Sailing downwind - faster than the wind?

chiefeng said:
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I can't see the cart would accelerate to even reach the constant wind speed as the relevent velocity of the wind over the blades reduces as the cart accelerates, then the force on the blades driving it reduces, until at some point where equilibrium is reached and the cart is travelling at a velocity lower than wind speed. Any extra energy derived from the wheels would then need to be converted to thrust at the propellor which would counteract the thrust already at the propeller! That would only be achievable if the blades could be turned to provide thrust in the opposite direction to which the cart is moving, so that the cart is being pulled along by the propellor
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You're on the right lines, but there is no need ever to have the propeller driving the wheels. It's always the other way round.

Imagine the cart arranged so that as it moves forward the fan blows backwards. Put it at a standstill in the wind. Windage will start moving it forward. As it accelerates, the back draught from the fan increases, effectively increasing the windage, so the net force increases, and so on.

Why doesn't it increase for every? Two reasons.

First of all, there is the windage of the the rest of the structure to take account of. Without the benefit of a fan blowing backwards the force on the rest decreases as the cart speed approaches wind speed and at wind speed it reverses.

Secondly there are drag losses in rotating the propeller and these increase with the square of the speed.

For any setup of this sort, therefore, there is an equilibrium point where the the work done by the wind pushing against the fan exactly balances the work needed to turn the fan and overcome other losses. The big question is whether this occurs above or below windspeed, and that depends entirely on the setup in question.

Once final point. Loads of people think that if the car/boat exceeds the windspeed, the propeller must be doing work on the wind. That's not true! Since the cart/boat is moving downwind, the fan is blowing the air back upwind: reducing its velocity and extracting energy from it. The mechanism for extracting the power is force in the direction of the wind: some of this power is then used to overcome drag losses in the propeller.
 
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mikemonty said:
Pick a reference which does not move - the seabed.
Now you have a energy source from the wind
And if you want to complicate matters, an energy source from the tide or river flow.
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There is energy in the wind, but you can only use it if you have something moving at a different speed to push against. It's a bit like th eSecond Law of Thermodynamics: you can only convert heat at a given temperature into mechanical work if you have something at a lower temperature into which you reject some of the heat.


A boat travelling along on a tidal stream at the same speed and direction as the wind still has energy in relation to the seabed, the only fixed reference.
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That's completely irrelevant.
 
Ubergeekian said:
That's a partial statement, but too many people are getting hung up on it.

Remember that there are two sorts of work associate with a propellor. There's the shaft work required to turn it, and there's the reaction work it does on the fluid through which it moves.

In the case of the trolley it's the wheels which turn the propeller (shaft work), but it's the wind pushing against the propeller (reaction work) which turns the wheels. There's a lot of fuzzy thinking going on about these two sets of work.
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Work is done on the WHOLE calliope

some of that work is converted to turn the prop
the prop in turn does some work on the air, sending it backwards.
at each stage some work is lost in friction
But even if ALL the work done to the cart was used to push air backwards all that would happen is that the air column from the prop would give more thrust.
And that would just mean that the "ping-pong ball" cart (vacuum cleaner analogy) would reach equilibrium at a later point. (higher in the air)

There is ONE train of energy, from the wind, through the drivewheel, to the prop, to the air being pushed by the prop, just to form a cushion against which the wind can blow!
Correction, Two actually, there is also the energy which is being used to actually drive the thing forward. the prop with its attendant parasitic losses is just a complex way of creating a backpressure.

They might as well have put a square rig in the thing - the effect would have been more immediate than a fan that has to spool up to speed and probably more efficient - no mechanical losses.

To use another analogy:-
it would still be like a waterfall gushing into the air (the props air column) just to fall back under gravity (the wind) - the waterfall would be a little higher

Or another one:-
Increasing the collective slightly on a helicopter hovering on ground effect.
 
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Ubergeekian said:
A stream of air blown back into the wind will generate more thrust than the same area of wind blowing against a flat plate (or sail). It's a higher stagnation pressure, see?
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Ok,
This is cr*p. You've seen my summary of what's happening with the real model...

I'm either right in principle or wrong
If I'm wrong and the thing can do downwind faster.... where is my summary wrong?

And never mind picking nits - give us the points of principle where I'm wrong.
So far all you've been able to say is that there was a "partial statement" somewhere.

Who cares!

Refute my summary and we'll leave it to our peers to make their own minds up.
 
Lots of odd statements here. " There is no energy in wind" Tell that to the weather people.
Comparisons with Oracle are useless. Tacking downwind is not the question, towing a water skier behind Oracle is just meaningless, the energy comes from tacking downwind.
Boats in baths in trains? Same as birds all jumping off their perches in a plane and flying. No effect.
BTW Gyrocopters do not alter the pitch of their rotors, they are always gliding down. ( I have flown in them)
The better arguments are coming from other than the OP, most of his are a bit opaque. The energy is at the interface between the air and water? Um? Keels taking the place of the wheels in the model?
A

I agree,but only as to why is this old stuff with no new developments?

Hey, I stop to type(slowly) a post and the world move on..
 
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mikemonty said:
If I'm wrong and the thing can do downwind faster.... where is my summary wrong?

And never mind picking nits - give us the points of principle where I'm wrong.
So far all you've been able to say is that there was a "partial statement" somewhere.
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Here are the points of principle where your summary is in error:

"There is ONE train of energy, from the wind, through the drivewheel, to the prop, to the air being pushed by the prop"

When talking about the energy transfer in the system, the source of the device's energy is the kinetic energy of the wind with respect to the ground. The rotating prop reduces the velocity of the wind thereby extracting part of its energy. The mechanism works because it has wheels on the ground and the prop in the air which are moving at different speeds

"just to form a cushion against which the wind can blow!"

This is a common fallacy. The thrust of a propellor, jet or rocket is obtained by accelerating the air or exhaust gas and using the reaction to that process. There is no 'pushing against', as proved by the fact that a rocket works as well in a vacuum as it does in air.

They might as well have put a square rig in the thing - the effect would have been more immediate than a fan that has to spool up to speed and probably more efficient - no mechanical losses.

And this is the really key point. It is totally wrong to equate this mechanism to a square sail on a trolley. That would only work on the difference in velocity of wind and sail which falls to zero as the trolley reaches wind speed. The mechanism works on the difference in velocity between the wind and the ground which is the same regardless of how fast the device moves.
 
DownWest said:
Comparisons with Oracle are useless. Tacking downwind is not the question, towing a water skier behind Oracle is just meaningless, the energy comes from tacking downwind.
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Those of us who have used Oracle as an illustration have done so to prove that it is possible to travel downwind at greater than wind speed, something that many doubters have claimed is not possible.

The mechanism of the device in question uses quite different principles and there is no attempt to suggest that Oracle works in the same way as the DDWFTTW machine.

What the two have in common is that they both get their energy from the same source: the difference between the velocities of wind and surface.
 
I know exactly what's going to happen here. Someone from British Gas or BP is going to stumble across this conversation and steal this new form of energy we have invented. I'll bet they make millions out of it and nothing to us - isn't that just typical.
 
snowleopard said:
Here are the points of principle where your summary is in error:

"There is ONE train of energy, from the wind, through the drivewheel, to the prop, to the air being pushed by the prop"

When talking about the energy transfer in the system, the source of the device's energy is the kinetic energy of the wind with respect to the ground. The rotating prop reduces the velocity of the wind thereby extracting part of its energy. The mechanism works because it has wheels on the ground and the prop in the air which are moving at different speeds
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Yes, and I said as much what's your point?

snowleopard said:
"just to form a cushion against which the wind can blow!"

This is a common fallacy. The thrust of a propellor, jet or rocket is obtained by accelerating the air or exhaust gas and using the reaction to that process. There is no 'pushing against', as proved by the fact that a rocket works as well in a vacuum as it does in air.
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A propellor does not work as well in vacuum as in air so its a weak point.
The prop is certainly being using reaction to get forward motion but it is also using aerodynamic principles - its a hybrid.
But assuming the prop gets the cart up close to the same speed as its reactive backward moving column of air (almost perfect transference of energy) it can't make it move any faster - any extra speed over this is down to the following wind - and THAT is limited to less than windspeed
.
There is a limit to how much energy the prop can gain from its drivewheel and that is going to be - at best - proportional to the area swept by the prop and the strength of wind (no losses) - you can't get more out of the prop than you put in so at best you get a column of air meeting an equal and opposite column of air.
Because the thing is moving at (again at best) downwind airspeed the best you can say of the following wind is that it is forming a "floor" like the ground under a helicopter.
So the best the machine can do is "hover" on the downwind air.
Even if the aerodynamic losses at the prop are proportional they will reach a point where the prop cannot be spun faster so there is a limit to the thrust and therefore the height at which the cart will "hover" on the following wind

snowleopard said:
And this is the really key point. It is totally wrong to equate this mechanism to a square sail on a trolley. That would only work on the difference in velocity of wind and sail which falls to zero as the trolley reaches wind speed. The mechanism works on the difference in velocity between the wind and the ground which is the same regardless of how fast the device moves.
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If you are talking about reaction it works on the difference in velocity between the CART and the air from the fan, and as I've said before there is a limit to how much of that is available due to the amount of energy that can be drawn from the wind, prop losses, and friction losses.

However that aside...
Actually the summary to which I referred is here
http://www.ybw.com/forums/showthread...53#post2452253

And again No nitpicking!
 
DownWest said:
Lots of odd statements here. " There is no energy in wind" Tell that to the weather people.
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Whether there is energy in wind depends on how you're moving. How much energy can a hot-air balloon extract from a gale? How much energy can the ram air turbine on an Airbus extract from still air.

Comparisons with Oracle are useless. Tacking downwind is not the question, towing a water skier behind Oracle is just meaningless, the energy comes from tacking downwind.
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On the contrary, they are entirely relevant.

Boats in baths in trains? Same as birds all jumping off their perches in a plane and flying. No effect.
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It's a good illustration of the need for different water and air speeds.
 
mikemonty said:
But assuming the prop gets the cart up close to the same speed as its reactive backward moving column of air (almost perfect transference of energy) it can't make it move any faster - any extra speed over this is down to the following wind - and THAT is limited to less than windspeed
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What fundamental limit do you think there is on the relative velocity of the air thrown back from the propeller. Bear in mind that the world speed record for prop-driven aircraft is 599mph.

here is a limit to how much energy the prop can gain from its drivewheel and that is going to be - at best - proportional to the area swept by the prop and the strength of wind (no losses) - you can't get more out of the prop than you put in so at best you get a column of air meeting an equal and opposite column of air.
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Why would the energy available to the prop depend on the size of the prop?

Because the thing is moving at (again at best) downwind airspeed the best you can say of the following wind is that it is forming a "floor" like the ground under a helicopter.
So the best the machine can do is "hover" on the downwind air.
Even if the aerodynamic losses at the prop are proportional they will reach a point where the prop cannot be spun faster so there is a limit to the thrust and therefore the height at which the cart will "hover" on the following wind
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Helicopters are not exploiting a velocity difference between two fluids. And they are perfectly capable of climbing.
 
Ubergeekian said:
What fundamental limit do you think there is on the relative velocity of the air thrown back from the propeller. Bear in mind that the world speed record for prop-driven aircraft is 599mph.
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Because in his case you don't have two thousand horsepower behind the prop - you have as much energy as you can get out the wind that actually hits the swept area of the prop at best.

Ubergeekian said:
Why would the energy available to the prop depend on the size of the prop?
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you have as much energy as you can get out the wind that actually hits the swept area of the prop at best.

Ubergeekian said:
Helicopters are not exploiting a velocity difference between two fluids. And they are perfectly capable of climbing.
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Yes - because they have excess power from the engine - this is not available when all the energy you can extract is defined by the swept area of the prop.

As I said - no nitpicking and obfuscation, the summary is here:-

http://www.ybw.com/forums/showthread.php?p=2452253#post2452253

Where is the principle wrong...
 
mikemonty said:
you have as much energy as you can get out the wind that actually hits the swept area of the prop at best.

you have as much energy as you can get out the wind that actually hits the swept area of the prop at best.

this is not available when all the energy you can extract is defined by the swept area of the prop.

Where is the principle wrong...
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Well, you've just made the same mistake three times above for a start.
 
This is funny. The great thing is that this machine opens up a world of possibilities. For instance, why use wind. Power it by light in the large hydronic collider. The same physics applies and now we have fastr than light travel. This opens up the possibility of time travel as well. Really you don't need light as a power source anyway because if you tandem a number of these machines together the wind will power you faster than light directly down wind. It is fabulous the lengths people go to explain something they don't understand. It is like listening to a child excitedly descibing what a magician has done. It is a terrible pity that the innocence of youth is lost. It will be interesting to reread this come April.

Something far more interesting and actually deserving of arguement is, what is magnetism and gravity? Both relevant to sailing and far more difficult to explain. Science is about maintaining the child-like affinity for the word 'why'!
 
Ubergeekian said:
Gibberish. That's not how a propeller works, you've confused shaft power and reaction power, you don't properly account for the effect of the water propeller, your model for a hovering ping pong ball is wrong as well as irrelevant.

There is nothing there worth commenting on.
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What water propellor? I was discussing the land version that was cited and shown on utube.

http://www.youtube.com/user/ayrshquk

You haven't even addressed the points, just a convenient dismissal - you are a troll,

As I thought,
 
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