Downwind faster than the wind. Poll

[Ubergeekian]

Saying it a dozen times does not make it right -
How does the cart gain energy from the movement of the wind relative to the ground?

Through interactions at the wheels and propeller, obviously.

Because it works on a different principle from the DDWFTTW machine - the operative word being DEAD DOWNWIND

Every single putative "conservation of energy" and "laws of thermodynamics" argument given here applies to a boat tacking downwind. Bit embarrassing that it works, eh?

I think we should leave out the windmill driven boat going upwind.

Getting a vehicle to go upwind is precisely the same problem as getting a vehicle to go downwind faster than the wind.

And leave out Oracle too, tacking downwind is not what is under discussion.

It blows all the theoretical arguments against out of the water.

 

[ianj99]

Apologies if this is simplistic...

If a runner is sprinting dead downwind slower than the windspeed, he'll feel the wind pushing him and will expend a little less energy than if there was no wind.

As his speed approaches the windspeed he'll feel less and less of a benefit from it.

What possible technique or device can he use to extract energy from the wind once his speed equals the wind's?

None that I can see and without the energy transfer he'd have to supply all the energy to go faster than the wind.

I

 

[DownWest]

Uh? Tacking downwind has been used by cats for ever. Because their resistance is low.
OK racing cats. Normal single hulls are limited by their hull speads.This still is not of much interest in the current chat.

 

[mikemonty]

Er ... it exploits the velocity difference between air and water. It's vmg exceeds windspeed. The fact that its foils (hard wing and daggerboards) are fixed, so that it has to gybe back and forth, rather than rotating - as a big air propellor connected to a water propeller makes no difference. In both cases, you end at the bottom mark ahead of any balloon that you released at the windward mark.

What part of "Dead Downwind" do you not understand?

Oracle tacks and gets its energy efficiently by utilising the apparent wind.

Were it to run dead downwind it would go slower than the wind.

Apart from the fact that, as I recall the statement is that the wheel is driving the prop. - which is contrary to your scenario where the airscrew is driving a water propellor....
Now you are suggesting that you can hook up a prop as a kind of sail to use apparent wind to drive the boat faster than the wind.

So I'll buy it -
your sail/airscrew has been towed up to a speed where it's downwind motion is fast enough
to create an apparent wind over the airscrew that it can use to drive its own water propellor fast enough to keep its down wind motion fast enough to create an apparent wind over the airscrew that it can use to drive its own water propellor fast enough to keep its down wind motion fast enough to create an apparent wind over the airscrew that it can use to drive its own water propellor fast enough to keep its down wind motion fast enough
to create an apparent wind over the airscrew that it can use to drive its own water propellor fast enough to keep its down wind motion fast enough ....

....Well you get the picture
So there you are - blasting away downwind at fifty knots and a thought strikes you...

well here I am ging downwind at fifty knots - why don't I just turn the tiller a little and head for Calais?
Well OK! now I can do a few doughnuts and noise up the Navy for a while because I'm going fast enough...
to create an apparent wind over the airscrew that it can use to drive its own water propellor fast enough to keep its down wind motion fast enough to create an apparent wind over the airscrew that it can use to drive its own water propellor fast enough to keep its down wind motion fast enough to create an apparent wind over the airscrew that it can use to drive its own water propellor fast enough to keep its down wind motion fast enough
to create an apparent wind over the airscrew that it can use to drive its own water propellor fast enough to keep its down wind motion fast enough ....

Three years later you finally discover how to stop.

Then you drive home without an engine in your car by pulling your own tie forwards hard.

 

[ianj99]

What part of "Dead Downwind" do you not understand?

Oracle tacks and gets its energy efficiently by utilising the apparent wind.

Were it to run dead downwind it would go slower than the wind.

Its hard to get the message across that the energy has to come from somewhere isn't it!

 

[Ubergeekian]

Apologies if this is simplistic...

If a runner is sprinting dead downwind slower than the windspeed, he'll feel the wind pushing him and will expend a little less energy than if there was no wind.

As his speed approaches the windspeed he'll feel less and less of a benefit from it.

What possible technique or device can he use to extract energy from the wind once his speed equals the wind's?

He can't. You need some mechanism for extracting energy from the ground as it passes by (fast) and transferring it to the air as it moves by (not as fast). That's why a sailing vessel can't do it on a dead run - it needs to tack (if it can do it at all) so there is some keel lift force in the direction of the wind.

 

[Ubergeekian]

So I'll buy it -
your sail/airscrew has been towed up to a speed where it's downwind motion is fast enough
to create an apparent wind over the airscrew that it can use to drive its own water propellor fast enough ...

Several people have pointed out that the water propeller drives the air propeller, not the other way round.

 

[Ubergeekian]

Its hard to get the message across that the energy has to come from somewhere isn't it!

The energy comes from the velocity difference between the wind and the sea. No problem there.

 

[snowleopard]

These are cute toys, but sadly the treadmill doesn't prove anything. The operator is simply putting energy into the system by holding it still on the treadmill; when released the prop has enough stored enery to overcome the friction and other losses between wheel and prop(which are minimised in construction) and drive it forward for a short time.

Note that there is some energy driven into the prop by the wheels, the stored energy only has to overcome losses, so in a well constructed model the stored energy doesn't have to be all that big to drive it a short distance.

That one has been clearly refuted here

 

[pmagowan]

If the energy is being used by the vehicle it should be relative to the vehicle. This whole stupid idea of the difference in velocity of the wind and the ground cant be maintained. This is just a way of confusing your own brain. In a scientific experiment you must keep your point of reference controlled. In this instance it should be the vehicle as the point of reference. I really really hope a significant number of you are trolls as this is very worrying that sailors are completely unable to grasp even the basics of what makes a boat move forward. All the arguements ahve been made and simplified as much as I think is possible. If people are still not able to see why this is impossible (and an April fools joke) then I feel sorry for them. It really is at the most basic level of physics and it is worrying that people will attempt complex calculations when the fundamental principle is impossible and therefore the whole thing can be disregarded without a second thought. It is a little like being asked if you can add 1 and 1 and make 4 and then spending the next decade argueing more and more elaborate ways to do so. Any mathematician would just laugh!

 

[snowleopard]

And leave out Oracle too, tacking downwind is not what is under discussion.

Have a look at this video. It achieves faster than the wind downwind using a more easily understood mechanism.

If it is possible to do it with this mechanism, how can it be so hard to believe the same might be achieved using a different mechanism?

 

[Ubergeekian]

If the energy is being used by the vehicle it should be relative to the vehicle. This whole stupid idea of the difference in velocity of the wind and the ground cant be maintained.

It is beyond any doubt that a sailing boat needs different wind and water velocities to work. If you think otherwise, tell me how to get a boat to sail in a bathtub on a train, when both wind and water are doing 100mph in the same direction.

This is just a way of confusing your own brain. In a scientific experiment you must keep your point of reference controlled. In this instance it should be the vehicle as the point of reference.

You also need system boundaries. Where do you think the system boundaries are.

It really is at the most basic level of physics and it is worrying that people will attempt complex calculations when the fundamental principle is impossible and therefore the whole thing can be disregarded without a second thought.

What fundamental principle do you think is being broken? Why?

 

[mikemonty]

Several people have pointed out that the water propeller drives the air propeller, not the other way round.

But not the guy I responded to - troll.

More of the misdirection an obfuscation that we've come to expect from you.

And if you reverse the roles for the water propellor and air propellor my parody still works.

 

[bbg]

Er ... it exploits the velocity difference between air and water. It's vmg exceeds windspeed. The fact that its foils (hard wing and daggerboards) are fixed, so that it has to gybe back and forth, rather than rotating - as a big air propellor connected to a water propeller makes no difference. In both cases, you end at the bottom mark ahead of any balloon that you released at the windward mark.

There is a difference in the two approaches. Based on your questions, it isn't clear that you understand how Oracle / Alinghi can achieve such high speeds. They are very easily and efficiently driven hulls with (particularla in the case of Oracle) very efficient mechanisms to converted the wind into forward motion.

So. The boats start to sail. Let's say on a broad reach. True wind on the beam. But as soon as the boat starts to move forward, the apparent wind moves forward and also increases. As the boat moves faster and faster (which it can do because it is easily driven, and doesn't face the same limitations as a displacement monohul), the apparent wind moves farther forward and continues to increase. Eventually the boat seems to be travelling very nearly upwind (compared to the apparent wind), and at several times the (very low) windspeed.

The boats are so efficent they can bear away significantly and the apparent wind is still forward of the beam. At some point, however, the boat is travelling too far downwind and there is not enough wind from the side in order to drive the boat. Imagine the boat travelling at 20 knots on a broad reach in 6 knots of wind. There is still some (true) wind coming over the side of the boat. The speed of the boat will bring that apparent wind well forward of the beam. But if the boat bears away to DDW, the vectors simply cancel each other out. The boat is now, in effect, head to (apparent) wind, and can't sail. Just as there is an optimum tacking angle upwind, for these boats there is an optimal gybing angle downwind (they don't tack downwind, they gybe downwind).

These boats do NOT use the mechanism on which the DDW advocates rely - which is going straight DW, and extracting energy from the kinetic energy of the vessel and impart it into the air to produce thrust.

 

[pmagowan]

It is beyond any doubt that a sailing boat needs different wind and water velocities to work. If you think otherwise, tell me how to get a boat to sail in a bathtub on a train, when both wind and water are doing 100mph in the same direction.



You also need system boundaries. Where do you think the system boundaries are.




What fundamental principle do you think is being broken? Why?

point 1: the wind is relative to the vehicle. This is how you sail. The point of reference should remain controlled.

Point 2: the system is a vehicle heading down wind with infinate flat ground and infinate wind of a steady force. I will even allow the road induce no friction losses for the purpose of simplicity.

Point 3: the fundamental laws of thermodynamics, conservation of energy and Newtonian physics. Essentially the laws of life, the universe and everything!

 

[ianj99]

It is beyond any doubt that a sailing boat needs different wind and water velocities to work. If you think otherwise, tell me how to get a boat to sail in a bathtub on a train, when both wind and water are doing 100mph in the same direction.

"..different wind and water velocities to work". That's ridiculous.

Its not between wind and water, its between wind and object driven by it. eg a land yacht.

The water velocity will just add or subtract to the speed over the ground.

It takes energy to drive the boat through the water, you can't extract energy fom the wind sailing dead downwind when the boat and wind speed are equal so please show the math that explains where the energy need to exceed the wind speed comes from. Dark Energy perhaps!

Now if the wind is gusting, you might gain a bit of momentum and briefly exceed the 'average' wind speed..

 

[Ubergeekian]

point 1: the wind is relative to the vehicle. This is how you sail. The point of reference should remain controlled.

The reference frame used makes no difference to the answer, although some make getting there easier than others.

Point 2: the system is a vehicle heading down wind with infinate flat ground and infinate wind of a steady force. I will even allow the road induce no friction losses for the purpose of simplicity.

That's not what I asked. I asked what system boundary you are using. That's a fundamental part of modelling any dynamics problem.

Point 3: the fundamental laws of thermodynamics, conservation of energy and Newtonian physics. Essentially the laws of life, the universe and everything!

OK, so now explain precisely why you think these are being broken.

 

[mobeydick]

These boats do NOT use the mechanism on which the DDW advocates rely - which is going straight DW, and extracting energy from the kinetic energy of the vessel and impart it into the air to produce thrust.

So you admit that a vehicle could be designed, in theory, to progress DDWFTW - using reciprocating aerofoils or similar to replace Oracle, or even consider just the 'waterskier case'? It is just this particular design you are unhappy with?

MD

 

[mobeydick]

Could somebody do a poll to see how many DDWFTW doubters are also MMGW doubters?


MD

 

[Ubergeekian]

"..different wind and water velocities to work". That's ridiculous.

It's true, though. I keep giving this example. Big bath filled with water. On a train. Doing 100mph. Wind speed 100mph. Water speed 100mph in the same direction. Bet you a tenner you can't get a model boat to sail using that wind.

Its not between wind and water, its between wind and object driven by it. eg a land yacht.

Without something else to push against, something moving with a different velocity (water for a yacht, the ground for a land yacht) all you can do is move with the wind, which is not sailing. How much steerage way does a hot air balloon have?

It takes energy to drive the boat through the water, you can't extract energy fom the wind sailing dead downwind when the boat and wind speed are equal so please show the math that explains where the energy need to exceed the wind speed comes from. Dark Energy perhaps!

OK, two points here. First of all, no, the wind will do no work on a sail moving with the wind. It will, however, do work against a fan blowing back into the wind, which is what we have here.

Second, the energy needed to exceed the wind speed comes from the wind. There is absolutely no problem with that.

A related question for you: Can a towed vessel use only the energy supplied by the tug to travel faster than the tug? The answer is "yes", by the way, and the mechanism is essentially the same.