Downwind faster than the wind. Poll

[Bosun Higgs]

BMW Oracle Racing's web site, videos, America's cup, race 1, about at 1 hr 58 minutes, both "platforms" gybe. Watch how the foresails behave and the speed through the water. When you there, note the time at the windward mark 1:39) and the time at the finishing line (2:42) and work out the average speed for the 20 nm down wind leg. Explain how a device driven by the wind can have a downwind VMG well above the prevailing wind speed.

The argument is about whether a vehicle can go downwind faster than the wind. ie the wind blowing in exactly the same direction as the boat is going. The video ( interesting - thanks for that ) shows quite clearly that the boats were not going dead downwind but tacking downwind. The boats velocity is about 30 degrees off the true wind vector. Thats an altogether different situation because it still leaves the sails able to get energy from the wind.

 

[RAI]

The argument is about whether a vehicle can go downwind faster than the wind. ie the wind blowing in exactly the same direction as the boat is going. The video ( interesting - thanks for that ) shows quite clearly that the boats were not going dead downwind but tacking downwind. The boats velocity is about 30 degrees off the true wind vector. Thats an altogether different situation because it still leaves the sails able to get energy from the wind.

Yes. But the VMG down wind that the boats are producing is 3x the wind speed. So taking their speed at 30 degrees off the wind (I think it's actually closer to 45) at 25 knots and splitting it between dead down wind and across wind components, you can see that the platform is slipping away from the wind at around 18 knots. How can it go away from the wind faster than the wind?

As the cart's propeller blade is spinning pretty fast compared to the wind when it is at speed, it is doing exactly what the yachts are doing, but in a 3D spiral instead of a 2D reach.

 

[Bosun Higgs]

So you are now saying that an aerofoil cannot have more lift than it has drag.

Grief Pierrome. Do you have any scientific training at all? Do you understand the difference between a force and work and energy?

If you take the case of a glider in level flight as an example of an aerofoil, you may well have greater lift than drag but they are working at two different angles. As the glider goes forward against the drag it is doing work and therefore consuming energy. Force x distance moved = work. Work x time = energy. Basic physics.

So the glider gradually slows down. Now as it slows down, so too does the lift decrease cos the relative wind speed has fallen. The glider has gravuity acting on it downwards so its progress down or up depends on the balance of forces between lift upwards and gravity downwards. Without an engine to increase flight speed and increase lift, gravity is always the more powerful which is why gliders have a glide angle downwards of maybe 1 in a few hundred.

Is that any help? I'm sorry if I sound frustrated but you are making suggestions that defy the laws of physics and at no more than A level level if that.

Anyway, this is my last post on this subject - its a bit like trying to argue with a bunch of Jehovas witnesses. Or nailing jelly to the wall.

 

[RAI]

Force x distance moved = work. Work x time = energy. Basic physics.

Ummm... work = energy, work x time = power?

 

[Pierrome]

Grief Pierrome. Do you have any scientific training at all? Do you understand the difference between a force and work and energy?

If you take the case of a glider in level flight as an example of an aerofoil, you may well have greater lift than drag but they are working at two different angles. As the glider goes forward against the drag it is doing work and therefore consuming energy. Force x distance moved = work. Work x time = energy. Basic physics.

So the glider gradually slows down. Now as it slows down, so too does the lift decrease cos the relative wind speed has fallen. The glider has gravuity acting on it downwards so its progress down or up depends on the balance of forces between lift upwards and gravity downwards. Without an engine to increase flight speed and increase lift, gravity is always the more powerful which is why gliders have a glide angle downwards of maybe 1 in a few hundred.

Is that any help? I'm sorry if I sound frustrated but you are making suggestions that defy the laws of physics and at no more than A level level if that.

Anyway, this is my last post on this subject - its a bit like trying to argue with a bunch of Jehovas witnesses. Or nailing jelly to the wall.

Dear Bosun

You give a long explanation as if you don't agree that an aerofoil has greater lift than drag. Do you agree with the statement or not?

As a matter of fact I have flown over 300 flights in gliders and 150 solo flights. So maybe I do know something about gliding.

You also said that you have a degree in physics and that a wind-driven vessel going faster than the wind downwind defies the laws of physics. Now it is proven. So much for a degree in physics!

I hope you have seen all the videos on the other thread.

It has now been proved to be possible and will shortly be ratified by an international sailing body NALSA. When it is ratified will you then admit you were wrong?

 

[shaunksb]

Dear Bosun

You give a long explanation as if you don't agree that an aerofoil has greater lift than drag.

Arn't the forces Thrust v Drag and Lift v Mass?

It's a good job you do most of your flying solo

 

[Pierrome]

Arn't the forces Thrust v Drag and Lift v Mass?

It's a good job you do most of your flying solo

Can you explain what you are talking about?

 

[Ubergeekian]

Grief Pierrome. Do you have any scientific training at all? Do you understand the difference between a force and work and energy?

If you take the case of a glider in level flight as an example of an aerofoil, you may well have greater lift than drag but they are working at two different angles. As the glider goes forward against the drag it is doing work and therefore consuming energy. Force x distance moved = work. Work x time = energy. Basic physics.

Force x velocity = work

Force x distance moved = work done (energy)

So the glider gradually slows down. Now as it slows down, so too does the lift decrease cos the relative wind speed has fallen.

Not if the pilot's any good it doesn't. You stick the nose down a bit to tilt the lift vector forwards and the drag vector upwards, thereby balancing lift, drag and gravity.

The glider has gravuity acting on it downwards so its progress down or up depends on the balance of forces between lift upwards and gravity downwards.

In normal gliding flight, lift is not upwards. Both lift and drag have components upwards, which together balance gravity downwards. Meanwhile the forward component of lift balances the backwards component of drag.

Without an engine to increase flight speed and increase lift, gravity is always the more powerful which is why gliders have a glide angle downwards of maybe 1 in a few hundred.

Gravity isn't more powerful - it can be balanced. The glider comes down because of drag. Typical glide angles are 25:1 for a metal+fabric two seater trainer up to around 60:1 for competitive exotica.

(About 300hrs P1 before I gave up)

 

[spork]

I'm the one driving the cart on the lake bed in Ivanpah. Is there anything I can do to help convince any skeptics that we were indeed going directly downwind faster than the wind?

 

[RAI]

Welcome spork

I'm the one driving the cart on the lake bed in Ivanpah. Is there anything I can do to help convince any skeptics that we were indeed going directly downwind faster than the wind?

Thanks for dropping by and thanks for all the entertainment. What multiple of wind speed are you aiming for now?

 

[spork]

Thanks for dropping by and thanks for all the entertainment. What multiple of wind speed are you aiming for now?

Pi

 

[RAI]

Pi

That 3.14159.... thingy? Very respectable.

I was wondering how far that might be from some theoretical limit. BMW Oracle racing seems to get about 3x wind speed down wind VMG, so I was betting on a wheel vehicle with a well designed variable pitch prop doing somewhat better.

 

[[32511]]

If you look at the start of this thread, you will see that I was a total unbeliever. Spork and his mates with the DDWFTTW cart made me think things through, and now I have a bit of a grasp of the theory, and have tried things out in practice in a small way So now a total believer.. As Galileo stated, "It moves!"

 

[spork]

That 3.14159.... thingy? Very respectable.

That's the one.

I was wondering how far that might be from some theoretical limit. BMW Oracle racing seems to get about 3x wind speed down wind VMG, so I was betting on a wheel vehicle with a well designed variable pitch prop doing somewhat better.

As a multiple of wind speed there is no theoretical limit (interestingly enough). But achieving more than about 4x wind speed becomes ridiculously difficult because it's extremely sensitive to internal losses (transmission efficiency, prop efficiency, rolling resistance, and aero drag).

If you look at the start of this thread, you will see that I was a total unbeliever. Spork and his mates with the DDWFTTW cart made me think things through, and now I have a bit of a grasp of the theory, and have tried things out in practice in a small way So now a total believer.. As Galileo stated, "It moves!"

JB and I are always very impressed with folks that keep an open mind and come around to see that this is real. We're even more impressed with the rare person that goes out and does his own experiments.

 

[Keen_Ed]

Do you think that building a boat capable of DWFTTW is a practical proposition?

 

[Tommyrot]

I'm the one driving the cart on the lake bed in Ivanpah. Is there anything I can do to help convince any skeptics that we were indeed going directly downwind faster than the wind?

Probably not!

There are still some on here (who go into highly involved maths) who think that the propeller is driving the wheels, despite the ratchet hubs!

Keep up the work on the vehicle Spork, and good luck with it.

 

[FistralG]

I'm sure it is powered by a cold fusion reactor as a back up when the wind dies.

 

[[32511]]

The only drawback with the poll is that once we are convinced, we can't alter our earlier 'no' votes!

 

[spork]

Do you think that building a boat capable of DWFTTW is a practical proposition?

Practical? No. But I'm sure it could be done. Ellison recently achieved a downwind VMG of close to 3X wind speed on the water by tacking. I suppose you could run two of those boats side by side, connect them with a telescoping pole, and call it a day.

Probably not!

There are still some on here (who go into highly involved maths) who think that the propeller is driving the wheels, despite the ratchet hubs!

Keep up the work on the vehicle Spork, and good luck with it.

True enough. We contemplated using a spool and takeup reel as a transmission. This has the benefit that it can be extremely efficient, and helps demonstrate that the wheels turn the prop (since you can't push on a rope). We may well do that before it's all over.

 

[spork]

The only drawback with the poll is that once we are convinced, we can't alter our earlier 'no' votes!

I wondered how different the poll might look today. Just reading the forum it seems a lot of folks get it. Perhaps it's time for a new poll?