Sailing downwind - faster than the wind?

[jwilson]

But still faster than the wind. VMG, remember.

Everyone now finally seems to accept that BMW Oracle and other really fast boats can get from A to B downwind faster than the wind by tacking downwind. To build a device that goes directly downwind faster than the wind just build two of them, join them by a long adjustable arm, and call it a single quadramaran. The two halves set off on opposite tacks, and gybe when they reach the end of the arm. The quadramaran is now one unit - going dead downwind faster than the windspeed. it might be more efficient done differently, but it is possible..

 

[RAI]

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

 

[Ubergeekian]

Everyone now finally seems to accept that BMW Oracle and other really fast boats can get from A to B downwind faster than the wind by tacking downwind. To build a device that goes directly downwind faster than the wind just build two of them, join them by a long adjustable arm, and call it a single quadramaran. The two halves set off on opposite tacks, and gybe when they reach the end of the arm. The quadramaran is now one unit - going dead downwind faster than the windspeed. it might be more efficient done differently, but it is possible..

I refer the Rt. Hon. Gentleman to my post #51.

 

[mikemonty]

Certainly the only force entering the system is the wind. But the force is generated by the difference in velocity between the wind and the land/water. As I said before, the craft cannot tell which one is moving. It's just as valid to say the craft makes progress through the air powered by the water; as to say the craft makes progress through the water powered by the air.

Doubters should check the videos already mentioned on youtube, and they will find many more of a similar kind there.

I'll bite - but only because I can't resist shooting fish in a barrel.

I suspect there have been no further responses to this post because the obvious nonsense of this statement kind of defines you.

It is NOT valid to say the craft makes progress through air powered by the water - quite ther reverse, the craft is (in still water) RETARDED by the water - not powered by it!

Your next statement re "just as valid...powered by water...powered by air" is just plain bollox.

The force is generated by the difference in velocity between the SAIL and the air.
The water does not come into it, except to provide a force in the opposite direction ensuring that the craft will not actually ever reach the same speed as the air (in still water)
Now if there was a current PUSHING the boat along at the SAME SPEED and DIRECTION as the air there would be NO force on the sails and they would flap.
If the current was pushing the boat FASTER than the air, but in the same direction, the sails would "back" and hold the boat back.

(I am, of course, referring to sailing dead downwind here)

If you are looking for company I suggest you have a look here:

http://www.physforum.com/index.php?showtopic=2417&st=9570

for a group of like-minded people who have been arguing a similar though-experiment about airplanes and conveyor belts for almost five years.

 

[Ubergeekian]

The force is generated by the difference in velocity between the SAIL and the air.
The water does not come into it, except to provide a force in the opposite direction ensuring that the craft will not actually ever reach the same speed as the air (in still water)

Well exactly. So - as you then acknowledged - you have to have a velocity difference between water and air for a sailing boat to work.

It's perfectly possible to sail in zero wind as long as there is enough current to move the boat enough to fill the sails. I believe the term is "drudging".

 

[mikemonty]

Strange as it may seem - (and some have difficulty accepting this). It is possible to devise a wind-driven craft that sail directly downwind faster than the wind. It's been done with models, and in the next few weeks a team in America are planning on demonstrating a man-carring version. They have a website on http://www.fasterthanthewind.org/
Admittedly, this is on land, but it is theoretically possible to do it on water too.

I've only just had a look at the website you linked to and it is, as I suspected, an airscrew.
your airscrew is doing exactly the same thing as a yacht sail on a reach.

I also found an example of your "DDWFTTW" machines on a treadmill.

I'm always amazed at how much wooly thinking goes on...
The forward propulsion (force forwards) is generated by the accelleration of mass (air) backwards
The resistance (force backwards) is generated by friction at the airscrew, bearings and tyres.
The whole lot has been balanced by gravity (force backward) using the incline of the treadmill.
The fact that the force produced by the airscrew overcomes the resistance of the wheels is no great breakthrough.
In fact the analog breaks down at this point because the force of the accellerated mass of air is roughly proportional to the accelleration from zero to post-airscrew speed. In a real faster than downwind machine the decelleration would be much less because the whole air mass surrounding the machine is also moving so the force in this case would be from windspeed down to "a bit less than windspeed" - not zero - so the whole thing would be less efficent in reverse.
There is nothing there that suggests that reversing the principle would result in "faster than downwind" speeds.

Reversing the principle would result in air movement turning the airscrew (producing a force forwards) and resisted by friction in gears wheels airscrew etc. (force backwards)
This force forwards depends entirely on there being an airflow over the airscrew.
The second this airflow stops - ie the second you are travelling at the same speed as the wind - the force stops and the machine cannot travel any faster.
In fact it's worse because the resistances and inefficiencis mean that you will never get as fast as the wind before the forward force and the resistances balance and you remain at constand speed.

I think your University professor is having a bet with his colleagues as to how far his weaker-minded students will go before they realise they are wasting their time. He must be cashing in about now.


Or maybe Pierrome - you are having a bet with your colleagues about how far you can troll a bunch of sailors.

 

[mikemonty]

Well exactly. So - as you then acknowledged - you have to have a velocity difference between water and air for a sailing boat to work.

It's perfectly possible to sail in zero wind as long as there is enough current to move the boat enough to fill the sails. I believe the term is "drudging".

No.
Even if the boat was going dead downwind with the tide in the same direction at the same speed as the wind the boat is still "working" as I have got to assume that this is the intended course - even though you will have no steerage.

Actually it would be more accurate to say that you have to have a difference in velocity between the boat and the water AND the boat and the air - not air and water - for the boat to work

I don't know the word drudging but I understand the principle
If the water and the air were moving at exactly the same speed and direction you will still derive a force from the sails and thus a velocity, as long as you are using your sails as an aerofoil. - but the minute you get some way on, using the tide to "drudge" you no longer have zero wind as far as the boat is concerned - so your assertion that it is possible to sail in zero wind is from one viewpoint untrue.
(perfectly logical since a boat sails on apparent wind - ie with reference to the boat )

But this goes to the heart of it, there is a frame of reference to be taken into account here, and we both have to ensure we are using the same frame of reference (and that it makes sense in the context) or it all ends in tears.

 

[Malo37]

To me the ignorance of basic school level physics and lack of plain common sense displayed in some of the arguments in this thread is really surprising. I assume most of this is just a wind up.

 

[magic-martinh]

I'm quite enjoying this thread as it is making me think about things I had not really thought about much. The polarity of arguement is great. I am now looking for treadmill driven fan for my yacht...

 

[Ubergeekian]

To me the ignorance of basic school level physics and lack of plain common sense displayed in some of the arguments in this thread is really surprising. I assume most of this is just a wind up.

I find it vaguely depressing to see how many people are claiming the laws of physics and thermodynamics to support their contention that exceeding the windspeed downwind is impossible when it has already been done.

There are lots of good arguments to be had about implementations, efficiency, theoretical limits but the basic idea is beyond any doubt, because it has already been done.

 

[saltwater_gypsy]

I thought the big deal with windmill driven "craft" was that they could be propelled directly into the wind (unlike soft sails where the theoretical limit is about 22degrees off the wind)
As for sailing directly downwind ...once you reached windspeed you would then be in perfect calm!!

 

[Ubergeekian]

No.
Even if the boat was going dead downwind with the tide in the same direction at the same speed as the wind the boat is still "working" as I have got to assume that this is the intended course - even though you will have no steerage.

Drifting doesn't count.

Actually it would be more accurate to say that you have to have a difference in velocity between the boat and the water AND the boat and the air - not air and water - for the boat to work

I'm afraid not, or at least not quite in that way. Sure, if you have a 10kt tide and a 10kt wind in precisely the same direction you can get a sailing boat released from rest to do something. It will only be transient, though: the steady state result is the boat drifiting downwind and downtide with the sails empty and the rudder useless.

If you want the sails to work steady state then you must have a wind-water speed differential.

I don't know the word drudging ... your assertion that it is possible to sail in zero wind is from one viewpoint untrue.

OK, it's a flat calm day and the tide is 10kt heading due south. You drift south with it and voila! you have a 10kt apparent wind from the south. Which you can use to sail. Why? Because there is a difference in velocity between wind and water. It wouldn't work in a 10kt northerly wind and it wouldn't work at slack water.

 

[Ubergeekian]

As for sailing directly downwind ...once you reached windspeed you would then be in perfect calm!!

The underwater bits wouldn't be. That's the point.

 

[Pierrome]

OK, it's a flat calm day and the tide is 10kt heading due south. You drift south with it and voila! you have a 10kt apparent wind from the south. Which you can use to sail. .

Yes, and this demonstrates that sailing is completely relative and only the utilisation of the difference in velocities between two mediums.
Sailing craft are technically "interface vehicles".

Several have claimed that it is impossible to power a craft if there is no wind.
The above example by Ubergeekian shows that this is not true.

What counts is the RELATIVE velocity between two mediums.

The craft in the above example is in fact powered by the water.

As far as the boat is concerned it doesn't know which medium is providing the power.
It is possible to move in either medium powered by the other medium. That is what the downwind faster than the wind cart does.

The non-believers need to explain why, in the above example, you could not sail in the apparent 10kt wind provided by the water current. I'm looking forward to their explanation!

I would like to point out that THIS HAS BEEN DONE and it's up to the naysayers to explain why what they see is not what it appears.

 

[DownWest]

I grew up next to a boat design board and worked later with aircraft. I am now having a good laugh. It is amazing how many people try to upend the laws of physics. Most here seem to ignore friction and other losses. Good read though.
A

 

[JomsViking]

Stumbled upon this thread, haven't read it all, so forgive me if I'm repeating what someone else has said.
There's a vast difference between a boat being pushed by a force (the tide) and a car on tarmac - For Pierrome's comparison below to be valid, someone would be physically pushing the car, and then it would obviously work.
While there's stuff we still cannot fully explain (like hot water sometimes freezing faster than cold), the example given here, is a good example of the opposite, namely that it requires additional power to go downwind faster than the wind. We've - hopefully - all experienced the change in relative wind when that happens.
The examples with VMG also does dectract from the FTTWDDW theory, but that is probably the way to go for different types of vessels, although they then travel a longer way.
Respectfully
/JomsViking

Yes, and this demonstrates that sailing is completely relative and only the utilisation of the difference in velocities between two mediums.
Sailing craft are technically "interface vehicles".

Several have claimed that it is impossible to power a craft if there is no wind.
The above example by Ubergeekian shows that this is not true.

What counts is the RELATIVE velocity between two mediums.

The craft in the above example is in fact powered by the water.

As far as the boat is concerned it doesn't know which medium is providing the power.
It is possible to move in either medium powered by the other medium. That is what the downwind faster than the wind cart does.

The non-believers need to explain why, in the above example, you could not sail in the apparent 10kt wind provided by the water current. I'm looking forward to their explanation!

I would like to point out that THIS HAS BEEN DONE and it's up to the naysayers to explain why what they see is not what it appears.

 

[Pierrome]

I grew up next to a boat design board and worked later with aircraft. I am now having a good laugh. It is amazing how many people try to upend the laws of physics. Most here seem to ignore friction and other losses. Good read though.
A

Hi, you haven't made clear which side you are on. But I can only assume you are a non-believer from your last comment.

Disbelievers are always very ready to claim their credentials. Which makes it all the more embarrassing for them when they have to retract.

This point has been argued ad infinitum on other forums. It's a good way of stretching your mind and finding out who is capable of abstract thinking and who is not.

 

[RAI]

You accept, I presume, that BMW Oracle can sail downwind - in the sense of have a VMG - higher than the windspeed?

Where is this actually demonstrated? I can certainly see that its speed usually exceeds the wind speed, but has it actually reached a downwind mark faster than the wind has?

 

[DownWest]

Nice insult, No, I do not follow your claims for alterations to the basic laws.

Why do activists not fill in their bios?
A

 

[wingsailor]

OK, it's a flat calm day and the tide is 10kt heading due south. You drift south with it and voila! you have a 10kt apparent wind from the south. Which you can use to sail.

You could certainly sail, but not directly into the apparent wind (caused by the current) but if you were using a rotary sailing craft which can sail directly into the wind - it raises the fascinating possibility of sailing downcurrent through the current, powered by the current - with no wind!
I'm now sure how that relates to downwind faster than the wind. It takes a lot of thinking about!