Wow...

[Davidasailor26]

at 28 seconds into the video he states there is no wind at all. that means no true wind. all other winds are apparent. i stand by my posts

Well, the water is moving relative to the land, right? So the air must be moving relative to one of them. I suppose that if the air over the land were stationary relative to the land, and the air over the water were moving exactly with the water, then you're right, none of it works.

 

[Davidasailor26]

By the way, there is one difference between the treadmill video Will found and the boat - The car on the treadmill has spinning blades. Those blades, as they spin, generate a sideways component of apparent wind over each blade which is converted to lift to pull the car forward. A boat doesn't have moving parts like the spinning blade, which is why it can't sail dead downwind. It must rely on sailing at an angle to the wind, a broad reach, to get that same sideways airflow that's converted to lift.

 

[Will Gilmore]

The car is moving dead down wind where a sailboat can't, but the individual blades of the propeller are not moving dead down wind. They are moving in an apparent wind of their own because of their perpendicular rotation. There is no wind at all until they start moving due to the initial inertia of the car as the treadmill rolls against the wheels underneath. This allows the propellers a moment to overcome the force of a tailwind generated by the movement backwards on the boat. The original and natural disposition of this system is a down wind scenario because the treadmill is attempting to pull the car in reverse.
If the wheels aren't driving the propeller, the propeller would turn in the reverse direction and it would fight against the wind. Instead, the car's inertia holds the car still long enough to allow the wheels to become the driver for the propeller. This changes the apparent wind on the blades that are moving perpendicular to the car's allowed direction. The pitch of the blades, as they spin, means they are actually sails trimmed to a close reach. They experience acceleration from that condition exactly the way a reaching high performance sail powered vessel does. Once they have achieved this condition, they take over as the driver of the wheels.
The movement of the car itself illustrates the DDW component of those forces at work.
As long as there is a difference in movement between two mediums, the gound/water and air, that energy can be added to the building energy of a downwind sailing system. Without any friction or outside resistance at all, whatever energy the vessel gathers and uses to move forward can be added to by that difference. Mathematically, there is no limit except that imposed by the pinching angle (iceboats can sail as close as 8 degrees to apparent wind).
If such a frictionless system were obtainable, the speed of the vessel and all it's component vectors, including the ddw component, is nearly unlimited (I think it would actually have a limit that it approaches, but like Zeno's arrow, it never gets there).
The real error in the video originally posted is in their simplistic illustration of Artemis turning from DUW to DDW. Neather of those orientations for the vessel are possible.
The question on the table isn't whether Artemis can do this, but whether ANY sail powered vessel can do this. The answer is yes.

-Will (Dragonfly)

 

[walt]

at 28 seconds into the video he states there is no wind at all. that means no true wind. all other winds are apparent. i stand by my posts

What exactly is it that you are standing by?

AC boats can make VMG downwind greater than the true wind. You need to understand polar plots to look at that actual data presented a few post back but. it all there. I dont think anywhere in the OP video did anyone say they were sailing straight down wind so Im not sure what the argument is. Remember that the boat had to sail downwind (wind created by the current) in order to sail back up the current flow.

However..
Some of you remember this from a few years back where some folks did prove that you can sail faster than the wind dead down wind.

 

[Cowpokee]

I won’t try to argue with someone who thinks that sailing upstream in zero wind is impossible.
For those that would like to understand how it could be possible, here are a few stumbling blocks to get past:

Terminology:
True wind is what is seen while stationary on the earth
Apparent wind is what is seen aboard the sailboat
Perceived wind or relative wind is the key to making this possible

Concepts:
True wind in this scenario is defined as zero
Perceived wind (caused by the current relative to true wind) is the motive force causing the apparent wind on the sailboat.

Common Knowledge:
No boat can sail directly up wind, or in this scenario directly up stream, we all know that. The boat has to be moving at some angle to the current. (which is 180 - the angle of the perceived wind)

Facts:
The boat has to be able to sail faster downwind than the wind is blowing. None of my sailboats can do this, (neither can most of yours) Don’t try to imagine yourself on your boat out there trying to attempt this. IT WON’T WORK!

 

[Scott T-Bird]

"sickness will surely take the mind where minds can't usually go,
come on the amazing journey and learn all you should know.

A vague haze of delirium creeps up on me.
All at once a tall stranger I suddenly see."

I have a nasty cold and I was just lying in bed, when I think @Davidasailor26 has a new convert! I must be on my death bed So it finally dawned on me that current induce "wind" actually is no different than a real wind. I had no problem with the concept that VMG exceeds True wind speed. I think somebody explained it once that it is like squeezing a pumpkin seed until it squirts out of your fingers. When resistance is significantly reduced relative to the forces in the opposing vectors, the speed is significantly increased. I'm not that bright so I think that I would still need to see it to believe it. But I'm pretty sure that if you could put an AC boat in a current of say 15 Knots, in no actual wind, assuming turbulence doesn't have a negative affect on reducing resistance when the boat needs to rise on the foils, I'm thinking that you could actually see about 20 knots of progress against the current with the boat sailing at about 30 degrees off the current! I doubt we could currently actually produce this experiment in a 10 knot current and finding a stable 15 knot current would probably be problematic.

I just skimmed thru a few posts since yesterday and I saw a number of interesting comments, but I can't get back to it now. I'm going to take more Mucinex and get back in bed.

BTW, I'm pretty sure that @Simon Sexton doesn't have the foggiest idea about the reference above without google!

 

[jon hansen]

GET WELL SOON

 

[Davidasailor26]

Wow, @Scott T-Bird, I think you've got it. It certainly is mind-bending, but you've got the two key ingredients -

concept that VMG exceeds True wind speed.

And

current induce "wind" actually is no different than a real wind.

That's really what it boils down to.

Now go feel better!

P.S. I know some Who songs, but had to Google that one myself.

 

[jon hansen]

i've never heard of "real wind" . 59 years of sailing and that means nothing to me. enlighten me. always willing to learn something new

 

[walt]

Facts:
The boat has to be able to sail faster downwind than the wind is blowing. None of my sailboats can do this, (neither can most of yours) Don’t try to imagine yourself on your boat out there trying to attempt this. IT WON’T WORK!

Well. maybe not exactly what I would call a fact. You have to have a vessel with very low D and sufficient L to do this which from the actual data in the links at the bottom, the AC boats do meet this.

I took some snap shots from those links which show the concept.. In the polar plot below (which is real data), I have highlighted that the VMG for race 6 (in this case for down wind) is 25 knots for down wind angles of a little over 150 degrees and somewhat beond that. The second plot shows the wind during race 6 at around 11 knots.

That seems pretty clear to me that you can make VMG downwind faster than the wind is blowing. Ive also experienced this myself ice boat and landsailing. But.. correct, our sailboats here will not do this.

http://www.cupinfo.com/cupstats/index-ac34-statistics-polar-plots-02.php#SOG
http://www.cupinfo.com/cupstats/americas-cup-2013-stats-wind-distribution-01.php#race-6

 

[Scott T-Bird]

Freshman year high school album … still in good shape. My mother confiscated the lyrics insert but didn't confiscate the album, subject matter was a little controversial in those days! Never got them back!

 

[Scott T-Bird]

i've never heard of "real wind" . 59 years of sailing and that means nothing to me. enlighten me. always willing to learn something new

Surprisingly, it took me some time to wrap my head around it. We think of True wind as the direction and strength of the wind, obviously. Apparent wind is what we feel. When we're drifting downstream in still air, the apparent wind that we feel is the same as True wind. When we turn around and advance against the current, the wind that I interpreted as true wind diminishing at our back until it becomes still when our boat speed matches the current (10 knots) is no different than sailing on water with no current with a 10 knot wind. When we sail downwind with VMG = 10 knots, the air around us is still, even though real wind is blowing 10 Knots. I was interpreting True wind diminishing to zero, when in reality apparent wind has picked up speed and shifts ahead of us. It is valid to consider the air relative to current as "true wind" but not really important in the final analysis. Apparent wind is the vector that matters.

 

[dziedzicmj]

We aren't taught lateral thinking.

generally, we aren't taught physics (or vector arithmetic)

 

[dziedzicmj]

Not overcome - just be redirected to move a piece of material in the other direction. Saying "the energy from the current can't overcome the current" is like saying "you can't sail up-wind, the energy from the wind can't overcome the wind".

Or saying that you cannot sail faster than wind ("the energy from the wind cannot overcome the wind"). But we know that you can.

 

[Will Gilmore]

generally, we aren't taught physics (or vector arithmetic)

If you took geometry or algebra two in high school, you had vector arithmetic. Physics is an elective that is currently being pushed by the STEM/STEAM program. On this site, there are a surprising number of engineers. My degrees are in computer science. University Physics was part of that program because of the kind of problem solving skills it taught. However, it still didn't teach lateral thinking.

The foundation for thinking about the problem at hand is all right here in this thread. Maybe there is something new out there that hasn't been brought to the discussion. I'm hoping that someone does introduce it.
I initially thought the video was making an absurd assertion. I didn't even think it was worth commenting on the absurdity except to post a laughing emoji. Within this thread, however, I have become convinced it is not. There are very few experiences I find more stimulating than when I have my paradigm shift so completely. How wonderful to have your mind peeled open so that it can expand. Right or wrong, my mind has expanded because of this thread.
Thank you, everyone who has participated in that. I owe you a debt of gratitude.

-Will (Dragonfly)

 

[Davidasailor26]

How wonderful to have your mind peeled open so that it can expand. Right or wrong, my mind has expanded because of this thread.

I agree completely. Although I didn't change my mind through the course of the thread, it has absolutely made me think about sailing and the physics of fluid dynamics in new ways. The feeling of satisfaction from having those "eureka" moments, like how all of these analogous models connect, is very rewarding.

Thank you, everyone who has participated in that. I owe you a debt of gratitude.

And thanks to you Will, as well as Scott, Dave, Jon, and all the others who have helped keep the thoughts flowing in a civil and constructive way!

Edit - And sorry, I forgot to thank @Simon Sexton for starting the thread in the first place!

 

[Jackdaw]

OMG no way I reading all of this. So sorry if I re-say something already said.

They often describe these AC boat as being able to sail 3x the windspeed. That's a bit of a simplification. To start, the boat can do this because of greatly reduced hydrodynamic drag. Like an iceboat, it sails above the water. But unlike an iceboat, the mechanism that holds it out of the water (foils vs runners) also generate LIFT. Lift both vertically (lifting up) and critically as a forward vector. So its not just about lift and speed created by the wind/sail, but the wind and water speeds combined.

This is why you cannot just tow this boat on a still lake to 10 knots and have it tack around forever. No such thing as a free lunch. The drag would eventually get you. It needs the true water speed (TWS!) to generate just enough hydrodynamic lift to augment the aerodynamic lift created by the wing to maintain a constant speed. Moving water is energy. You just need to know how to harness it.

 

[jon hansen]

the OP's video was dispelled on page one. however the entire thread was politely fun and that 'Alberta clipper' had many of us huddling inside near the fire typing away and smiling

 

[Will Gilmore]

Foils are for everyone now.

-Will (Dragonfly)

 

[Jackdaw]

the OP's video was dispelled on page one. however the entire thread was politely fun and that 'Alberta clipper' had many of us huddling inside near the fire typing away and smiling

Jon I disagree. There is no true wind speed (TWS), but there is true water speed (TWS). This boat is efficient enough that it can use that as energy. They didn’t choose the Amazon river by accident. It flows with a 10 knot current.. Anything less would not sustain the up current boat speed.

You really can't think of these things as boats. They have more in common with a glider than a displacement keelboat.

I reread the entire first page and see nothing that refutes this.