Sailing downwind - faster than the wind?

[HoratioHB]

Any one gonna comment on my point about the treadmill example??
cos you can't!!

The real problem here is that people are arguing from two different standpoints. Going down the general direction of the wind and using relative wind angles to get increased VMG - fair enough.

Dead downwind with NO other wind vector - not possilbe to go faster than the wind.

Oh and I could talk about impulse and reaction methods of generating lift/thrust but there lies madness.

 

[RAI]

Dead downwind with NO other wind vector - not possilbe to go faster than the wind.

The blades of the propeller are not going dead down wind. They are on a broad reach.

 

[HoratioHB]

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.

Whether the blades are 'on abroad reach' whatever that means or not - (and what you really means is that they are generating lift by reaction rather than impulse) does not counteracct my argument above - come on lets see you refute it.

 

[RAI]

Whether the blades are 'on abroad reach' whatever that means or not - (and what you really means is that they are generating lift by reaction rather than impulse) does not counteracct my argument above - come on lets see you refute it.

They put that little tricycle to the test in open air on the ground and it ran away. Have a look at the videos again. On the treadmill, it could climb the slope when they put full slope against it. Yes, I would have liked to see it on a longer treadmill. Let's see what the man carrying version can do when they get it to work without breaking.

 

[snowleopard]

Dead downwind with NO other wind vector - not possilbe to go faster than the wind.

I looked at the evidence and said to myself: intuitively it is impossible but if this is true, how would it work? I figured it out from a fairly basic set of mechanical principles. I now see that there is absolutely no need for trickery.

Read what has been said here, look at my last video posting - the ruler/wheel one - and open your mind to viewing in another frame of reference. What you should see, if you have a reasonable grounding in mechanics, is that this is a machine for converting the difference in wind and ground velocities into forward motion.

Let me give you an analogy.

For centuries it was assumed that a heavy weight would fall faster than a light one. No one bothered to test it, it was obvious. Then along came Galileo who actually tried it and found they fell at the same rate. Nowadays everyone understands that but try this one: take two cylinders, one is solid, the other is hollow. They weigh the same and the external dimensions are the same. Now put them on a slope and, to most people's amazement, the solid one rolls faster. Most people 'know' they should go down at the same speed. Now many people would be looking for trickery, others observe the results and look for an explanation - which type are you?

 

[HoratioHB]

They put that little tricycle to the test in open air on the ground and it ran away. Have a look at the videos again. On the treadmill, it could climb the slope when they put full slope against it. Yes, I would have liked to see it on a longer treadmill. Let's see what the man carrying version can do when they get it to work without breaking.

Yes of course it did it was simulating the 'into wind condition!!!. My argument is if they put it on the treadmill the other way round - which is analogous to dead down wind it COULD NOT travel faster than the treadmill. The outside demonstration was not a true test as the vehicle was not dead straight and the wind almost certainly not either so there was some crosswind component.

And Snowy I am an engineer with a degree in Aeronautics and if you are right REFUTE my argument with reason. The wheel demo is a classic demonstration on how gears work no more.

 

[RAI]

Yes of course it did it was simulating the 'into wind condition!!!. My argument is if they put it on the treadmill the other way round - which is analogous to dead down wind it COULD NOT travel faster than the treadmill. The outside demonstration was not a true test as the vehicle was not dead straight and the wind almost certainly not either so there was some crosswind component.

I don't think so, the movement direction shown, against the treadmill direction, is the equivalent of going with the wind.

 

[snowleopard]

Yes of course it did it was simulating the 'into wind condition!!!. My argument is if they put it on the treadmill the other way round - which is analogous to dead down wind it COULD NOT travel faster than the treadmill. The outside demonstration was not a true test as the vehicle was not dead straight and the wind almost certainly not either so there was some crosswind component.

Look again. The treadmill is travelling right to left and the air is still. Change the frame of reference to the surface of the treadmill and you will see that the air is travelling LEFT TO RIGHT relative to the surface of the treadmill.

 

[HoratioHB]

Right then if you are correct then you're saying that putting the vehicle the other way round, which you could do, is analogous to the into wind condition?? Then do you agree that if you did this the vehicle would not accelerate because on release it was moving slower than the belt?
Because if you do you have just refuted that such a system can move into the wind - about the only thing that everyone has agreed to so far!!!!

In the treadmill example the wheels are driving the prop not the other way round so the treadmill is effectively the relative wind and the vehicle is most definitely going into that movement.

Anyway I said earlier that I would have one more try at this and I now give up. Not because I have defeated by reasoned argument but in fact the opposite.

Right now I'm going to invent a real perpetual motion machine, cold fusion and watch the aliens make a nice crop cirlce. Bye.

 

[snowleopard]

In the treadmill example the wheels are driving the prop not the other way round so the treadmill is effectively the relative wind and the vehicle is most definitely going into that movement.

Oh, a troll. Sorry, I can be a bit slow at times.

 

[mikemonty]

No need to. Once you have a boat which can have a VMG downwind higher than the windspeed, a simple though experiment - which I have already done for you - gets you to a boat sailing directly downwind.

Well that's very considerate of you!
But I'll do my own thinking thanks.

You will not get a traditional sailing boat to go on a dead run at faster than windspeed.
You will not get a propellor driven contraption per the ACTUAL road demonstration to work on a boat because the drag on a boat is too high.
(the road demo works - and I accept it works - because the friction on a wheeled craft is much lower than a boat - and you will never get enough power out the water, due to losses, to power the prop).

And I don't see the logic in your leap of faith from a boat with a VMG greater than windspeed to a boat sailing directly downwind.

 

[Ubergeekian]

Well that's very considerate of you!
But I'll do my own thinking thanks.

<bites tongue>

You will not get a traditional sailing boat to go on a dead run at faster than windspeed.

So what You won't get a square rigger to sail at forty degrees to the apparent wind either, but that doesn't mean it can't be done.

And I don't see the logic in your leap of faith from a boat with a VMG greater than windspeed to a boat sailing directly downwind.

<shrug> That's your problem, I'm afraid. Unless you pay me my normal teaching rates, in which case it will become mine.

E pur si muove

 

[mikemonty]

<shrug>

So what You won't get a square rigger to sail at forty degrees to the apparent wind either, but that doesn't mean it can't be done.

<sigh>

It can't be done on with traditional sails because the closer you get to dead downwind the closer you get to the point where you are "in irons" on the apparent wind.

You are sailing Oracle at her best downwind speed - the apparent wind is ideal and the sails are drawing perfectly.

You bear away a litle bit further downwind.

The sails are sheeted right in but cannot get quite as much lift out of the wind.

The forward (downwind) vector of the force has reduced for two reasons - the boat is facing further downwind so the downwind vector decreases, and the actual power the sails produce is reduced because they are now less efficient.

The boat starts to slow because of this.

The apparent wind drops because of the drop in boat speed.

The sails are now over sheeted - don't ease - or you will end up sailing a dead run and lose your argument!

The boat is now dead downwind and slowing, sails flogging - the apparent wind - now right on the nose, continues to drop until it reaches zero. You are now, as far as the sails are concerned, "in irons".

Oh No! the boat is now going slower than the wind, which is right up the chuff, and sheeted in - watch out for the gybe!

<shrug> That's your problem, I'm afraid. Unless you pay me my normal teaching rates, in which case it will become mine.

E pur si muove

I won't be paying you your normal teaching rate to teach me to sail!

 

[Ubergeekian]

<sigh>

It can't be done on with traditional sails because the closer you get to dead downwind the closer you get to the point where you are "in irons" on the apparent wind.

Nobody, absolutely nobody, has claimed that this can be done with traditional sails.

http://en.wikipedia.org/wiki/Straw_man

 

[mikemonty]

Nobody, absolutely nobody, has claimed that this can be done with traditional sails.

http://en.wikipedia.org/wiki/Straw_man

You did by implication...

...
You will not get a traditional sailing boat to go on a dead run at faster than windspeed.
You will not get a propellor driven contraption per the ACTUAL road demonstration to work on a boat because the drag on a boat is too high...

...And I don't see the logic in your leap of faith from a boat with a VMG greater than windspeed to a boat sailing directly downwind.

<bites tongue>


So what You won't get a square rigger to sail at forty degrees to the apparent wind either, but that doesn't mean it can't be done.

The clear implication was that you though that it was within the bounds of possibility for a traditional boat to... yada yada

 

[Kelpie]

You can't sail downwind faster than the wind on any kind of conventional craft.
However, the fact that high-performance craft like Oracle can make a better VMG downwind than the wind itself- and I now believe this is a fact- demonstrates to me that it is possible to reach a downwind point faster than the wind itself does. That is enough to remove all this talk of conservation of energy, thermodynamics, etc, because, counterintuitive as it seems, these laws do not seem to bind us in the way which you would assume.
Now, exactly how the prop/wheel device does what it claims to do, I don't know. But I think that the principle may actually be sound.

 

[snowleopard]

I have now figured out where the 'impossible' contingent are coming from.

We all (or mostly) understand the rig for going directly upwind. The apparent wind drives the prop, the prop drives the wheels, the wheels make the vehicle move, directly upwind.

Now you turn it round so the wheels drive it downwind. As you approach wind speed the apparent wind dies away to zero, drive power falls to zero and you can go no faster.

That is entirely logical and it would indeed require magic to make it go faster than the wind - if that were the mechanism used.

Now look again at this video - don't worry, it's only the first 20 seconds that are significant - let it run to 18 seconds and pause it where they make the second attempt to get started. At this point you would expect the wind to turn the blades clockwise but look again - they turn anticlockwise! That is the crucial point - this is quite a different mechanism in action. The confusion is that both types use prop, gears and wheels but the gear ratios and direction of rotation are different.

The problem is that the mechanism used in this case is far from intuitive. It draws its energy not from apparent wind but from the difference between true wind and road. The easiest analogy to illustrate it is the roller under the ruler (fast forward to 1:45 to skip the patronizing bit). In the wind-operated version, the prop effectively grips the air in the same way as the rubber wheel grips the ruler.

 

[DJE]

Think of each propeller blade being one of those boats and spiralling down wind in 3D instead of tacking back and forth in 2D.

I refer the Rt. Hon. Gentleman to my post #45.

 

[Ubergeekian]

The clear implication was that you though that it was within the bounds of possibility for a traditional boat to... yada yada

Only for people with poor comprehension skills and there is little I can do about that.

 

[Ubergeekian]

Now, exactly how the prop/wheel device does what it claims to do, I don't know. But I think that the principle may actually be sound.

'Zactly. It's clearly not an easy mechanical problem to solve, but the laws of thermodynamics have no problem with this.