Downwind faster than the wind

[Badger]

I was wondering whether it is possible to design a sailing boat which, in theory at least, is capable of sailing directly down wind at more than the wind speed. This would mean that if you stand on the deck you would feel a direct headwind. Any thoughts ?

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[AndrewB]

In theory only ...

... wind turbine driven. I suspect it would have to be an impractically large turbine, and the yacht would have to be initially accelerated to above wind speed to get it started.

Err ... on second thoughts perhaps this would break the law of the conservation of energy as it implies a yacht could 'sail' in no wind at all! Is using kinetic power from the waves allowed?
<P ID="edit"><FONT SIZE=-1>Edited by AndrewB on 06/02/2004 14:50 (server time).</FONT></P>

 

[webcraft]

Badger,

This is an patently impossible . . . Once you are sailing at the same speed as the wind you cannot possibly go any faster as there is no unequal force acting to accelerate the vessel to a greater speed.

What IS perhaps theoretically possible is to sail upwind faster than the actual wind speed, as the faster you go the faster the apparent wind becomes - and the closer to the wind you sail. However, my gut feeling is that due to the frictional forces involved this is never likely to happen witha waterborne craft - although it might with an ice yacht, where the frictional forces are much smaller.

- Nick

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[boatmike]

Yeah!
Fit an engine......

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[Jacket]

Open 60's, large multihulls, maxi yachts, dinghies etc are all capable of sailing upwind and on reaches at speeds exceeding wind speed.

If you can reduce friction enough, you can sail at many times wind speed. I sail landyachts, which have relatively low friction. As a result, its possible to hit 40 knots in 10 knots of wind, and 70 knots (or more if you're brave enough, and have a large enough runway to play with) in 20 knots of wind.

Once you're moving, the apparent wind is always from directly ahead- a large class 3 keeps the wind within 15 degrees of the bow on all headings. As a result, you can exceed wind speed even down wind. You can't go directly downwind, but have to sail in a series of broad reaches (I say broad reach, but the apparent winds still from ahead). despite having to zigzag, your speed made good downwind would still be around 30 to 40 knots in 15 knots of wind.

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[jamesjermain]

I know land and ice yachts are very fast and use apparent wind to generate enormous speeds, but I cannot believe that even they can actually achieve a VMG dead down wind in excess of wind speed - even allowing for the fact that they are broad reaching at 20 to 30 knots with the apparent wind well forward of the beam. As soon as they start pointing below 90 degrees to the true wind, apparent wind will become harder and harder to generate.

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[StephenSails]

>>Once you are sailing at the same speed as the wind you cannot possibly go any faster as there is no unequal force acting to accelerate the vessel to a greater speed.<<

Mari Cha IV sails close hauled downwind. Not only that she was sailing at 20 knots in about 10 knot of wind!! Plenty of multi-hulls and larger craft do the same.

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[Badger]

Re: In theory only ...

I was fascinated by the picture, it's a kind of windmill yacht.Surely if you had the wind up your back hitting a big turbine like that, you could go downwind faster than the true wind ?

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[Sans Bateau]

Re: In theory only ...

Oh thats what it is! I thought it was an early type of outboard motor, in the raised postion of course.

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[Jacket]

In a class 3 (The larger of the two main racing classes), once you get going you can sail as low as about 140 degrees to the true wind, and keep the apparent wind within 20 degrees of the nose, allowing you to sail at typically between 3 and 4 times the true wind speed (though it does take a lot of skill to prevent this speed bleeding off on a long downwind leg)

It does sound unbelivable, and takes a lot of getting used to if, like me, you enter the sport after learning to sail in dinghies and yachts, but they really are that fast.

Why not come landyachting one weekend, and do a report for YM? Its a perfect sport for yachtsmen, as the best sailings during the winter, and it really gives you a different outlook on the power of wind!

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[Badger]

What about this on the pushpit ?

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[Sans Bateau]

Re: What about this on the pushpit ?

That must be a mobile wind farm for emergancy supply during power cuts, is'nt it?

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[DeeGee]

i disagree with everybody...

The wind is not a steady laminar flow, but a puffy turbulent flow. Thus, if you could gain enough acceleration from the strongest puff (think ice-yachts here) then you could get up to the speed of that puff. When the wind dies off, you keep going at faster than the decayed wind, until the next puff. So, your ice-yacht can travel at faster than the AVERAGE wind speed.

IMHO

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[AndrewB]

Re: In theory only ...

Unfortunately not. As the yacht approached the speed of the wind, the pressure on the blades would fall, and the power generated would drop to zero. On the other hand it is possible to use a wind turbine to drive a yacht directly INTO the wind: theoretically up to the speed of the wind but in practice very slowly. A number of experimental catamarans have been powered this way.

I just remembered seeing reference to a faster-than-wind-downwind turbine driven craft in an AYRS publication on display at LIBS. <A target="_blank" HREF=http://www.dcss.org/ayrs/AYRSpubs/ayrs120.html>SEE HERE</A>, Chapter 5. Haven't read it, so no idea what principle is involved.

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[Badger]

Re: In theory only ...

Thanks for the info. I also found one of the experimental cats you mentioned ....

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[Twister_Ken]

Heroic failure

Somehow, that beret says it all! And in combination with the two small wheels in front of the prop hub, you just know it's never going to work.

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[Twister_Ken]

So that\'s what happens...

...if you sail into a wind farm.

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[Twister_Ken]

Directly downwind you said

in which case no, because the sails would only be able to act as parachutes, not as foils

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[webcraft]

I stand corrected . . . I was aware that big multis, open 60s and the like nearly always have the apparent wind forward of the beam**, but was not sure if they regularly exceeded the true wind speed.

However, I still maintain that this is patently not possible on a dead run - which was the original postulate - as any increase in apparent wind would alway be 'on the nose' and will not drive the boat. I see the theory behind sailing on a series of broad reaches and still making a VMG downwind greater than the average wind speed.

(Sadly it's not going to happen on my boat . . .)

- Nick

** Thus proving that racing is not a gentleman's sport . . .

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[MainlySteam]

Absolutely agree with you webcraft. It is against all the laws of physics for a yacht to sail DEAD (as was the question) downwind faster than the speed of the wind.

As James says, no apparant wind giving a vectored increase over the wind true speed can be generated. To the suggestion that gusts provide the answer, that implies a frictionless (at best) system.

If anyone has a provable contrary view, PM me and I will ensure that we profitably protect and sell the secret to the race boat designers /forums/images/icons/smile.gif.

John

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