Tacking Downwind (faster than the wind)

Yes, but ...

Ah, one of the eternal boating controversies raises its confused head! Take it outside boys! Leaving aside the eternal cat vs skiff argument and praying that I won't trigger another one from the foiler community etc: -

Yoavraz: I took the liberty of copying your polar diagram (temporarily) so I could superimpose a circle whose radius equals windspeed (I don't trust the monitor aspect ratio). It is clear that the faster boats routinely exceed windspeed on a broad reach, no surprise there.

However, I noted that for a course directly into or down wind, only the skiff actually makes good a speed over the course (not the heading) that is faster than the wind, and that is over a very narrow range of headings, by a mere 20% margin. Fall of by 3 degrees either side and that margin vanishes.

So did we answer the question? It was a theoretical one so the answer is yes, it's possible. In theory. For practical purposes it's not going to happen unless you have a damn good boat and perfect conditions. Foilers, land sailers and iceboats and other extreme types can meet the challenge but I don't yet consider them practical.

DG, welcome back! I think I now know why I have never seen, and am highly unlikely to see it happen.

 

strictly downwind - never

At 180 you are always slower than the wind. A kite and perfect planing can take you very close to wind speed, but you can get there only if you are dust above water. You need some side component of wind force to generate lift downwind to overcome water drag (even minor) and pass wind speed.

 

Agreed; but the thread was called tacking downwind. That's why I was so careful to distinguish between heading and course.

 

Got you at last...

Indeed. If the polars are correct for 10 knots wind:

At best VMG the Tornado will zigzag to its target downwind while making ~8 knots in the direction of target (slower than wind, "the wind will get there sooner"; though being speedier (~10) on the zigzag itself).
The 18ft Skiff is zigzagging making ~12 knots to target (faster than wind; and even faster (~14.5) on the zigzag itself)

 

I think we area all on track here. At least for the 10 knot case. This has been an interesting discussion, which went far beyond the "it obviously can't be done because" answer that I was expecting. It's rather like human-powered flight; it has happened but not on my lake ...

 

Not just 10 knots. For any boat and any wind speed, if you have the polars, you have the answer! More specifically, if you have the VMG, you have the answer. It has been interesting, something I have never checked before. I saw your question yesterday, but exactly understood what you meant only today... One needs a very fast (substantially beyond wind speed) boat at 140-160 degree winds for this. I wonder who else except the 18ft Skiff and alike qualifies.

 

With conventional sails yes, but there are ways and means. But the efficiency needs to be so high that its almost certainly unattainable on water, but its been demonstrated on dry land. Basically it involves a contraption that is propelled by means of an air screw suitably geared to the wheels.

 

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Ah, well, ah hmmmm.......

 

gggGuest: one would do better with an airfoil than a conventional sail perhaps although some are very good. It is attainable on water, see Doug's post. There's a thread on turbine driven boats with information on boats powered from a wind turbine through a water prop.

Doug: sorry, sorree! I know they WORK, but practical? A boat has to work everyday unless the weather is just plain too threatening, not when the wind is in the right quarter at a minimum speed while the waves aren't too high and the crew has to be weighed in and forget about cargo ... but they look like great fun. I wish I were young enough to risk life and limb on one ...

 

Just having some fun AK...But your kayak is using foils for stability,right? Seems eminently practical to me. Sorry,couldn't help myself!

 

The point was not whether the Tornado was faster than the Skiff; the point is that there is no way the Tornado is way slower as the polars indicate; therefore the polars are obviously not all accurate; perhaps none of them are accurate. Therefore the lessons that can be drawn from them, if any, should be treated with extreme reservations.

I've never measured the speeds downwind on a T or Skiff, but I'm certainly not sure that "for practical purposes it's not going to happen unless you have a damn good boat and perfect conditions." However, that's just a gut feeling, from being aboard or watching the occasional T or Skiff going downwind in light stuff. Formula windsurfers could also be candidates for having a VMG exceeding windspeed downwind.

 

Gotta agree with that.

Not sure it's wise to get into that whole Tornado/Skiff thing, but it seems logical to me that a fast cat like Big T should toast the competition on a reach with its ability to hold a lot of sail up to a stiff crosswind and cock a hull. The polars don't show that so you might have a point. Conversely, as the apparent wind approaches the axis of the boat either up- or down-wind a planing monohull should have the legs of the cat through reduced drag.

The last time I rode the T was a long while ago before I learned to appreciate the nuances of sailing. All I had to do was hang on grimly to the jib sheet and avoid falling off, but it seemed like it was going a whole lot faster than 12 to 13 k. We were chasing and catching squalls. I stuck my hand down into the water to Braille the speed and created a roostertail that soaked the skipper. When we caught a squall the cat accelerated like a motorbike and the apparent wind shifted noticeably forward. The damn skipper kept yelling "pull harder, damn it!"

 

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.

Comment to gggGuest:

At 180 degrees when getting to wind speed, the apparent wind speed is 0. Thus I cannot see how 0 wind can move any wind turbine, and steady state or some oscillations occur just before getting to 0. Looks to me that some auxiliary power source is needed here to pass wind speed.

 

Thanks people, I enjoyed the read, and appreciate the contributions/learning experience provided by you all....

 

Consider this simplified case: the boat is sailing at 10 k NE; the wind is 14 k toward the North. The boat has a windward speed equal to the windspeed and can keep up with the wind by tacking.

The boat's speed creates an apparent wind component on deck of 10 k from the bow. The wind creates an apparent wind component on the deck of 14 K off the starboard bow. Together the two components form an apparent wind on deck of 22 k fine off the starboard bow. The sail can be set to use this to keep the boat moving.

Obviously if the boat is turned directly downwind it will slow down. equally obviously the boat can be trimmed to move downwind faster than the wind when tacking if it is efficient enough.

You should draw a vector diagram to illustrate this.