Challenge: MythBusters and Cruisers Forum

[skookum]

A great display of engineering and craftsmanship.

I was quite skeptical about this experiment and then I thought about it for a while, did some research, and came up with an answer that will leave me with both a good night's sleep and what little hair I have left intact.

Ice boats can travel much faster than the windspeed on a reach because they transfer lateral apparent wind into a force parallel to the direction of travel.

Logically one can reason that on a dead downwind course there is no lateral force. Therefore as the machine speeds up, the apparent wind decreases until equilibrium and the net sum of the forces causes acceleration to cease somewhere just shy of the wind's speed.

By jibing from one reach to another, ice boats can maintain a net DDW course made good at a speed higher than the wind. But at any moment when the boat is actually traveling DDW, the boat will decelerate.

At first I thought the treadmill test had nothing to do with DDW travel. It only demonstrated the ability of the author's device to transform the energy supplied by the electric motor in the treadmill into an aerodynamic force strong enough to overcome the weight of the vehicle on a very small incline. I figured that the experiment didn't account for things like aerodynamic drag or other unknown factors that invalidated the claim.

Then the light bulb in my head turned on.

The device demonstrated here CAN travel dead down wind at a groundspeed greater than the local wind speed because the propeller blades themselves are not actually travelling dead downwind.

A thought experiment:

Pretend an ice boat is sailing not on a flat sheet of ice, but instead on the surface of a large horizontal ice cylinder that is pointed directly downwind. Also pretend for the moment that our iceboat can travel around this cylinder without falling off at the bottom. Our iceboat skipper could maintain a broad reach indefinitely without jibing by simply sailing a straight course (like a rhumb line on a mercator projection turned on its side). At any given point the boat would be pointed on a broad reach relative to the wind. The vessel's course on the surface of the cylinder would be at an angle to the cylinder's axis. But the vessel's net velocity would be dead down wind (at up to 6 times the windspeed!!! See the link at the end)

The blades of the device depicted at the begining of this thread rotate around a central axis and act like the iceboat's mainsail on a broad reach. They transfer the lateral apparent wind into a force that, when applied to the wheels, allows the whole device to travel dead downwind SLIGHTLY faster than the windspeed. Not a whole lot faster mind you. And as soon as the windspeed relative to the ground drops to zero, the device decelerates and eventually stops.

The motive force relies on the difference in relative velocity between the ambient air and the ground. If this difference euqals zero, then there is no potential for movement.

On the treadmill, the difference between the velocity of the ambient air (zero) and the velocity of the treadmill (something) is enough to overcome the drag from the device's wheels and allow it to travel slightly faster than the apparent winsdpeed relative to the treadmill's surface.

This is certainly not a perpetual motion machine since energy is not created from the ether. The device will certainly not accelerate indefinitely. It simply travels fast enough to briefly defy common sense.

Cheers

www.nalsa.org/Articles/Cetus/Iceboat%20Sailing%20Performance-Cetus.pdf

[ThinAirDesigns]

Originally Posted by skookum

The device demonstrated here CAN travel dead down wind at a groundspeed greater than the local wind speed because the propeller blades themselves are not actually travelling dead downwind.

The above is true skookum.

The blades of the device depicted at the begining of this thread rotate around a central axis and act like the iceboat's mainsail on a broad reach.

Still true.

They transfer the lateral apparent wind into a force that, when applied to the wheels, allows the whole device to travel dead downwind SLIGHTLY faster than the windspeed. Not a whole lot faster mind you.

Just to be clear, the blades do not transfer force *directly* to the wheels ... that is they do not act as a turbine and drive the wheels through the shaft. The wheels actually drive the prop.

And as soon as the windspeed relative to the ground drops to zero, the device decelerates and eventually stops. The motive force relies on the difference in relative velocity between the ambient air and the ground. If this difference euqals zero, then there is no potential for movement.

This is correct -- if the wind stops blowing across the ground, the cart stops moving.

On the treadmill, the difference between the velocity of the ambient air (zero) and the velocity of the treadmill (something) is enough to overcome the drag from the device's wheels and allow it to travel slightly faster than the apparent winsdpeed relative to the treadmill's surface.

The device works *exactly* the same on a treadmill as in the street -- jogging on a treadmill at 6mph is the same as jogging on the street at 6mph with a 6mph tailwind. Same with the cart ... if it holds it's own on the treadmill, it's going the same speed as the wind. If it can advance, it's going faster than the wind and conversely if it can only retreat it demonstrates that it can't beat the wind.

This is certainly not a perpetual motion machine since energy is not created from the ether. The device will certainly not accelerate indefinitely.

Correct -- not indefinitely, but if geared ideally it will accelerate until the drag forces balance out the thrust forces -- at well above wind speed.

It simply travels fast enough to briefly defy common sense.

There's nothing "brief" about it -- it will maintain a speed faster than the wind, steady state.

JB

[ThinAirDesigns]

Originally Posted by Little Otter

Look Scotty This is actually very possible and I can tell you how it is possible,
First the propeller is not driving the car it is turning the weels and that is what is driving the car

Otter, it is possible and the cart is doing it -- but the prop does not drive the wheels as you describe. If this were the case it could never equal the wind, let along beat it.

The force on the wheels is a braking force and this force is used to turn the prop and to generate thrust.

JB

[ThinAirDesigns]

Originally Posted by Robertcateran

An ice boat can et down wind faster than windspeed by using the energy generated from reaching to go faster than the wind, then using the momentum to go down wind faster than the wind.

Robert, ice-boats, land-yachts and top line racing sailboats can maintain downwind VMGs (velocity made good) greater than windspeed indefinitely and steady state -- they have no need to "use momentum" to make this happen.

I strongly suspect there is some perpetual motion problem here. I'd love it if I was wrong.

That's a common suspicion, but if you do you research and find it a fact that ice-boats regularly achieve downwind VMGs of 3x+ steady state you will see that there is no PM monkey business involved.

I do know that at windspeed it has zero power input but it my be possible to go through this deadspot if there is a sufficiently efficient flywheel to help it through this powerless area.

When an ice-boat or any other craft with a traditional sailing rig on a broad reach reaches a VMG of 1.0 (meaning that the downwind component of it's angled path is the same speed as the wind that is powering it) there is no dead spot. As with any other change in speed, there is only a minor shift in apparent wind angle with each speed increase.

JB

[ThinAirDesigns]

Originally Posted by GreatKetch

Let's do a thought experiment....

I have a propeller mounted on my little cart. It is a left handed propeller, geared so that when the propeller turns counter clockwise the wheels drive the cart forward.

Hi Ketch -- right off the bat you've got the operation of the carts in the video backwards. The spinny pinwheel thingy on the back of the cart does not act as a turbine, turning the wheels. The wheels drive the prop which generates the thrust to move the car forward.

When the cart is moving at 10 knots the apparent wind switches from behind the cart, to forward of the cart...

Nope, the apparent wind on the *chassis* of the cart reverses direction, but the apparent wind on the sail changes only incrementally as the speed increases.

NOW.... with the apparent wind from IN FRONT of the cart my propeller now starts to turn CLOCKWISE and the wheels turn backwards...

Right?

No right -- the prop never acts as a turbine, never sees a reversal of apparent wind and never reverses direction.

JB

[ThinAirDesigns]

Originally Posted by Robertcateran

I have been thinking a bit about the situation. It won't work.

Except that it does.

The energy has to come from slowing down air molecules relative to the water.

This is correct -- without this exchange in kinetic energy it would be breaking the laws of physics.

The air ends up traveling even faster if the boat is travelling faster than the wind dead downwind.

This is not true -- whether it be our cart or a traditional sailing rig, the air is always slowed relative to the ground/water.

This is very different from ice boats cutting through a very large area of moving air and gaining energy from it in the form of kinetic energy, and then using that energy to go directly into the wind with minimal interference, and then bearing away again to cut across the wind again to get some more energy.

But that's not how ice-boats achieve VMG's much greater than the wind speed -- they do it by generating lift/thrust from the apparent wind. They can do this steady state with a fixed bearing and do not need to "bear away again to cut across the wind".

JB

[ThinAirDesigns]

Originally Posted by anotherT34C

I think the makers of the video were confused. They probably didn't realize they were testing two different physical principles with the outdoor cart and the indoor cart on the treadmill.

Nope -- it has been established since the days of Galileo, that without external reference there is no test one can do which will differentiate steady state motion ... that is to say that there is no way to know if the air is moving and the road is still or if the road is moving and the air still.

An example of this would be riding a bike on a treadmill set at 10mph in a still air room is *exactly* the same as riding a bike on the street at 10mph with a tailwind of 10mph. In both cases you are pedaling at 10mph and in both cases there is no wind relative to the frame of the bike.

I think they meant the indoor treadmill test to be a frame of reference control.

There's no "control" being set -- both cases are identical to the cart.

Of course, if you think about it for a few seconds, and imagine making the prop arbitrarily large, you'll see why it fails as a test. A large prop, even at low rpm (or small prop with appropriate gear ratio to provide high rpm) can easily provide enough force to push the cart forward against the treadmill.

And however easy it is to make a cart advance against the belt on a treadmill, it is equally easy to make a cart move faster than the wind on the street. It is however easier to document the feat on the treadmill.

JB

[ThinAirDesigns]

Originally Posted by Little Otter

The tread mill was turning the opposite way the wheels were turning and therfore can't be powering the car.

Nope -- in all treadmill videos, the belt is running as you would expect by just looking at the device ... from right to left. The wheels also turn just as expected given the above -- clockwise from the side of the filming.

JB

[anotherT34C]

Originally Posted by ThinAirDesigns

Nope -- it has been established since the days of Galileo, that without external reference there is no test one can do which will differentiate steady state motion ... that is to say that there is no way to know if the air is moving and the road is still or if the road is moving and the air still.

If the wheels are not coupled to the propeller on the outdoor cart, then you are testing two different physical principles.

What I don't understand is why you seem to be behaving as if you've discovered some new physics, rather than set up a freshmen mechanics lab.

[Robertcateran]

Originally Posted by ThinAirDesigns

Robert, ice-boats, land-yachts and top line racing sailboats can maintain downwind VMGs (velocity made good) greater than windspeed indefinitely and steady state -- they have no need to "use momentum" to make this happen.


That's a common suspicion, but if you do you research and find it a fact that ice-boats regularly achieve downwind VMGs of 3x+ steady state you will see that there is no PM monkey business involved.




MG of 1.0 (meaning that the downwind component of it's angled path is the same speed as the wind that is powering it) there is no dead spot. As with any other change in speed, there is only a minor shift in apparent wind angle with each speed increase.

JB

That is certainly news to me that ice boats can end up with a downwind VMG of greater than wind speed with out relying on momentum to gain on the gybe. It would be interesting to know where the energy comes from. I can only see resistive forces dissipating energy once dead downwind VMG exceeds windspeed. When I do the vector diagrams of the forces this just doesn't happen

[ThinAirDesigns]

Originally Posted by anotherT34C

If the wheels are not coupled to the propeller on the outdoor cart, then you are testing two different physical principles.

The wheels are couple to the propeller with the essentially the exact same ratios in all of our carts -- same as Goodman's cart, Mark C's cart, Pelesi's cart and Bauer's cart from the '60s. All the same -- indoor, outdoor.

What I don't understand is why you seem to be behaving as if you've discovered some new physics, rather than set up a freshmen mechanics lab.

What I don't understand why you're accusing us of behaving as though we discovered *anything*. This device uses basic sailing principle used for next to forever and was first done in this exact form (prop cart) near 50 years ago.

We make absolutly NO claims of "discovery" and in fact give ample credit to those who have gone before.

JB

[ThinAirDesigns]

Originally Posted by Robertcateran

That is certainly news to me that ice boats can end up with a downwind VMG of greater than wind speed with out relying on momentum to gain on the gybe. It would be interesting to know where the energy comes from. I can only see resistive forces dissipating energy once dead downwind VMG exceeds windspeed. When I do the vector diagrams of the forces this just doesn't happen

Hi Robert. Perhaps you wish to engage landsailors on the topic -- I did:

Yahoo! Groups

I've got more links on that topic that I'll gather for you.

Once you come to the realization that a traditional sailing rig is capable of downwind VMGs much higher than the wind, it's not a far journey to understanding how the prop cart works.

JB

[Tropic Cat]

Well, it looks to me that this works pretty well on a light weight model. Despite the critics, the video is convincing though the horse power to weight ratio seems to be significant.

Early on in the video are photos of boats rigged with this type of propulsion system. Where are the videos of these boats successfully propelled by this rig against the real world of water resistance?

[anotherT34C]

Originally Posted by ThinAirDesigns

What I don't understand why you're accusing us of behaving as though we discovered *anything*. This device uses basic sailing principle used for next to forever and was first done in this exact form (prop cart) near 50 years ago.

We make absolutly NO claims of "discovery" and in fact give ample credit to those who have gone before.

JB

Did you do the narration on the video? Seems pretty grandiose for demonstrating the principle of "motor and propeller".

As for different principles, what's the direction of propeller rotation on the street cart relative to the treadmill cart (essentially a windmill)? What does this tell you about the forces involved?

[skookum]

Thin Air Designs wrote:

There's nothing "brief" about it -- it will maintain a speed faster than the wind, steady state.

JB

Yes, but sometimes common sense takes a little time to catch up.