Re: OT Here is an example of pseudo science.

[Andy Burns]

Google have been sponsoring development of one effort

search for other DWFTTW projects, less useful if you want to go in a different direction to the wind ...

[Ronald Raygun]

No, that's a faulty conclusion. It correctly answers the wrong question.

The question was whether it is possible for a boat to sail directly downwind faster than the wind, powered only by a wind turbine. That means whether it is possible for a boat *to sustain* such a speed.

The question you've answered, however, is whether it's possible for the boat *to reach* that speed, subject to the constraint that it must start from rest and only ever head directly downwind whilst accelerating. That's a completely different question, doomed to a negative answer by the fatal constraints imposed.

Anyway, your answer correctly observes that the relative wind speed approaches zero as the boat's speed approaches that of the wind, and so the turbine is faced with the impossible task of extracting power from a dying wind. Therefore crossing the barrier of actual wind speed is going to be impossible in those circumstances, and in practice there will be a top speed which it is possible to reach but not exceed.

Suppose this practical top speed is about 90% of wind speed, so that a

10% difference or so is enough to provide the power needed to keep up with water resistance etc.

But if a 10% difference is enough, then travelling at 110% of wind speed should also be sustainable.

The only problem remaining is how to reach that state to begin with. Clearly it can't be done subject to the constraint of your scenario, but it could be done by other means, including by "cheating" and using stored energy.

I dare say another way would be to use the wind turbine to accelerate to beyond wind speed by going downwind *but not directly downwind*, and then quickly turning directly downwind, relying on inertia to lose not too much speed in the turn.

[dennis]

The only way it can work is if they are storing energy somewhere. I expect its a flywheel, maybe even hidden in the design . To travel faster than the wind (down wind) for any length of time requires you the extract energy from somewhere and it can't be the wind. maybe its cold fusion? ;-)

[Andy Burns]

They jokingly refer to one of their fairings as "the engine cover"

Obviously the wheels and the propeller will act as flywheels, you can't avoid that, but they travelled an average of 2.8x the tail wind speed, accelerating over the measured run.

I don't know who NALSA are, other than what is says on their website, but I'd expect their observer to be able to spot a heavy flywheel, a set of pedals or batteries and motors hidden within the polysytrene ...

[Andy Burns]

There is an explanation of sorts on their website, more or less it's not the propeller turning the wheels, but the other way round ...

"The reason the car works is that the propeller is pushing on air that is moving slower than the speed of the car because there is a tailwind. This allows energy to flow into the system for propulsion, something that would be impossible if there were no wind."

Clearly between you, Harry and TNP the details can now be thrashed out for weeks.

[dennis]

Where do the nuts come from? I bet he has a car that runs on water too.

[dennis]

an effective zero wind speed, which they must do. It has to be done by storing energy or the whole thing is just a joke/fraud. They can't even claim its the sails (turbine) crossing the wind like a land yacht as its completely different. In fact once they pass the wind speed the turbine should reverse as the wind flow has reversed. I notice it didn't.

[dennis]

No need its just not true.

[Andy Burns]

They claim to be doing the opposite, driving the propeller from the wheels.

The specific record the NALSA were overseeing does seem *so* tightly defined and measured as an average over just a 10 second period, that momentum of the rotating bits might account for it.

During most of the runs they give it a push start, but they do also have some videos showing that it can get up to the push-start speed just from the prevailing wind (it's not exactly fast off the line).

An easier claim to "sell" to Joe Public would be over a longer period, i.e. let it start from zero on the line and reach a final speed that's faster than the average (or even better the peak) wind speed over the course.

They have a picture showing a flag on land blowing in one direction with the wind, while a streamer on their vehicle blows the opposite way as it travels past ...

[The Natural Philosopher]

Oh no. In this case I am staying out. I merely mentioned the site: Dennis found it. It will appeal to him.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Tim Lamb saying something like:

The energy input I refer to is the turbine(s), but I can't see it working from start height without some battery power onboard. I'd be quite happy to be proved wrong - if it can be made to work on a small scale it would be a hoot.

[Ronald Raygun]

Indeed. Drag increases with speed, but available power (and hence thrust) reaches a minimum (of zero) at 100% of wind speed. I postulated that if equilibrium might be achievable at 90%, that, on the basis that the relative wind speed is then 10%, equilibrium ought *also* to exist at about

110% when the relative wind speed is also 10% (I appreciate that water drag at 110% will be a bit more than at 90%, so the difference might need to be a bit more than 10%).

Or a bigger turbine in this case. But if doing this on a dead downwind course, you still can't break the 100% barrier, you will merely push your equilibrium a bit nearer 100%.

My point, which you may have missed, was that you could exceed 100% wind speed by making your initial acceleration on a course which is *not* directly downwind, and where therefore there is no barrier effect at which relative wind goes to zero. And then suddenly turn dead downwind where you should then be able to sustain the 110% you had already reached earlier.

[Ronald Raygun]

The same place as the bolts?

Yeah, right. You think this is perpetual motion? It isn't. There is no conservation-of-energy reason why this shouldn't work. It is in theory completely possible. Whether it's possible in practice merely boils down to whether you can get the turbine-to-propeller power transfer efficient enough. Could be a struggle.

[The Natural Philosopher]

Kinetic energy extractable from the air is a function of the *relative* velocity between the machine extracting the energy, and the air.

If they are moving at the same velocity, that is zero.

you can do better than 70%, but that cant negate the laws of physics.

Next stop of course, the case of the vehicle being pushed into still air. and deriving enough power from spinning its blade in its own self generated wind to drive the back wheels..so it goes faster..and faster. ;-)

Proper overall system analysis of the energy balances will always highlight the specious.

>

[Alan Braggins]

Apart from the bit where the whole point is to go directly downwind, faster than the wind, powered by the wind, and a sail can't do that. Do try and keep up.

to be the most appropriately titled article on the subject.

a more suspect title, but does have a good explanation of how it gets past the wind speed (i.e. through zero relative wind) without relying on energy storage or switching mode of operation.

[Andy Champ]

Odd, my post from last night seems to have gone missing. Oh well, I kept a copy...

I ain't Einstein. Luckily this is far easier.

As you so delicately told Harry, you need a turbine. Or some such.

The one I've seen uses wheels and an airscrew - the principle is the same for a prop. in water and an airscrew.

In order to generate a force against the wind the airscrew, which is not moving relative to the wind, requires only enough energy to overcome friction losses and inefficiencies. Polish it enough, and that won't be much.

Put that force down to the bottom of the vehicle, where it's moving against the substrate, and you have a force moving through a distance. From that you can extract substantial amounts of power. Certainly enough to keep the airscrew spinning in what it sees as still air.

For the numbers, imagine this: Airscrew is generating 1kN of force, and as the air is still relative to it it requires minor amounts of power only.

The vessel is moving downwind at 1m/s. Half of that kilonewton is used to shove it through the water; the other half is pushing a prop through the water. 500 newtons at 1 m/s is half a kilowatt. Not enormous, but as much as a racing cyclist... OK you won't get it all out of the prop, and you won't get it all up to the airscrew, but certainly some!

Andy

[ThinAirDesigns]

As to the DDWFTTW vehicle in question:

-- it does not utilize a turbine, it uses a propeller. Several people have correctly deduced that if it were a turbine, the extractable power available to the turbine with the vehicle at windspeed would be exactly zero and thus the vehicle could only attain some fraction of windspeed.

-- The vehicle has no capabilities of utilizing onboard stored energy for acceleration -- this was a strict requirement from NALSA (as it would be with any other good scientist)

-- For record purposes, the NALSA rules impose strict requirements on the vehicle from the moment it begins to move and not just during a 10 second period as some have surmised. The 10 second period is merely the length of time that is averaged to come up with the record speed.

[ThinAirDesigns]

@harry:

You are absolutely correct Harry -- in your link with that upwind race for university students, held it Europe each year, there are no specific requirement for crosswinds. The races are held head to head between the competitors and they use whatever wind Mom Nature gives them on race day.

The Blackbird directly downwind vehicle had no such flexibility. To set the NALSA (and soon to by published Guiness) record, 18 recording sensors were placed on the vehicle and the course and during the record run it was kept within ~3degrees of the wind.

JB

[Alan Braggins]

Noticable if you actually read it, "participating teams were challenged to drive directly into the wind, without tacking". But I've seen photos of a wind turbine powered boat doing that years ago.

Sailing downwind faster than the wind is a different challenge (and one where I'm not convinced you could make a boat efficient enough to do it, sticking a turbine in the water is going to be more lossy than having wheels on the ground, and there will be more drag to overcome). (Well, in one sense you can do it trivially - have a very light wind and a strong current or tide, and drift. That doesn't really count though.)

[ThinAirDesigns]

@ The Natural Philosopher >

This is true for a device extracting energy through a turbine mechanism, but not for a device extracting energy from the air using a propeller.

First, let=92s summarize the difference between a prop and a turbine: In short and simplistic terms, a turbine slows air down relative to itself, and uses that extracted energy to provide an *output torque* on the rotor shaft. A propeller requires an *input torque* at its shaft, and uses that input torque to accelerate air relative to itself.

One way to highlight the differences in the two mechanisms is to imagine being on a skateboard on a busy city sidewalk (where for our purposes everyone is walking the same direction).

If I'm on the skateboard and I just cross my arms and stand still on the board, I will gradually get pushed down the sidewalk as people bump into me one by one. I will be extracting energy from each of the people who contact me - slowing them down just a bit, and obviously I can never quite reach the speed of those who bump into me. If (though other means) I manage to attain the speed of the travelers on the sidewalk, it=92s obviously impossible to extract any energy from them at all. This is the turbine analogy =96 like the people on the sidewalk, the air molecules strike the passive blades just as the people strike the passive skateboarder.

The propeller device works quite different and has a different set of limitations.

Now imagine me on the same skateboard on the same busy sidewalk, but instead of simply crossing my arms and passively waiting for pedestrians to strike me and bump me down the walk to my destination I quite literally take matters into my own hands =96 I reach out and push people backwards while propelling myself forwards. In this scenario, I can extract *more* energy from each person (I can push back on them hard enough to bring them to a complete stop, unlike the turbine). I can also continue to extract energy from people even when I am going faster than they are as unlike the turbine, I am no longer just waiting for them to hit me. As I rip down the sidewalk say twice as fast as everyone else, there=92s nothing stopping me from grabbing everyone I come up on and pushing them backwards to a stop =96 and taking all their KE in the process.

So, taking us back to the the assertion quoted at the top of the post

-- one can see that it's a true assertion when applied to a passive turbine device which *does* rely on the relative motion between air and machine to extract energy. It's not a true assertion however when applied to a device which can actively push back on the molecules of air -- such a device can continue to extract energy from the air even when it's relative motion is exactly the same as the average speed of the molecules.

The primary limitation on the propeller device? -- we have to find the energy somewhere to provide the input torque to turn it. To put it back in the skateboard example, the energy to power my arms has to come from somewhere and in this case, it can come from the skateboard wheels.

If anyone wishes see the math which shows there is ample power available at the wheels of the skateboard to accomplish this task, I=92m quite happy to walk through it.