the wind and/or water - no paddles, motors, etc.

If you can answer the above correctly, you will see that it IS exactly the same -- that is if your claim that the cart (or sailboat) cannot outrun it's power source was in fact valid, a sailboat would never be able to tack upwind.

You might want to think about it carefully, 'cause it's easy to look silly when dealing with the real world.

JB

 

[OmCheeto]

I must admit that I'm a bit new to this forum.

Is this a perennial Halloween joke kind of post?

 

[spork]

I must admit that I'm a bit new to this forum.

Is this a perennial Halloween joke kind of post?

I don't think so. I think Topher actually doesn't understand inertial reference frames.

 

[schroder]

So, for everyone but Topher, I'm still at a loss on what's left to discuss. Is there honestly any real doubt that ice-boats are doing as claimed?

If so, how can you explain the sailboat tacking into the 5 knot relative wind (on a calm day with a 5 knot current) in the middle of the ocean?

Look, sailboats tack into the wind at an angle, typically 40 degrees. This makes the wind curve around the sails, resulting in a delta velocity, in other words, acceleration. This acceleration is mainly to the side but does have a forward component as well. Also, the keel of the boat tends to redirect the sideways acceleration into a forward acceleration resulting in forward motion. Note, however, that the boat does not move directly forward into the wind as it is always tacking at an angle. Let’s say you have a twenty knot headwind and you tack into it at a forty degree angle. You might typically generate a twenty-four knot speed at an angle of forty degrees into the wind. This is why it can be said that a sailboat can go faster than the wind. But the velocity, as a vector is never faster than the wind. In the above example, the forward component would be about 18 knots, which is less than the 20 knot wind. The boat cannot sail directly into the wind. But I fail to see what all this has to do with a cart on a treadmill. The cart is initially getting ALL its drive power from the moving tread driving its wheels while the cart is held stationary against a backstop. Then the wheels drive a propeller through some sort of drive train. Now you expect me to believe that the force of the propeller can drive the cart forward on the treadmill? If that was true, you could turn the treadmill off and the cart would keep going! In other words, you are asking me to believe in a perpetual motion machine. This thread is an insult to the intelligence and it is about time that a moderator put it out of its misery.

 

[spork]

Look, sailboats tack into the wind at an angle, typically 40 degrees...

Yeah, I'm pretty familiar with how sailboats tack into the wind since I've been sailing for over 30 years.

This makes the wind curve around the sails...

Yeah, I'm pretty familiar with how the wind over an airfoil creates lift seeing as I have in M.S. in aero.

But I fail to see what all this has to do with a cart on a treadmill.

And I really can't imagine how it could possibly be made any simpler. I'll walk you through it. I'm going to number the steps, because I want someone to actually point out where this goes wrong.

1) I think we can agree that a sailboat can tack into the wind.

2) I hope we can agree that if you're in the middle of the ocean and you feel a 10 knot wind over your deck, you can tack into that wind. It doesn't matter whether you're in a 10 knot current and there's no "wind", or there's a 10 knot wind and no current. Afterall, what do we measure the wind relative to - the bottom of the ocean?

3) So... if we are in a 10 knot current, with no wind, and we tack into the relative wind, we'd have to say that we're tacking down-current rather than upwind. But we're going faster than the current. In other words we can tack faster than the fluid that's pushing us.

4) If a sailboat can tack faster than the fluid that's pushing it, an ice-boat can sure as heck do the same (which we already know from GPS data and testimony of the ice-boat racers).

5) Put an ice-boat or two in a huge frame with a seat in the middle and let them tack downwind faster than the wind all day long - while you sit in your seat going straight downwind faster than the wind.

There you go. A vehicle that goes straight downwind faster than the wind, powered only by the wind. Forget about the cart. The object is to build a vehicle that goes straight downwind faster than the wind - and this one does it.

It's now been shown with GPS data, vector analysis, testimony from the very ice-boat racers that do it every day, and finally we can see that exactly the same thing happens whenever a sailboat tacks into the wind.

Now you expect me to believe that the force of the propeller can drive the cart forward on the treadmill?

I would've said yes - but now I think it's a trick question.

If that was true, you could turn the treadmill off and the cart would keep going!

Now you're just messing with me - huh?

In other words, you are asking me to believe in a perpetual motion machine.

I'm curious to know how many times we'll have to explain that this is NOT a perpetual motion machine. How can it be so complicated to understand that this vehicle exploits the motion of the air relative to the ice or water?

This thread is an insult to the intelligence and it is about time that a moderator put it out of its misery.

You actually find this so challenging to your world view that you think a moderator should make it go away? That is an insult to "the" intelligence. But if it's REALLY so obviously impossible, why not take the bet that seems to be annoying everyone? You can claim the $100K and shut me up.

So without simply telling me this is a perpetual motion machine, or that it breaks every single law of physics, someone please point out the step where it all goes wrong. If a sailboat can tack upwind, then a wind-powered vehicle can go directly downwind, faster than the wind - in 5 easy steps.

 

[schroder]

3) So... if we are in a 10 knot current, with no wind, and we tack into the relative wind, we'd have to say that we're tacking down-current rather than upwind. But we're going faster than the current. This tells us that we can tack faster than the fluid that's pushing us.—spork--

This is where you are going wrong. When you tack into the relative wind, you can go faster than the relative wind, but you cannot go faster than the current! Can’t you see that? The sails resist the motion of the boat in the air, which will slow it in the water. But, at the angle of attack the slipstream of air will now be moving faster than the original relative wind. For example: the water current is ten knots. The boat is moving at zero relative velocity to the water but 10 knots relative to the wind. Now you hoist the sails and tack at a 40 degree angle. The wind coming across the sails will be at a greater angular velocity than ten knots (maybe 12 knots) but the boat will actually have a negative velocity in the water of about 1 or two knots. You will never outrun the current which is driving you in the first place!

You actually find this so challenging to your world view that you think a moderator should make it go away? That is an insult to "the" intelligence. But if it's REALLY so obviously impossible, why not take the bet that seems to be annoying everyone? You can claim the $100K and shut me up.—spork--

Where are the details of this bet you keep mentioning? I just might take you up and you can donate the money to Physics Forums after I win. About that video: why is there a need for a tensiometer to be attached to the rear of the cart? After all, if the cart can move forward on the treadmill, that should be very obvious for all to see. The tensiometer adds no useful information. However, the tensiometer might actually be a compressed spring which pushes the cart forward when the prop provides enough lift. That would explain the hoax!

 

[spork]

This is where you are going wrong. When you tack into the relative wind, you can go faster than the relative wind, but you cannot go faster than the current! Can’t you see that?

Of course you can. You're in the middle of the ocean. The current is moving you toward the north at 5 knots. There is no wind - but you feel the 5 knots of relative wind over your deck. You throw a buoy overboard as a current marker, put up your sails, and tack into the relative wind - leaving the buoy in your wake. You're now going north faster than the 5 knot current that's pushing you north. Can't you see that? This is where you are going wrong.

The sails resist the motion of the boat in the air, which will slow it in the water. But, at the angle of attack the slipstream of air will now be moving faster than the original relative wind.

Forget all about the details of how a boat sails. We agree it does. The question is this... how can you even tell whether you're in a 5 knot current with no wind or in a calm current with a 5 knot wind if you're in the middle of the ocean with no GPS etc? Serious question. Tell me what you do aboard that sailboat to tell me which situation you're in.

Where are the details of this bet you keep mentioning?

PM me with your contact info. We'll set it all up.

I just might take you up and you can donate the money to Physics Forums after I win.

Fine. Just don't sign that check until you actually see the experiment fail.

...That would explain the hoax!

Yup - there are lots of ways to explain the hoax - if there were a hoax. But why post a video hoax of an everyday event? This has been proven in just about every way imaginable - including GPS data, vector analysis, and testimony of the people that do it.


No one seems willing to answer the question any longer as to whether they believe ice-boats tack downwind with a downwind velocity component greater than the wind speed (as the racers, GPS data, and vector analysis clearly shows).

 

[Trond]

The bet is for people that are positive this is impossible - people like you.

Then you have misunderstood me completely, I said it was impossible at first on RR, then I pondered about it and changed my mind. However I have a different approach to the proeblem than you do.

 

[spork]

Then you have misunderstood me completely...

I think you're right. What is your current stance?

A) Sure it can be done (and happens every day)
B) Sure it can not be done
C) Unsure and interested in the analysis and evidence
C) Other

 

[Trond]

What is it? I'm trying to have a discussion about a cart and you want to discuss sailing. I'm not sure it means the same thing to you and me.

If you in A) as usual mean if an ice yacht can reach a point downwind faster than the ballon, then yes sure it can be done. Not sure how often tho

But as you know, I'm more interested in the cart.

How is your stance on whether the cart can advance on a treadmill with no wind relative the treadmill.

A) Sure it can
B) No it can't
C) Unsure
D) Other

 

[spork]

How is your stance on whether the cart can advance on a treadmill with no wind relative the treadmill.

I assume you mean no wind relative to the treadmill itself - not relative to the moving belt of the treadmill. In that case the cart can definitely advance on the moving belt.

 

[Trond]

Yes, that is what I mean. No wind relative the room, no wind relative the treadmill itself and consequently no wind relative the cart until it starts moving.

So it's rolling, at the start it's standing still on the belt, propeller turning, driven by the rolling wheels as the belt rolls underneath the cart. We get to the point where it starts to budge.

I trust we can agree that to continue from that point the blades will have to spin faster?

 

[schroder]

Of course you can. You're in the middle of the ocean. The current is moving you toward the north at 5 knots. There is no wind - but you feel the 5 knots of relative wind over your deck. You throw a buoy overboard as a current marker, put up your sails, and tack into the relative wind - leaving the buoy in your wake. You're now going north faster than the 5 knot current that's pushing you north. Can't you see that? This is where you are going wrong.





No one seems willing to answer the question any longer as to whether they believe ice-boats tack downwind with a downwind velocity component greater than the wind speed (as the racers, GPS data, and vector analysis clearly shows).

Yes, you leave the buoy in your wake because you are moving away from the direction of the current at an angle. Let's say the current, the boat and the buoy are moving directly from North to South at ten knots. You hoist your sails and tack into the apparent headwind at a forty degree angle. You are moving away from the buoy at the angle of tacking. Now, there is absolutely no way you can ever get back to a point directly South of where you started before the buoy does. If you think you can, that is the end of discussion! But I admit the same cannot be said about an iceboat and a balloon. However, much of the balloon’s velocity has a vertical component. Remember, the balloon is rising while the buoy drifts horizontally with the current. Also, while the buoy will drift along with the current, the balloon will allow some of the airstream to slip around the surface; it will not necessarily be carried along at wind velocity. Finally, the iceboat does achieve a higher peak velocity than a sailboat. But comparing the iceboat velocity with the balloon apparent velocity is not a direct comparison with the wind. But my part of this discussion is concerned primarily with that contraption moving steadily forward against the treadmill while being powered by the treadmill. I still say that is impossible.

 

[spork]

the boat and the buoy are moving directly from North to South at ten knots. You hoist your sails ... Now, there is absolutely no way you can ever get back to a point directly South of where you started before the buoy does. If you think you can, that is the end of discussion!

I don't just *think* I can get back to that point - I *know* I can get back to that point. In fact I can easily get back to a point further south than the buoy. That's what tacking is all about. So I guess this is the end of the discussion - but before you leave let me offer you a $100K bet on this as well. Frankly I can't imagine why anyone would take me up on a bet so blatently obvious - but it's there if you want it.

But my part of this discussion is concerned primarily with that contraption moving steadily forward against the treadmill while being powered by the treadmill. I still say that is impossible.

No need to go anywhere near the vehicle on the treadmill if we can't even agree that a sailboat can tack into the wind - particularly if the discussion is already over before it really began.

 

[spork]

For those still in the discussion...

Are we generally on board that the ice-boat racers aren't lying to us with their testimony, their GPS data, and their diagrams?

Do we not agree that a typical sailboat can tack into the wind - and thus it's easily shown that we can make a wind powered vehicle that can go directly downwind faster than the wind?

 

[ThinAirDesign]

Now, there is absolutely no way you can ever get back to a point directly South of where you started before the buoy does.

schroder, before you walk away from the conversation, humor me and answer the below question. I'd really appreciate it.


You and I are on a sailboat in the fog. We know that harbour is exactly South of us. As we pop our heads up above the deck, we see that we have a 10knot wind coming directly from the South. You and I both decide that we'll tack upwind to shore. (at this moment, we also drop a bouy into the water to mark our start point).

As we power away from our bouy, tacking zig-zag South towards land, will the bouy get there first, or will we?

Thanks

JB

 

[Trond]

I'll answer
That depends on your boat, sailing skills and whether there is a strong current going south directly to shore. If there were no current at all the buoy wouldn't go south as they seldom go directly against the wind...

And even if there were a 10 knot current going directly to shore it would be hard to beat you even with bad skills as you would then have both the current and the wind helping you...

 

[ThinAirDesign]

I'll answer
That depends on your boat, sailing skills and whether there is a strong current going south directly to shore. If there were no current at all the buoy wouldn't go south as they seldom go directly against the wind...

And even if there were a 10 knot current going directly to shore it would be hard to beat you even with bad skills as you would then have both the current and the wind helping you...

Until schroder gets back to us all I will say is that our sailing skills are perfectly adequate.

JB

 

[Trond]

I don't doubt that as it wouldn't take much to outsail a bouy tacking up wind now would it Jayson

 

[DaveC426913]

All this talk about a buoy is silly. A buoy is not wind-driven. A sailboat can sail circles around it.

We're getting off onto faulty analogies again.

 

[ThinAirDesign]

All this talk about a buoy is silly. A buoy is not wind-driven. A sailboat can sail circles around it.

We're getting off onto faulty analogies again.

At this point I couldn't care less about any analogies --- I just simply want to know the level of physics understanding of the folks in the exchange. If they don't understand simple inertial frames, I have to approach any explanation much differently.

JB

 

[schroder]

schroder, before you walk away from the conversation, humor me and answer the below question. I'd really appreciate it.


You and I are on a sailboat in the fog. We know that harbour is exactly South of us. As we pop our heads up above the deck, we see that we have a 10knot wind coming directly from the South. You and I both decide that we'll tack upwind to shore. (at this moment, we also drop a bouy into the water to mark our start point).

As we power away from our bouy, tacking zig-zag South towards land, will the bouy get there first, or will we?

Thanks

JB

I don’t know why I bother, but I will answer your question, and then ask one of my own. If the headwind is a true wind relative to still water, then the boat will get to port before the buoy. In fact, the buoy will never get to port! But, if the wind is a relative velocity caused by a ten knot current flowing towards the port, then the buoy will absolutely reach port first, regardless of how high performance the yacht is or how clever the skipper. Remember, there is no wind, only the relative wind to the current! And the discussion I had with spork was in reference to the latter scenario.

Now, let me ask you this: Suppose you have a very well designed auto rotation glide copter, the very best there is. You drop it in still air under the force of gravity. As it descends, the blades start to turn due to the relative velocity of the air as gravity pulls it towards the earth. Is there ever a point where the lift from the blades can stop the downward fall, or even reverse it so the copter goes up?

 

[ThinAirDesign]

I don’t know why I bother, but I will answer your question, and then ask one of my own. If the headwind is a true wind relative to still water, then the boat will get to port before the buoy. In fact, the buoy will never get to port! But, if the wind is a relative velocity caused by a ten knot current flowing towards the port, then the buoy will absolutely reach port first, regardless of how high performance the yacht is or how clever the skipper. Remember, there is no wind, only the relative wind to the current! And the discussion I had with spork was in reference to the latter scenario.

Thanks schroder. I will answer your below question and then I'll get back to the above.

Now, let me ask you this: Suppose you have a very well designed auto rotation glide copter, the very best there is. You drop it in still air under the force of gravity. As it descends, the blades start to turn due to the relative velocity of the air as gravity pulls it towards the earth. Is there ever a point where the lift from the blades can stop the downward fall, or even reverse it so the copter goes up?

I believe I understand the question, but let me be sure -- I assume by "glide copter", you are referring to say the equivalent of a gyrocopter without a motor. Am I right?

Assuming "Yes" to the above:

In a steady state situation, there is absolutely no version of this craft which could halt its descent or reverse it. The better designed the machine ... the slower the descent could be, but it will never get to zero or be able to reverse.

Hope I got that right.

Thanks for the quid pro quo. I hope it can continue with my next post.

JB

 

[schroder]

Thanks schroder. I will answer your below question and then I'll get back to the above.




I believe I understand the question, but let me be sure -- I assume by "glide copter", you are referring to say the equivalent of a gyrocopter without a motor. Am I right?

Assuming "Yes" to the above:

In a steady state situation, there is absolutely no version of this craft which could halt its descent or reverse it. The better designed the machine ... the slower the descent could be, but it will never get to zero or be able to reverse.

Hope I got that right.

Thanks for the quid pro quo. I hope it can continue with my next post.

JB

Thank you for your response, which is correct. Now can you please explain how a cart which is powered (through wheels and drive belt and propeller) running on a treadmill, can possibly reverse direction and move against the movement of the tread?

 

[ThinAirDesign]

Ok schroder, thanks again for the exchange (feel free to ask your own questions as before and I will do my best to answer them). I have posted my original question at the bottom of this post, just for reference.

Another question related to the original:

After you and I decide that we will tack upwind toward shore (South) and we drop our marker bouy, we set sail as planned. We start with a ~45 degree tack (or reach depending on whose terms we're using) to the SW and after a few hundred yards we tack back to the SE and return to a point directly South of our bouy. Just for conversation, let's say that the progress me made directly south was 500ft relative to the bouy.

Do you agree that the above progress relative to the bouy is possible?

Thanks

JB

You and I are on a sailboat in the fog. We know that harbour is exactly South of us. As we pop our heads up above the deck, we see that we have a 10knot wind coming directly from the South. You and I both decide that we'll tack upwind to shore. (at this moment, we also drop a bouy into the water to mark our start point).

As we power away from our bouy, tacking zig-zag South towards land, will the bouy get there first, or will we?

 

[ThinAirDesign]

I have to begin my commute schroder. I will continue a bit later (and will do my best with your question)

Thanks again.

JB

 

[uart]

Here's one train of thought that might help convince that the "treadmill in still air" situation is a least feasible.

Imagine for a moment that the rotating a propeller was replaced by a large “cork screw” (or similar). That is, the wheels were coupled through a suitable drive train to turn this large “cork screw”.

Now imagine that the corkscrew is started into a large block of soft foam attached to the front of the treadmill. If the treadmill is run then the wheels turn and the corkscrew turns and the vehicle will move forward as the corkscrew screws into the foam.

Perhaps we can think of the propeller in a similar was as "screwing" itself forward into the stationary air. I’ve got to admit that when I first saw the video I thought it was a hoax/impossible but right now I certainly think it’s at least feasible.

BTW. I also sail (just windsurfer ) and I’ve found this a very interesting discussion.

One last thing. Way back in this thread I posted a mathematical derivation of the maximum downwind speed as a function of the acute angle “phi” that a (sail or ice) boat could head into the apparent wind. The post was effectively lost to the forums current Latex problem so I’ll just briefly repost the main results here.

From the vector diagram attached,
W = wind velocity.
V = craft velocity.
A = relative (or apparent) wind velocity

Applying the sine rule (and simplifying) you get.

V / W = cos(theta) + cot(phi) sin(theta)

(note that “theta” is your boat heading off the wind and "phi" is your boats highest heading into the apparent wind)

From the above the VMG is

V cos(theta) / W = cos^2(theta) + 1/2 cot(phi) sin(2 theta)

Differentiating wrt theta the optimum heading theta was found to be,

theta_best = 45 – phi/2 degrees (assuming phi is acute)

And the resulting maximum VMG was found to be,

VMG_max = (1 + cosec(phi)) / 2

Clearly this is greater than W and I believe this proves that as long a phi is acute (which is the same as saying that the craft is capable of tacking up wind) then the maximum downwind VMG is greater than W.If anyone wants to see more details of the derivation take a look at post #62 in this thread when the latex bug is eventually fixed.

 

[spork]

All this talk about a buoy is silly. A buoy is not wind-driven. A sailboat can sail circles around it.

We're getting off onto faulty analogies again.

The sailboat tacking into the wind is not an analogy at all. It IS the device we seek to build. If you don't understand intertial reference frames, you won't see that.

However, you claim the sailboat can sail circles around the buoy. And I agree with that of course. In that case we know that the moving current can move the boat in the current's direction - faster than the current.

If the headwind is a true wind relative to still water...

We're in the middle of the ocean - 100's of miles from land, thousands of feet from the ocean floor. What IS "true wind". Do you honestly believe that what the air is doing relative to something so far away has any bearing on anything? Why not measure the wind speed relative to the moon in that case? Einstein tells us quite clearly that if we put a giant box around our boat (the fog for example) we have no way at all to distinguish between moving wind and still water vs. moving water and still wind. In fact Einstein would point out that even referring to either one as "moving" ONLY even makes sense relative to the other. After all, how fast is the "true" wind moving in an absolute sense? How fast is the Earth moving in an absolute sense? Ans: there is no answer - velocity is not absolute - it's relative.






I'll ask again. Are most of us on-board that the ice-boat can do as claimed by the ice-boating commnity (and backed up by GPS data and analysis)? At the start of this thread, most if not all said the ice-boat could never achieve a downwind VMG faster than the wind. I'd like to believe the evidence

 

[uart]

Hey I've always believed they could a achieve a VMG of W( 1 + cosec(phi) )/2