Downwind faster than the wind

[peterb]

Ross Garrett, in his book "The Symmetry of Sailing" postulates a wind turbine boat able to sail downwind faster than the wind speed. But in place of the usual wind turbine driving the water propeller, he is using the water propeller to drive the air propeller.

Think of a boat with an air propeller geared to a water propeller. Imagine that initially both propellers are held stationary. Point the boat downwind, and she will move, not as fast as the wind but she will move. The faster she moves, the greater will be the torque on the water propeller but the smaller will be the air propeller torque (because of the fall in apparent wind). With correct design, on releasing the propellers the water propeller can drive the air propeller, producing thrust. The thrust will exist even when the apparent wind has been reduced to zero, and on a low-drag hull it should be possible to go downwind faster than the wind.

Garrett also shows the measured performance polar diagrams of several classes of boat, including the 18 ft Sydney Harbour skiff. In a 10 knot true wind, sailing at 143 degrees to the wind (i.e. at 37 degrees off a dead run) the skiff can make 14 knots, giving a speed made good downwind of 11.6 knots. Under these conditions the apparent wind speed is about 8.8 knots and the apparent wind angle is only 44 degrees.

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[Sybarite]

Re: In theory only ...

I remember seeing a photo of what remained of a man's head after he walked into a helicopter rotor.....

John

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[BrendanS]

PM sent

<hr width=100% size=1>There is no such thing as "fun for the whole family."

 

[MainlySteam]

I think that once one considers that the only source of energy is the wind, then for the DEAD downwind case Garrett's hypothosis (wild joke) does not work. No matter how big the air propellor it will not provide any energy once the vessel is at wind speed. At that time, even in an imagined lossless situation, the water propellor will be generating but it can then only get energy by taking it from the boat's motion so the boat will slow, the air propellor will start turning as the boats slows, the boat will speed up, etc , etc but never past the speed of the wind.

But, however, see next post.

John

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[MainlySteam]

Brendan\'s broken the problem!!

Following my invitation to anyone who thought they had a ground breaking idea enabling sailing dead downwind faster than the wind, Brendan has sent me a PM which sets out a very (VERY!) clever idea. I can now see why he has a PHD - real clever little chappy he is. As some of you no doubt have come to realise, I am a real cynic and apply the acid test to things - well his idea has stood that easily. I have to rescind my claim that the laws of Physics forbade such a thing being possible /forums/images/icons/blush.gif.

I am sure that he has something that can really be exploited, and we just have to decide if we will just sell it to an America's Cup syndicate first, keeping all rights, or put it straight into the intellectual market for exploitation in yacht design. Fortunately, I work in the industry and have some contacts who will be excited to run with the idea.

The real worry is that the concept has come from a Mobo and not a saily person but when the idea becomes public peeps will see that it is one of those things that is just so obvious that all are blind to it. All I can say is that when Brendan was last out here he flew on a plane ride developed in NZ called "Fly by Wire" - the North Island one is just down the road about 15 km from our house - and from that and the question raised by this thread he has put 1 and 1 together and came up with a very clever idea.

Will keep you informed.

John



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[jamesjermain]

I still believe it impossible - in any way.

I am neither a mathematicial nor a physicist but I still believe that, to sail faster than the wind, down wind - ie to sail with a VMG at 180 degrees to the wind direction greater than the velocity of the wind - is impossible, even if the boat tacks down wind in order to increase the apparent wind speed.

The extra distance sailed will mean that if a boat and a notional pocket of air left the same point at the same time, the boat will arrive at a given point dead down wind after the pocket of air; much sooner that it would if it had travelled dead down wind, but still later.

As I understand it the wind has a certain amount of energy - which can be expressed as the ability to move a given mass a given distance. Unless you have invented a perpetual motion machine, you will not be able to get out of the wind more energy that it contains and therefore you will not be able to move further than the wind. Sailing across the wind may increase apparent speed, and therefore apparent energy, but the extra distance will always outweigh the extra speed.

I cannot prove this but feel it instinctively to be true.

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[peterb]

Re: I still believe it impossible - in any way.

James

I think the point is that neither the air nor the water on its own has extractable energy. You get energy from the difference in speed between the air and the water. You can extract that energy either by using the sails (boat held by the water, air going past the sails) or by using a water propeller (boat held by the air, water moving past the propeller/turbine). There is a degree of symmetry about it (hence the name of Garrett's book).

There is not that much difference between a wind or water turbine and a boat's sails. Remember that the blades of a windmill are usually called sails. By revolving them round an axis it's possible for the blade/sail to move obliquely through the air while the axis itself is lined up with the airflow.

 

[MainlySteam]

Re: I still believe it impossible - in any way.

<<<the blades of a windmill are usually called sails. By revolving them round an axis it's possible for the blade/sail>>>

But if you move the windmill DEAD downwind at the same velocity as the wind you will find that you are plain out of luck. Buy one of those kids toys with a windmill on a stick, hold it in the wind and it will spin, then move it DEAD downwind and then you will find it will slow down until when you move it at the same velocity as the wind, no matter which way you face it you will find that it will not revolve.

As a yacht approaches the velocity of the wind when sailing dead downwind, or with windmill bades or whatever, the same happens.

There is a solution, but that is the now intellectual property of Brendan and myself.

John

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[qsiv]

Re: I still believe it impossible - in any way.

It is absolutely straight forward to travel downwind faster than the TWS - it depends on your limits as to distance travelled, and what limits you set for aero and hydrodynamic friction.

Reducto ad absurdam, imagine a floating body with a kite attached -- all that is required is to fly the kite at an angle that generates a component of lift downwind (thats all most kites do), and steer the vessel in an arc, around the pivot provided by the kite. The centripetal acceleration gained is a function of the tow line length, and as such is user selectable - and can easily result in a speed that is a multiple of wind speed. Perhaps an easily understood analogy is that of a skier crossing the wake of ski boat - they travel much faster than the speed of the tow boat, as is evidenced by the tow line becoming slack - until friction takes it's toll.

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[MainlySteam]

Re: I still believe it impossible - in any way.

If I understand you correctly QSIV (one uses the lift vector from the kite, which is fundamentally the same in the vertical as a sail not on a flat run in the horizontal) I agree, and by steering an arc one may go even faster again if there is sufficient lift. However, it is not sustainable because you will very quickly run out of string and/or end up under the kite (if it is climbing the kite will be travelling downwind slower than the wind), in the limit you will end up at the wind speed - which I think you have the same view of as is probably why you mentioned limits.

Or have I misunderstood you and you have thought of some other "Schocking" /forums/images/icons/smile.gif way?

For any solution involving steering the yacht, note that I have carefully always said DEAD downwind as was the original question.

John



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[qsiv]

Re: I still believe it impossible - in any way.

I agree - but the original statement did not place limits on a duration of run, or of theoretical or practical limits to lift drag ratrios of either aerofoils or hydrodynamic surfaces or wave making resistance of the vessel, all of which are distinctly relevant.

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[MainlySteam]

Re: I still believe it impossible - in any way.

Why do I always find it difficult to find a way to disagree with you /forums/images/icons/frown.gif?

And there are probably other ways of doing the faster than the wind bit if the duration is limited - perhaps a very fast "pump" and a quick roll to windward {edit: "windward" loosely being side opposite the boom, seeing we talking of flat run} in a fast, light dinghy could do it for a brief moment?

PS How is your shocking monster getting on?

Regards

John

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by MainlySteam on 08/02/2004 11:07 (server time).</FONT></P>

 

[jamesjermain]

Re: I still believe it impossible - in any way.

Yes, But you will never overtake the boat. This is what you would have to do if you are to exceed wind speed dead down wind

<hr width=100% size=1>JJ

 

[qsiv]

Re: I still believe it impossible - in any way.

The Schocking monster is progressing (although mybe not so monstrous). Hull is laminated, interior structures built, and the interior due to be in place within two wweks, meantime deck will be constructed and in place within 5 weeks - then there is the fit out. Sails are ordered (ouch!), and Southern Spars are winding a mast, so it is all coming together. Due to be shipped to UK in May, hopefully in time for the Round the Island (more of a jolly as it may be first outing), and then a work up period before Cowes Week.

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[qsiv]

Re: I still believe it impossible - in any way.

The original question never suggested that the speed had to be sustained for a specific length of time. There is also a fallacy that assumes that the true wind direction is of paramount importance - all the aerofoils see is the apparent wind wind angle and speed.

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[MainlySteam]

Re: I still believe it impossible - in any way.

Thanks for the feedback. Will be very interested to follow progress, especially on the water.

Regards

John

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[Twister_Ken]

Re: I still believe it impossible - in any way.

I don't think it will be possible to follow it on the water, not without a 150hp RIB anyway.

Unless going diirectly downwind of course.

<hr width=100% size=1><A target="_blank" HREF=http://www.writeforweb.com/twister1>Let's Twist Again</A>

 

[MainlySteam]

Re: I still believe it impossible - in any way.

Maybe if QSIV (is the new one QSV ??) sails directly downwind under bare poles and I fly a fast climbing kite I may get close enough to see him disappear over the horizon!

John



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[jimi]

Re: I still believe it impossible - in any way.

I have difficulty with the concept of ourunning the wind unless sailing downhill. Surely if one can sail faster tahn the wind this means that one can go in any diection you want. Otrunning the wind would mean having the wind coming from dead ahead so surely there is no difference between this and starting from scratch and sailing directly into a headwind?

<hr width=100% size=1>.. whit way roon should it be again ..

 

[qsiv]

Re: I still believe it impossible - in any way.

No chance. My Old Man tried running under bare poles once in 60' prewar day racing boat. Unfortunately he was racing offshore (Brixham-Santander), and was still doing 17 knots. He described the weather in the log as 'wind freshening from NW'. The destroyer shadowing the fleet had her bridge windows stove in, and suggested to him that it was rather more than 'fresh'.

On this basis I shall try to avoid bare poles - they seem too much sail for the conditions.

I think the RIB will always catch us - but I do fancy frightening a bigger stinkie sometime! There is a 1.5oz chicken chute being built for just such an opportunity...

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