Apparently ice and land base boats can do this. The source of power is the difference between wind and ground speed, which is independent of the speed of the sailboat. There are limits to downwind speed, but Vmg/Vt (Velocity made good (in the direction of the wind in this case) / Velocity of true wind) can be greater than 1, and it's related to something called "apparent wind"

There's is a factor called Beta based on lift versus drag of sail and blades or wheels. If Beta is about 14, then Vmg/Vt can go up to about 2.56 with a heading about 38 degrees to the side of the wind.

More controversial is this propeller based device that claims to get a similar "apparent wind" effect (using the difference between wind and ground speed as a power source) that can go downwind faster than the wind.

youtube link: ddfttw

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

Where is the myth?

quote:

Originally posted by phage0070: Where is the myth?

The wiki article mentions faster than wind speeds, but not a speeds where the downwind component is faster than the wind.

The propeller based device shown in the youtube video. Propellers operate in their own induced wash, limiting efficiency, and question is can the propeller like device operate in the equivalent apparent wind scenario so that the propeller device moves downwind faster than the wind?

Note the wheels are connected to the propeller axle via a thin belt.

quote:

The wiki article mentions faster than wind speeds, but not a speeds where the downwind component is faster than the wind.

But any sailing site (or iceboating site) will show you vector diagrams and/or GPS tracks that show the downwind component to exceed the wind.


And Jack Goodman's cart was already the subject of about 40 pages of arguments.

quote:

Originally posted by roofingguy:
And Jack Goodman's cart was already the subject of about 40 pages of arguments.

Which I feel makes Jack's cart worthy of a MystBusters investigation.

Ah yes! The old bogus "downwind cart" video again. Take a close look at the direction the tell-tale is streaming. That cart is NOT going downwind. It is going ACROSS the wind.

I do not dispute that it is possible to go downwind faster than the wind, but that video proves absolutely nothing of the sort!

quote:

Originally posted by denverd0n:
Ah yes! The old bogus "downwind cart" video again.

The guy repeated the test on a treadmill.

Link to pdf file: DWFTTW.pdf

It looks interesting enough to be worthy of a MythBusters episode, plus they could include some land sail boats to test the down wind faster than the wind theory.

Originally I thought this was definitely a hoax also, but now after reading thousands of posts by the pro people I have to acknowledge that it may be possible. jeffereid your link did not have any video of him redoing the test on a treadmill.

quote:

Ah yes! The old bogus "downwind cart" video again. Take a close look at the direction the tell-tale is streaming. That cart is NOT going downwind. It is going ACROSS the wind.

Actually, denver, consider one thing...

Let's be silly and assume the cart *is* going faster than the wind. What would we see? The wind indicator still pointing forwards? No. If the wind was, let's say 10mph, and the cart was doing 12mph, then the wind that the indicator would see and react to would be 2mph from the front.

That is how a boat can tack into still air when drifting with the current -- there is a *relative wind* to be made use of.

Before Jack, there was Bauer: Bauer Cart .jpg

When someone posts a YouTube perpetual motion machine, we don't even have to click the link... we *know* it's fake. We don't care *how* the video was faked -- we just know it was. Our task isn't to study the video and try to spot the fakery; we can just explain why physics doesn't work like that.


Yes, in a traditional sail, you must always travel directly downwind slower than the wind.

But, by making use of relative wind, sailboats *do* have their downwind component exceed the wind speed on a regular basis. Or, in still air with a current, a sailboat that was drifting with the current can/will tack into the relative wind and beat the current. In both of these cases the boat appears to be outrunning its power source, but the sailboat doesn't seem to mind -- it just goes ahead and does it anyway. Until you look at the math and the vectors, and/or see it done for real, it does smack of over-unity or perpetual motion or magic. But it is a common occurrence nonetheless, despite appearances.

So, with that in mind, there's no reason in physics why Goodman's cart *can't* do what is claimed of it. Whether the video is accurate or not is a much lesser point in my eyes -- I personally see any possible fakery as irrelevant. The big point is that a wind-powered vehicle can do what it is claimed that the Goodman cart does.

Here's a link to the previous 45 page thread:

http://community.discovery.com/eve/forums/a/tpc/f/9701967776/m/4561931559/p/1

SDZ, good to hear that you've reconsidered. Now if we can only get denverdon on board ...

Denverdon, I think I asked once before how much angle to the wind is needed to be considered not directly downwind for a sailing vessel. The tell-tale does register the slight cross wind component when the cart is reaching and passing the wind speed but the wind is very close to parallel to the cart's path.

So if the wind is a cross wind and the cart can't actually go directly downwind faster than the wind, why doesn't the cart slow down when it is traveling through the part of the road that has the trees right beside it?

>> Which I feel makes Jack's cart worthy of a MystBusters investigation.

EXCELLENT! We've come full circle. My original intent has been to get this demonstrated on Mythbusters. I've been all over the place, debating this on R/C heli forums, hang gliding forums, kitesurfing forums, and most recently a physics forum. I can't believe it came back here. But yes, this seems as controversial as the evil "plane on a treadmill". Let's get it on TV.

>>
More controversial is this propeller based device that claims to get a similar "apparent wind" effect (using the difference between wind and ground speed as a power source) that can go downwind faster than the wind.
<<

Jeff, there are countless approaches to explain/prove that DWFTTW is completely possible, but it sounds like you are buying that now. Since your current concern seems to be "can the prop cart do it?" let me offer the following...

Let's take one of those ice boats on a 45 degree downwind tack - with a downwind velocity component faster than the wind. I think you accept this - yes?

Now rather than having it tack back and forth, let's imagine the earth is a cylinder rather than a sphere. The wind is steady, continuous, and parallel to the axis of this giant cylinder.

Now our ice-boat can follow a continuous downwind tack tracing a big helix around the cylindrical earth.

But now instead of making the whole earth a cylinder, let's just make a really truly huge cylinder, weld two land yachts together so their sails are pointing 180 degrees from each other, and their wheels are toward each other. We place this monstrous land-sailing-yacht rig on our monstrous cylinder, and let them do the same thing as the ice-boats on the cylindrical earth. They'll just cruise on down that giant cylinder faster than the wind - looking like a giant prop.

All this huge cylinder is doing for us is supporting the prop, and helping to enforce the kinematic constraint between the downwind motion vs. the cross-wind motion (as the keel or skates would normally do). So let's scrap the giant cylinder and replace it with a cart that holds the giant prop up, and enforces the same constraint (through it's wheels, belt, and pulleys).

And there you have it - a giant Bauer cart.

Spork, what physics forum is that on? Do the arguments against differ substantially from the 45 page thread here?

Did you manage to find any time to check the float drag stuff on POAR (plane on a river)? I'm afraid I concentrated on one aspect (not even the one I think is important), blathered on to an inconsequential conclusion and essentially killed the thread from what I can see.

>>Spork, what physics forum is that on?


http://www.physicsforums.com/showthread.php?t=265521

But it's been locked now. I'm not sure if it's because they concluded that it's completely and obviously impossible, or because some of us (like me?) weren't civil enough. I'm too easily baited.

quote:

Do the arguments against differ substantially from the 45 page thread here?

I know this is going to sound silly, but I honestly can't even follow the arguments against. I always hear "we want solid evidence" and "give us real analysis". So I provide plots of GPS data, ThinAir provides testimony from the ice-boat racers that do it all the time (along with links to their forums, and I draw up what seems like the most straightforward vector analysis.

The cries against are things like "it's obviously impossible", "only an idiot would try to tell us about a perpetual motion machine", and "no vehicle can outrun its power source".

I mean honestly, how do you even respond to such things. None of that is math, analysis, examples, or even genuine physical laws. Now don't get me wrong. People do invoke real physical laws like "conservation of energy". But that's their entire two word argument. No indication of how or why they conclude such law is violated.

Most of the arguments against don't even seem to form proper sentences that I can parse any meaning at all from. Of course I'm biased, because I know this thing is just a simple novelty that violates no laws of physics, and frankly has no real purpose. I guess I have to leave it to others to see if they can make any sense at all of the arguments against.

quote:

Did you manage to find any time to check the float drag stuff on POAR (plane on a river)?

Nope. To be honest with you, too often I write a simulation, or do an analysis to put a question completely to rest. I post the data, and I get no response at all. Nothing. Not a "cool so that's how it works" or an "idiot - that's completely wrong". I can't quite understand it, but people seem to simply enjoy arguing in the abstract.

Case in point... check the link on the physics forum. Most people swore that no vehicle could ever possibly outrun the wind downwind - even tacking. We provided every form of evidence known to mankind - the response... they simply stopped talking about ice-boats. No one would answer the question "do you at least now believe ice-boats can do this while tacking?". It's as if that argument is gone, but they still KNOW the cart is impossible - so they simply won't respond in any way to the real evidence of the ice-boats.

Sorry, that's the long-winded answer. If anyone responded asking to see the results of the analysis I'd be happy to write the sim. It'd be reasonably straightforward. Just not quite worth doing to be completely ignored. Incidentally, I have done many of these for various threads in the past.

quote:

Originally posted by jeffareid: There's is a factor called Beta based on lift versus drag of sail and blades or wheels. If Beta is about 14, then Vmg/Vt can go up to about 2.56 with a heading about 38 degrees to the side of the wind.

I wasn't paying attention here, Beta is also dependent on heading and not constant so my numbers are off.

Jack's cart apparently wasn't the first. There's also the Bauer cart: Bauer cart

The first potential problem I see is that the prop area is much smaller than the sail area would be for a similarly sized landsail or icesail. Some high end catemarans use semi rigid airfoils instead of sails, but those "wings" are very large compared to propellers. There are other issues with propellers such as induced drag and relative air speeds that vary with radius of the prop.

I need to do more research on this stuff to get a handle on what the limits really are.

quote:

Originally posted by roofingguy:
Actually, denver, consider one thing...

I understand perfectly all about relative wind. Have you actually looked at the video? Pay attention when the cart is not even moving, or barely moving at all. The tell-tale CLEARLY indicates that the wind is NOT coming from anywhere close to directly behind. The wind, in fact, is almost directly abeam.

Again, I do not dispute that it is possible for the cart to go downwind faster than the wind. But that video--which has probably been posted on this forum at least a dozen times--SHOWS NOTHING OF THE SORT!!!

I just watched that again for the umpteenth time and the wind was coming from behind the cart and for lack of a better term its port side. One thing that bothers me about the video is that up the down the wind whatever, the rotating prop is supposed to be a sail equivalent but it looks way to small to get any significant power from the wind. If this is possible then he should be able to make a land yacht with the same sail size in a more traditional shape and just go with the wind. But every land yacht that I have ever seen has a sail that is at least an order of magnitude larger than this one.

quote:

Have you actually looked at the video? Pay attention when the cart is not even moving, or barely moving at all. The tell-tale CLEARLY indicates that the wind is NOT coming from anywhere close to directly behind. The wind, in fact, is almost directly abeam.

It doesn't look that way to me... it seems to be essentially straight forward when the cart is given its initial push, and as the cart starts moving, it starts hanging limp and then trailing behind.

http://s253.photobucket.com/albums/hh67/RoofingGuy/Opening.jpg

If it were "clearly" directly abeam, would it not be pointing either towards or away from the camera?

quote:

Again, I do not dispute that it is possible for the cart to go downwind faster than the wind.

In that case I think we're pretty much on the same page. In my mind this whole thread is about whether or not it's possible. Not about whether that specific video proves it.

quote:

But that video--which has probably been posted on this forum at least a dozen times--SHOWS NOTHING OF THE SORT!!!

For what it's worth, I have a completely different feeling about the video. I think it shows exactly what I'm looking for. But I certainly wouldn't argue the point very strongly since I wasn't there to see it filmed. What I do know is that such a cart CAN be built to operate correctly.

quote:

...the rotating prop is supposed to be a sail equivalent but it looks way to small to get any significant power from the wind.

But keep in mind that a windmilling prop has the effective area approximately like that of a drogue 'chute of the same diameter. It's FAR more effective than the actual area of the prop blades would indicate. Consider a helicopter in autorotation. It comes down quite nicely under what amounts to a really tiny amount of airfoil area - and counter to popular belief can then make a feather light touchdown.

quote:

If this is possible then he should be able to make a land yacht with the same sail size in a more traditional shape and just go with the wind.

This confuses me for two reasons. First, a traditional sail operates VERY differently than a rotating prop, and secondly, we know that a traditional sail will never let him sail directly downwind faster than the wind.