Sailing downwind - faster than the wind?

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Pierrome

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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.
 
Pierrome said:
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.
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Fine on land - it will generate lots of torque. But as soon as the platform is allowed to be driven - either on water or on land - then as soon as the vessel is moving downwind at wind-speed there will be no relative wind, and so nothing to drive the propeller. I don't see how it can travel faster than the wind directly downwind.

There is evidence such a system can make progress directly upwind!
 
Yes, it sounds a bit like perpetual motion. Jim Wilkinson has a wind-vane propelled cat which has been discussed before and it certainly doesn't exceed wind speed.

I was a bit surprised a couple of years ago when motoring through the Solent in wind that was too light for me with no spinnaker to find that I had to motor at 6 1/2 knots and failing to keep up with a couple of large yachts tacking downwind who must have been sailing close to wind speed.
 
Yes. I'm used to the idea of boats sailing faster than the wind (not mine!) but it's the idea of a negative VMG greater than the wind speed that takes a bit of getting used to.
 
Its all down to relative wind speed innit. I believe the record of vessel speed exceeding wind speed is with ice yachts in the order of 10-15 times higher, but as said on a reach where the relative wind slowly moves forward as the vessel accelerates.
Dead downwind just aint gonna work once the boat is travelling at the wind speed, cos the relative wind is gonna stop.
 
HoratioHB said:
Dead downwind just aint gonna work once the boat is travelling at the wind speed, cos the relative wind is gonna stop.
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Yes, but everything is relative. The wind will stop. But the water flowing past the boat does not stop. If you want to go faster than the windspeed you have to get power from that (water turbine) that's how it works with the land vehicle - the road drives the wheels which drive the air-prop. The craft doesn't know whether the land is moving or the air is moving - it just gets its power from the relative movement.

As you know "windmill boats" can be made to go directly into the wind. Going downwind faster than the wind is exactly the same - but in reverse.
 
I just know I'm going to regret this but I think the second law of thermodynamics has a say in this.
So what you are saying is that once the boat speed approaches wind speed the relative wind speed is zero but a water turbine then uses the boat speed to drive the air prop - but surely then the drag of the water turbine will slow the boat down and the relative wind will pick up again. Er so the best you will get is something approaching wind speed for the boat.
Please prove me wrong - like with a u-tube video. otherwise I think you have just invented perpetual motion.
 
It's a very impressive student "design and build" project, but the idea of going downwind faster than the wind is just barmy. It's a perpetual motion machine, and has nothing whatsoever to do with apparent wind sailing. It won't work.
 
Along the same lines I knew a mechanic who, recognising the torque generated by an angle grinder, was totally convinced that if you were to tie two angle grinders together at exactly the right angle, they would levitate - he spent several winters in his shed trying to make it work.
 
HoratioHB said:
So what you are saying is that once the boat speed approaches wind speed the relative wind speed is zero but a water turbine then uses the boat speed to drive the air prop - but surely then the drag of the water turbine will slow the boat down and the relative wind will pick up again. Er so the best you will get is something approaching wind speed for the boat.
Please prove me wrong - like with a u-tube video. otherwise I think you have just invented perpetual motion.
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Theoretically it's possible exceed the windspeed downwind, even on water. But it's more easy on land. This was all started by Jack Goodman of the Amateur Yacht Research Society who posted a video on Youtube showing his model could exceed windspeed directly downwind (powered by the wind) you can see it here http://www.youtube.com/watch?v=aJpdWHFqHm0 - whilst at it, it's interesting to have a look at all the other videos on the subject, including some where a treadmill was used to simulate relative movement between the air and the road. Moving road - stationary air being exactly the same as moving air - stationary road. One such is http://www.youtube.com/watch?v=1BRvYZd81AQ
 
Even if it is possible - which I still doubt - I have to ask: "Why?" Alinghi and to an even greater extent Oracle proved (as if there were any doubt) that it is possible to make a downwind VMG that is much higher than the windspeed. Without sailing DDW.
Maybe there will be some practical applications that come out of this, but I doubt that sailing will be one of them.
As I see it, they are putting a lot of time and effort into finding a solution to a problem that has already been solved.
 
Homer Simpson: "In this house we obey the laws of thermodynamics".

If the only force entering the system is the wind then you can not use it to generate a larger force in opposition to itself. This would require the creation of energy which is impossible under the afformentioned laws. This requires no experimentation and any apparent success stems from a failure to calculate the forces involved or from trickery. IMHO
 
pmagowan said:
Homer Simpson: "In this house we obey the laws of thermodynamics".

If the only force entering the system is the wind then you can not use it to generate a larger force in opposition to itself. This would require the creation of energy which is impossible under the afformentioned laws. This requires no experimentation and any apparent success stems from a failure to calculate the forces involved or from trickery. IMHO
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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.
 
The video is interesting, but I suspect changes in wind speed and momentum have a lot to do with the "faster than the wind" appearance.
 
Right I've looked at the U tube vid - first question, is that propellor geared to the road wheels or is it using thrust?

Secondly if you look at the streamer it starts off with a relative wind from astern and the blades are turning anticlockwise (when seen from astern) as it speeds up the relative wind comes from ahead yet the blades contunue to turn in the same direction. You cannot autorotate a propellor in the same direction with opposite airflows.
I are now totaly confused!!!!
 
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