Sailing downwind - faster than the wind?

[Ubergeekian]

In the video of the model, the moving road is turning the wheels and the wheels are turning the propeller. In fact they go to some trouble to show that there is no wind. All that is happening is that the energy transmitted from the wheels to the propellor is giving thrust which is just about balancing the frictional forces at the wheels.

Almost but not quite right. Most of the force at the wheels is doing work turning the propeller. Friction is there, but is - or should be if they've made it well - a relatively minor factor.

In the "road " model the wind turns the propeller and this transmits drive to the wheels.

No.

In your confusion you have picked out the best of both worlds and devised a machine where the wind drives the propeller which drives the wheels which drives the vehicle which causes wind which drives the propellor which drives the wheels...ad infinitum.

No. Simply no. You're wrong. As far as I can see, that's because you are getting confused about the sort of work done by a propeller and done by the wind. What happens as the cart or boat moves downwind is that the wheels (or water turbine) start turning the air propeller. That blows back against the moving wind, effectiively increasing the windage of the thing so the wind exerts a greater force against it. If that greater force is more than the drag at the wheels then you're in business. Whether it is depends on many things, but mainly the gearing and pitch angle of the propeller or propellers.

The really important thing to remember is that although the wind cannot do work on the structure of the vehicle when it has achieved windspeed, it can do work on the propeller, because, to put it crudely, the propeller is trying to blow back against the wind.

Shall we wave willies a little here? I've got two research degrees in mechanical engineering and I've been teaching both mechanics and fluid dynamics at universities for around twenty five years. How about you?

 

[pmagowan]

'If that greater force is more than the drag at the wheels then you are in business'!

The whole point is that force can not be greater than the drag of the wheels as it IS the drag at the wheels. The drag on the wheels turns them round and this energy is relayed to the propellor through cogs and stuff. As their are inherant losses to create more force out than you put in is not possible.

Your last point worries me

 

[DJE]

Sailing Anarchy's thread is over 3600 posts long.

I gave up caring about this a looooong time ago.

From the San Jose Group's web site:-

We've had an unspoken (blog wise) goal for the last two months -- the vehicle running on the Ivanpah lake bed during the last full week of March. It's not a random date we picked ... the NALSA (http://www.nalsa.org/) is holding their premier yearly 'Americas Cup' landsailing event on those dates and in that venue.

Ivanpah is the playa where almost exactly a year ago the sensational Greenbird (http://www.greenbird.co.uk/land-record) set a new world wind powered land speed record of over 126mph. NALSA is the ratifying organization for such records here in North America.

Starting last year, we have been in discussions with the NALSA BOD regarding the possible addition of a DDWFTTW landsailing class and record ratification procedures. In January they suggested that we bring our vehicle to the event in Ivanpah where a good number of the BOD members will be. This would give them an excellent opportunity to see the vehicle first hand for themselves and provide a more hands on knowledge base for further discussion.

We accepted the challenge even though it meant moving up our construction schedule by almost a full month. We've been charging hard to make this date and it's all looking good at the moment. One week from now we should be well into the first open surface shakedown runs of the vehicle.

So why don't we all just wait and see?

 

[colingr]

From the San Jose Group's web site:-

We've had an unspoken (blog wise) goal for the last two months -- the vehicle running on the Ivanpah lake bed during the last full week of March. It's not a random date we picked ... the NALSA (http://www.nalsa.org/) is holding their premier yearly 'Americas Cup' landsailing event on those dates and in that venue.

Ivanpah is the playa where almost exactly a year ago the sensational Greenbird (http://www.greenbird.co.uk/land-record) set a new world wind powered land speed record of over 126mph. NALSA is the ratifying organization for such records here in North America.

Starting last year, we have been in discussions with the NALSA BOD regarding the possible addition of a DDWFTTW landsailing class and record ratification procedures. In January they suggested that we bring our vehicle to the event in Ivanpah where a good number of the BOD members will be. This would give them an excellent opportunity to see the vehicle first hand for themselves and provide a more hands on knowledge base for further discussion.

We accepted the challenge even though it meant moving up our construction schedule by almost a full month. We've been charging hard to make this date and it's all looking good at the moment. One week from now we should be well into the first open surface shakedown runs of the vehicle.

So why don't we all just wait and see?

Because that would spoil a perfectly good long running argument.

 

[snowleopard]

So why don't we all just wait and see?

And if the tests show it works, how many sceptics are going to change their opinions?

I can see the arguments now...

'It's just a conspiracy, the evidence is faked'
'It can't have been going dead downwind'
'There was a hidden motor'
'It's impossible'....................................................

 

[halfway]

Ubergeekian: "Shall we wave willies a little here? I've got two research degrees in mechanical engineering and I've been teaching both mechanics and fluid dynamics at universities for around twenty five years. How about you?"

Oh my, and you say concepts like "That blows back against the moving wind". I would sack you instantly as a total embarrassment.

Wind is NOT behind you to push against, it is in front of you. It is beside you, it is everywhere. You do NOT run away from wind, the bit behind has gone, it would be a better story to say you run forward INTO the wind and overtake it.

But then you could tell the real story and stop all this nonsense about efficiency and friction.

 

[Pierrome]

From the San Jose Group's web site:-


Starting last year, we have been in discussions with the NALSA BOD regarding the possible addition of a DDWFTTW landsailing class and record ratification procedures.

That's fascinating, it opens up the prospect of DDWFTTW carts competing against each other to see who can get downwind fastest. Competition is the best way of improving the breed. So we will see what the ultimate possibilities are.

An interesting thought. When you are sailing downwind, (in the normal way slower than the windspeed) what happens if you apply a turbine into the water and connect it to an air propeller (fan) blowing wind towards the stern of your boat. Does it increase your speed or slow you?

In other words can you take power from the passing water and use it to push you along?

 

[RAI]

In other words can you take power from the passing water and use it to push you along?

My sails flog too much as it is, unless the wind speed is high. I've taken to gybing down wind, much more fun.

P.S. mark Drelas's papers analyse your case, see DDWFTTW on the boat design forum.

 

[Keen_Ed]

The one thing that strikes me is that there are several supporters of this subject that argue incredibly vociferously for their case. Sorry guys but it almost seems like fanaticism, you seem so upset that peeps can't see the (what to you) is obvious that you have to keep restating your case ad nauseam.

Because a belief that it doesn't work looks so very much like a belief in a flat earth. And believers think that with just a little explaining, they can get the flat earthers to see how they're wrong and where their misunderstanding lies. Unfortunately, they run up against the whole horse/water thing.

 

[Keen_Ed]

Oh, and for those who think Mark Drela doesn't understand what he's on about...

there are some details about him here: http://web.mit.edu/aeroastro/news/magazine/aeroastro-no3/2006drela.html

and his little (!!) airfoil analysis program, Xfoil. http://web.mit.edu/drela/Public/web/xfoil/

 

[halfway]

Keen_Ed: "Mark Drela doesn't understand"


Equation 2 of the proof: Pp = Fp (V −W)/p

So he starts his proof off by saying that the machine when it is travelling at the same speed as the wind has no shaft power even if the efficiency is 1.


I would say that he started on a good note and the rest was pure embroidery!


To be fair to him he NEVER claimed it was a proof. It was even titled an "analysis". It is others that claim it is a proof.


Even the wonderful V/(V-W) has a slight issue at vehicle speed equals wind speed. As a mathematician, he should have noticed that.

So one page in, and anyone with a slight brain has been turned off. The credibility as a proof is down the toilet. No one would read the second page.

 

[RAI]

Keen_Ed: "Mark Drela doesn't understand"


Equation 2 of the proof: Pp = Fp (V −W)/p

So he starts his proof off by saying that the machine when it is travelling at the same speed as the wind has no shaft power even if the efficiency is 1.


I would say that he started on a good note and the rest was pure embroidery!


To be fair to him he NEVER claimed it was a proof. It was even titled and "analysis". It is others that claim it is a proof.


Even the wonderful V/(V-W) has a slight issue at vehicle speed equals wind speed. As a mathematician, he should have noticed that.

So one page in and anyone with a slight brain has been turned off. The credibility as a proof is down the toilet. No one would read the second page.

I think you need to read up a bit on propellers and see what happens to the air prop total efficiency factor. The special case of hovering or static propellers has to be dealt with separately. Mark's analysis is clearly for the case V not equal to W and mathematically the division by (V-W) in equations (4) and (5) would be an invalid operation if V=W. On page two he addresses this issue by substituting for the air prop total efficiency factor.

 

[halfway]

RAI: "substituting for the air prop total efficiency factor"


So you are saying he starts off with an impossibility, and then uses models of inefficiencies to show that the impossible condition could never happen. So he bases his proof on it only working in a lossy world.

Absolutely amazing. A technique that relies on imperfection to prove its concept. Why not get the kid next door to design the prop.


So look at the equation from (10) and its value at V=W. Whoops! It seems to be infinitely small. Now that helps. We now have an efficiency so small it counters the infinite problem.

This is meant to be a proof not an approximation.

What is being missed here that these equations are models of a prop performance at, and around, the ideal use of that prop. They do not characterize the full use of the prop across all conditions. These are approximations to be used at and around the ideal use of the prop.


It is not me that needs to start reading.

 

[RAI]

So look at the equation from (10) and its value at V=W. Whoops! It seems to be infinitely small. Now that helps. We now have an efficiency so small it counters the infinite problem.

Work out the value of inviscid efficiency for V=W again (try multiplying top and bottom of equation (10) by (V-W) first). What value would you expect it to have at V=W? It's easier to see the effect in Equation (11).

 

[halfway]

Just to put you out of your misery: if you really want an equation to prove this then you are looking at

ArcTan((V-W)/V) < 45 - (angle of attack) - (aerofoil resultant force angle deviation back from the chord normal).

There is also another equation to do with the stall angle for the blade when you are trying to start. But this is irrelevant when using a variable prop. Which is a must, if you do not want people saying you were pushed to get going.

This equation behaves very well and since ((V-W)/V) limits at 1, which makes 45 degrees, it never gets to the tan curve's infinity point.

Also the critical crossover results in zero which is smaller than the right hand side and shows no problems crossing the boundary to faster than the wind.

There is no discontinuity at the crossover.

Have I convinced you yet that the proofs are all wrong? Let see if you are now alone? Or does the world think proofs need to have discontinuities at critical points and rely on losses in a system to survive. I doubt it. Only time will tell.

See you later.

 

[spork]

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.

Surprisingly enough, the two actually work on exactly the same principle. If you imagine the world as a giant cylinder rather than a sphere, you can imagine a sailboat on a 45 degree downwind tack with downwind VMG greater than wind speed. Put another boat on the opposite side of this cylindrical world and they both make their way downwind faster than the wind. Their sails are our prop tips. From there it's simply a matter of scale and an engineering problem to create the same kinematic constraint for our prop blades that the boat's keel creates for the sail.

Instructions to build your own copy are here

http://www.rtfa.net/tag/dwfttw

Interesting. Those are my hands and voice. I just noticed the blogger asks:

"Well, now that we know DDWFTTW works, I suggest that it’s time to make some carts and figure out what cool things can be accomplished. For example, how big can these things get? What about a salt flats race? "

It seems he predicted the future. Our BUFC stands over 20' tall with the prop vertical and weighs about 350 lbs. So "pretty big" I suppose. We haven't put it on the salt flats yet, but the dry lake in Ivanpah, NV is a reasonable facsimile.

 

[snowleopard]

Surprisingly enough, the two actually work on exactly the same principle.

True but at that very early stage of the discussion the doubts were all about the different courses and the sceptics were all hung up about boats and apparent wind. We got onto the similarities a couple of threads later. This one is best left dead.

 

[spork]

True but at that very early stage of the discussion the doubts were all about the different courses and the sceptics were all hung up about boats and apparent wind. We got onto the similarities a couple of threads later. This one is best left dead.

Fair enough. I should have been smart enough to read the rest before jumping in.