Tacking Downwind (faster than the wind)

[CT 249]

Comment to CT 249:

Tornado got performance boost in 2000 (see http://en.wikipedia.org/wiki/Tornado_(sailboat) ), which cannot be reflected in its polars in a book of 96. This supports your and others' comments.

The T's performance boost is only about 4%, if I recall correctly, and even adding 4% to those polars still under-rates the T's speed.

Anc K, without going into detail, maybe one should not rely on one's own impressions when one has not sailed a Skiff or a modern T. The available evidence (yardsticks, race results etc) indicate that the cats do very well indeed, even on courses without any reaches; in fact at our cat club the Tornadoes complain when there are too many reaches. Most T and skiff race courses have no reaches.

 

[ancient kayaker]

The T's performance boost is only about 4%, if I recall correctly, and even adding 4% to those polars still under-rates the T's speed.

Anc K, without going into detail, maybe one should not rely on one's own impressions when one has not sailed a Skiff or a modern T. The available evidence (yardsticks, race results etc) indicate that the cats do very well indeed, even on courses without any reaches; in fact at our cat club the Tornadoes complain when there are too many reaches. Most T and skiff race courses have no reaches.

I knew it was a mistake to get into a cat vs mono punch-up ...

Agreed. The Tornado is and always has always been a fast boat, boosted or not. it would be nice to have actual numbers though; those polars are all we have here. My Tornado experiences predate GPS by a depressing number of years.

 

[Boston]

here here

 

[sigurd]

hi yoavraz, @DDW and @WS there is still a relative motion between air and water even if not between air and vehicle. Thus you can apparently continue accelerating if you have a suitable contraption. there are some wheeled designs at youtube illustrating it and a stupenduous amount of funny discussions here and at every other sailing forum about it.

I hear you can also do it with a kite buggy on land but when I fly a kite I am typically more concerned with elevation than keeping up with WS DDW.

I was looking for a polar to kill the notion with a friend that cats are slow upwind, when I found this thread. Would appreciate inputs, but this is probably not the correct thread for such a discussion;pm's are welcome.

 

[ancient kayaker]

I thought this thread was at least moribund if not totally dead. reading back over it, everyone did a pretty good number on it. Myself, I was just interested in the theoretical aspects of how it was done; I'm more into comfort than performance.

With a sail on a boat you need to tack to beat the wind at 180 deg, but it has been done with a vehicle using a turbine to drive the wheels, and I have read reports of it being at least theoretically possible in a boat with a turbine/prop combination.

With a kite at sufficient height, you have the intriguing possibility of beating the surface wind at 180 deg since the wind at height will be faster ... but I'm sure that's not what you meant.

There's life in the old thread yet, but you can probably find better ones with a search, if you include "turbine" in the search topics.

 

[sharpii2]

I have read several sailing articles that advocate tacking downwind to gain speed, but is it possible to exceed the wind speed in the course made good (i.e., directly downwind) direction? It sounds crazy, but a quick bout of vector analysis suggests there is no theoretical reason why this is impossible.

As a simplistic example, a wind of speed W due North and a heading of 30 deg at speed W.sqrt(3)/2 ... would create an apparent wind speed on deck of W/2 due East, which could be harnessed by a sufficiently efficient sailboat to further increase speed. Sounds like something a foiler or cat would have done by now. Or is there a reason why not?

For the most part you are correct.

The main limitation is the maximum speed the hull can handle.

Most modern yachts are capable of planing. In some conditions, it is concievable that they might actually sail faster then the wind. Especially high performance dinghies.

More traditional hulls can barely exceed "hull speed", which is somewhere in the nieghborhood of 1.34 to 1.5 times the square root of the waterline measured in feet and knots, respectively. When these hulls try to exceed this speed, the tend to be pulled down rather than be pushed up by the passing water.

Working sharpies were probably the first boats to plane (when empty, with just crew)

When I sail, I rarely sail dead down wind. I always tack. This way, the wind is on just one side of the boat and its pressure on that side keeps the boat from rolling.

I once took a guy out sailing who boatsted how tuff he was.

A four or five foot sea was running, so I steered dead downwind. The boat rolled, yawed, and pitched like a mechanical bull.

The guy got a little pale faced after that.

 

[sigurd]

oops ak, I forgot to check last post date. anyway, I forget how the wind shear is usually but I think it is supposed to be logarithmic close to surface so there is surely alot to gain there. From experience on snow dunes the best flying height is highly variable.
But that just confuses the theoretical argument where classically one part claims going ddw faster than the wind is impossible because it breaks some physical rule.

To muck up the argument more, with the kite I often find myself "paragliding" downwind, in which case I guess at least temporarily I might go faster downwind than the WS. If I am careless I get the kite to ww of me and it falls out of the sky.
Probably in theory it would be possible to use jumping as a means of going FTTW DDW on average too..

Otherwise, accelerating on a beam reach and then gaining downwind vmg through the jibe would probably do it.
It's not relevant for anything "steady state" though since it trades inertias, speeds, courses all over the place over time.

 

[ancient kayaker]

Sigurd; hush, hush, or you'll wake up the Sailing Anarchy guys! Although with a kite you're probably one of them!

The FTTW-DDW theory and practise is well established of course. No problem with the date, lots of otherwise sensible people forget to check the posting date (little jab at Bob there). I'll go back to sleep now ...

 

[bistros]

Sigurd; hush, hush, or you'll wake up the Sailing Anarchy guys! Although with a kite you're probably one of them!

The FTTW-DDW theory and practise is well established of course. No problem with the date, lots of otherwise sensible people forget to check the posting date (little jab at Bob there). I'll go back to sleep now ...

I've gone faster DDW than windspeed many times - under parachute. My parachute can reach around 30 miles per hour forward speed. I haven't jumped in years (although I still miss it), but it is certainly possible to exceed windspeed with a modern "square" aerofoil canopy.

Any time anyone needs to prove this to themselves, just find a local dropzone and watch someone good. In winds under 20 knots, it is possible to exceed windspeed going directly into the wind - this is how we land. Front risers are yanked downward to increase angle of attack, and the speed gained is amazing.

--
Bill

 

[sigurd]

corrected:

Front risers are yanked downward to decrease of attack, and the speed gained is amazing.

--
Bill

sounds fun

 

[ancient kayaker]

I've gone faster DDW than windspeed many times - under parachute. My parachute can reach around 30 miles per hour forward speed. I haven't jumped in years (although I still miss it), but it is certainly possible to exceed windspeed with a modern "square" aerofoil canopy.

Any time anyone needs to prove this to themselves, just find a local dropzone and watch someone good. In winds under 20 knots, it is possible to exceed windspeed going directly into the wind - this is how we land. Front risers are yanked downward to increase angle of attack, and the speed gained is amazing.

--
Bill

I recall launching model gliders on a tow line when I was a kid. It was easy to get the glider almost overhead before the line released, and if it was moving fast enough the ring on the end of the line would snap away from the hook and gain more altitude than the length of the line theoretically permitted. This became more important when competition rules slashed the legal line length. The same principle permits wingsails to sail very close to the wind, the ultimate limit is the life to drag ratio of the wing or sail, for any given hull. However, if the line ring did not clear the hook on the glider at the right time the glider would auger in upwind of the launcher, still attached to the line; or you could release the line, wasting one of your limited launch opportunities. Even if you had a spare glider it was an unpleasant decision, although if you did it right the wings and stab survived.