Sailing downwind - faster than the wind?

snowleopard said:
Indeed. If they had released a balloon as they rounded the top mark it would have still been drifting down the course 2 hours after the boats had packed up and gone home!

Of course that's on the water. A land yacht tacking downwind can do as much as 80 knots.
Click to expand...
Excuse me while my mind boggles.

While sailing down wind on a broad reach, BMW Oracle's speed could be regarded as having two orthogonal components, one down-wind, parallel to the wind direction and one perpendicular to it - cross-wind. The vector combination of the down-wind and cross-wind components gives its speed through the (assumed still) water.

You are saying the down-wind component of the boat's velocity got to about 3 times the actual wind speed over water on that leg of the race?
 
snowleopard said:
The date must have been wrong. It happened last month. Yes, quite genuinely a vmg >3x wind speed. Staggering.
Click to expand...
The references to Sailing Anarchy seem to have messages and links from November 2008, as do the Youtube links.

I'm trying to draw a vector diagram for the forces on BMW Oracle while it runs away from the wind at 3 times the wind's speed. It's proving difficult. When it's on a beam reach, all is fine. As it turns down wind and its downwind speed increases, it runs out of thrust from the wind at wind speed. How does it get from there to three times wind speed down wind? Does it see 2 x wind speed in the opposite direction to the wind?
 
OK, here's how it works....

The essence of the situation is that you have an airscrew which grips the air and wheels which grip the ground. They are connected by a gear mechanism. The air and the ground move relative to each other and the gearing between the two results in the vehicle moving. The key to the faster-than-the-wind motion is the gearing.

This is an excellent analogy, if a little patronising.

So - no cheating, no perpetual motion, no rewriting of the laws of physics. Simples
 
RAI said:
I'm trying to draw a vector diagram for the forces on BMW Oracle while it runs away from the wind at 3 times the wind's speed. It's proving difficult. When it's on a beam reach, all is fine. As it turns down wind and its downwind speed increases, it runs out of thrust from the wind at wind speed. How does it get from there to three times wind speed down wind? Does it see 2 x wind speed in the opposite direction to the wind?
Click to expand...

This diagram should explain it. The combination of the true wind over the quarter and the boat's forward motion produce the apparent wind, the red arrow.

When that acts on the sail/wing, it produces a thrust at right angles to the apparent wind - the green arrow. You can think of that as the sideways component - yellow- that is resisted by the dagger boards and the purple forward component that pushes the boat along.
Apparentwind.jpg
 
Thanks for taking the trouble to draw the diagram SL.

However, it doesn't go far enough for me. Can you draw in the down wind and cross wind components of the boat's velocity. Whereby the downwind velocity component is three times the wind velocity. How does the wind keep up?
 
RAI said:
Thanks for taking the trouble to draw the diagram SL.

However, it doesn't go far enough for me. Can you draw in the down wind and cross wind components of the boat's velocity. Whereby the downwind velocity component is three times the wind velocity. How does the wind keep up?
Click to expand...

No, that isn't how it works. The true wind doesn't 'keep up'. It moves the apparent wind sufficiently out to the side to give a useful thrust component.
 
I'm going to have one last try on this then I think I'll shoot myself.

VMG downwind faster that the wind - fine its called down wind tacking. On my boat I could always go faster down wind that way. This is because the boat is reaching and is it speeds up it generates a forward component of wind speed that gradually brings the realtive wind forward. It is actually possible to end up with a boat (or more likley an ice yacht) hard on the the relative wind - when travelling ALMOST dead down the true wind.

Now lets take that propellor driven trolley and assume that at one instant its travelling at a constant speed. The wheels are turning and connected via gearing to the propellor. To continue travelling at a constant speed the thrust generated MUST be equal to the drag otherwise it will speed up or slow down.

If its going into wind or has an into wind component the relative wind component from ahead will always be greater than the vehicle speed, thrust will be generated and the trolley accelerate until equilibrium between thrust and drag is achieved. If it is doing 10 knots and the true wind is 10 knots the relative wind is 20 knots and it is this speed difference that generates the energy to push the vehicle along.

BUT if it is EXACTLY downwind the relative wind component will always be less than the vehicle speed ie if its is doing 10 knots and the true wind is 10 knots then the relative wind is ZERO and no thrust can be generated and it will slow down. If its doing 20 knots and the true wind is still 10 knots (ie its travelling faster than the true wind) the relative wind is only10 knots and that cannot generate sufficient thrust to overcome drag so it slows down.

And I think you'll find its the second law of thermodynamics not the first. edit - Oops just checked up on that and actually its both laws!!
 
Last edited:
snowleopard said:
I think this one deserves a look.

I see two possible responses: (a) I don't see how it could work so the evidence must be false or (b) I wonder why it works.

We have proved, for those prepared to read the evidence, that downwind faster than the wind motion is possible, so as far as I am concerned the question is - how does this particular device work? It has to be drawing its energy from the difference in velocity between air and surface. My first thought would be to find out whether the prop or the wheels are providing the forward motion.
Click to expand...


There is a third response, the scientific response: If something must break the fundamental laws of physics in order to work then it is not working. If it appears to work then there is a flaw.

The simplest analogy is David Blane. He APPEARS to do things which we know are impossible. Scientific people do not think he has 'magic'. They may not understand how he APPEARS to do these things but they do know that it is an illusion.

If people cant grasp themost basic principles then it is impossible to have a reasoned argument about this. It is impossible for a vehicle powered by the wind to move dead downwind faster than the wind. The wind is the only source of energy entering the system and therefore to create more energy would break the laws of conservation of energy.

There are litterally hundreds of these types of devices around the internet, all starting circular arguements with people who believe their eyes more than their brains. I must assume this is a troll
 
I think, but I'm not sure, that the weakness in your argument HB, is that you are not looking at the aerofoil of the propeller blade and its parallel to the BMW Oracle wing. Rather than thinking of propeller disk as a parachute.

If the BMW Oracle wing can go at 3 x wind speed down wind, then the blade of the propeller on this contraption might be able to do the same.
 
Just had a very nice bath and whilst ruminating (as one does) I thought of another way of looking at this.
If you take the treadmill video, the speed of the treadmill is effectively the true wind speed. Imagine holding the little propellor driven trolley as shown - it is facing in the opposite direction to the travel of the treadmill. While the propellor speeds up you will feel a force dragging it back until the propellor thrust overcomes drag and then the force will reverse - therefore when you let go it accelerates away and therfore its speed relative to the belt will INCREASE if the treadmill were infinitley long it would reach equilibrium at some speed higher than the treadmill - this is the into wind example.
Now turn it round (the vid does not show this for some reason!) this is the downwind condition you will feel the same forces but when you let it go, although it will initially start to move it will be going slower than the belt and so it will not have enough thrust to continue to acclerate and will eventually slow down. If it were infinitely long the end result would be slowly moving forwards relative to the belt as the propellor would extract some energy from the air BUT not as fast as the belt.
 
RAI said:
I think, but I'm not sure, that the weakness in your argument HB, is that you are not looking at the aerofoil of the propeller blade and its parallel to the BMW Oracle wing. Rather than thinking of propeller disk as a parachute.

If the BMW Oracle wing can go at 3 x wind speed down wind, then the blade of the propeller on this contraption might be able to do the same.
Click to expand...

No a propellor is not a parachute but if it is generating torque it is also generating drag and quite a lot of it. A helicopter in autorotation does not fall out of the sky at uncontollable speeds as the drag from the rotor is quite significant.

And anyway I am not arguing against the Oracle example but she was not DEAD downwind she was on a very broad reach and that is very different and you can make a lot of VMG that way.
 
mikemonty said:
Why not? a boat making way dead downwind with wind and tide as described might be drifting but its still sailing within the context of the OP - YOU don't get to make the rules.
Click to expand...

Drifting ain't sailing. End of.

If you have a 10kt tide and a 10kt wind in precisely the same direction you can sail across it perfectly happily, why do you think it would only be transient.
Click to expand...

Nope. Can't be done. To the boat that's exactly the same as a flat calm and no tide, and you can't sail across that.

Nope! if you want the sails to work steady state (produce a force) you need sails -air differential. To be able to STEER or not simply drift downwind you need water - boat differential
Click to expand...

You're almost right, but miss the most important point.

Yes, to get the aerofoils to work requires a wind/boat differential
Yes, to get the hydrofoils to work requires a water/boat diifferential

But to get the boat to sail continuously you need to maintain both differentials and the only way to do that is to have a wind -water differential. Otherwise you will inevitably end up drifting downwind and downtide with no forces on the sails, keel or rudder.

Each of the two sets of forces are controlled independently of each other (within bounds) by the sailor. Don't try to bundle them up into one statement and then jump through hoops trying to justify it - what's the point?
Click to expand...

Sailing boats require both wind and water forces to work. It's completely pointless to try to separate them. How well do you think a hovercraft - with effectively no water forces - would sail?
 
OK .. simple expanation.

Forget all about wind speed and boat speed, consider, instead, the energy of the system.

A body of wind will "contain" energy. A moving boat will have energy. The sails and keel combination are simply a device which extracts energy from the wind and delivers it to the boat. If the sails can extract enough energy you can achieve any desired speed up to a point. Now the whole point of the vector diagrams is to take the explanation to the next level and explain what happens to the apparent wind as we accelerate the boat .
 
pmagowan said:
If people cant grasp themost basic principles then it is impossible to have a reasoned argument about this. It is impossible for a vehicle powered by the wind to move dead downwind faster than the wind. The wind is the only source of energy entering the system and therefore to create more energy would break the laws of conservation of energy.
Click to expand...

I'm afraid you're way off beam there. (a) I do have a very sound grounding in physics to university level (b) it's not impossible, it's just that you don't understand what makes it work (c) The source of energy is not the wind, it is the difference in velocity of wind and ground or wind and water.

All sailing works only by exploiting a difference in wind and water velocities which is why a boat can't sail if the wind and tide are moving in the same direction at the same speed.
 
saltwater_gypsy said:
Please give a more understandable response!
What I wrote was that if you were going directly downwind at windspeed, you would effectively be in calm conditions.
I don't see how your reply relates to that or how you might get propoulsion from the motion of a boat through water if the boat was getting propulsion from the apparent wind which was causing it to move through the water at all!!!!!!
Click to expand...

OK, let me explain in a little more detail.

There's a 10kt north wind and no current. You're moving downwind - south - at 10 kt. Apparent wind is 0. Apparent current is 10 kt heading north.

To go faster south, you simply have to use the energy in the apparent current to move into the head current.

This is exactly the same problem as sailing direct into a headwind! And we all know that that can be done with the right gearing between two turbines.

All you have to do is start with such a boat making its way into a headwind, turn the picture upside down, change the "air" and "sea" labels and you've done it. In practice, of course, there will also be changes needed to the gear ratios and turbine sizes.

But it's the same problem
 
pmagowan said:
If people cant grasp themost basic principles then it is impossible to have a reasoned argument about this.
Click to expand...

How true. How very, very true.

It is impossible for a vehicle powered by the wind to move dead downwind faster than the wind. The wind is the only source of energy entering the system and therefore to create more energy would break the laws of conservation of energy.
Click to expand...

Who has said anything at all about creating more energy? For that matter, what, precisely are the systems boundaries you are using?
 
You must log in or register to reply here.

Share this page

Top