Once again, reports are out about a vehicle powered solely by the wind that can outrun the wind. And once again, we expect continued debates on the subject.

Rick Cavallaro and his merry band of enthusiasts recently improved the upwind and downwind speeds of their Blackbird, their DDWFTTW (directly downwind, faster than the wind) vehicle, according to Wired.

Using New Jerusalem Airport in Tracy, CA as the testing ground, the vehicle hit an upwind velocity of 2.01 times wind speed, which is counterintuitive until you consider the vehicle's design of having two large turbine blades catching that wind and propelling the wheels. Better yet, Wired reported that the Blackbird reached downwind speeds of 2.86 times wind velocity. Going downwind, the process is reversed, in that the rolling wheels spin the propeller, which further drive the vehicle.

Cavallaro and his cohorts have been at this for at least a couple of years, claiming as far back as June 2010 that the Blackbird's downwind speed could hit three times wind velocity. They're getting close to that mark. Remember, this isn't a direct sail thing, and no perpetual motion is being claimed. Still, there are plenty who doubt the veracity of the DDWFTTW claims. Discuss.


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    • 1 Second Ago
  • 111 Comments
      Joeviocoe
      • 2 Years Ago
      Okay, fine. No, this is not exactly "Intuitive". But if you think of it this way, maybe some people will get it. Regarding conservation of energy: Velocity does NOT equal energy! It is all about mass times velocity (or momentum). And momentum is conserved. Notice how the sweep area of the wind turbine blades is MUCH larger than the area of drag fro the vehicle itself. (also the blades are shaped so that its own drag is very low). Regardless of how fast the wind is moving, the sweep area converts that power into forward motion of the wheels.... so the only thing that needs to be overcome, is the much smaller drag forces of the vehicle. That 17-foot-tall propeller (A = pi x 8.5^2) = 227 square feet or 21 square meters of swept area. (I love google's new converter) Power of wind = 0.5 x Swept Area x Air Density x Velocity^3 http://www.reuk.co.uk/Calculation-of-Wind-Power.htm (air density given as 1.23) With a wind speed of about 6.2 m/s or 14 mph... Power of wind = 0.5 x 6.2 x 1.23 x 2744 Power of wind = 10,463 Watts or 10 KW http://www.reuk.co.uk/Betz-Limit.htm With the Betz limit (assuming efficient blades)... let's say only 50% of that wind power can be captured and sent to the wheels. 5 KW of power to the wheels at a 14 mph head wind. http://www.fasterthanthewind.org/ (they mention a 13.5 mph wind and 2.85 times faster) ♦ Now, the vehicle travels at 2.86 times as fast... so that is about 40 mph! ♦ So if the vehicle tops out at a ground speed of 40 mph and a wind speed of 14 mph... and my calculations estimate 5 KW of power captured from the wind and put toward the wheels... then the assertion should be that there is about 5 KW drag on the whole vehicle at 40 mph. According to: http://ecomodder.com/forum/tool-aero-rolling-resistance.php this is a very reasonable assertion. ------------------------------------------ Bottom line... it is not intuitive. But it can be. If you consider how wind turbines extract power. As long as the sweep area is MUCH larger than the drag area... you will get more power out than you put in. Not perpetual motion either, since there must be actual wind energy relative to the stationary ground to extract from. But still, not a practical vehicle for road use.
        • 2 Years Ago
        @Joeviocoe
        When you say; "As long as the sweep area is MUCH larger than the drag area... you will get more power out than you put in", that is just totally wrong. You can NEVER get more power out than you put in! Otherwise, please explain where the additional power comes from? You also seem to be mixing up the upwind and downwind cases. Please be more clear about which case you are talking about. Finally, your statement here: "Notice how the sweep area of the wind turbine blades is MUCH larger than the area of drag fro the vehicle itself. (also the blades are shaped so that its own drag is very low). Regardless of how fast the wind is moving, the sweep area converts that power into forward motion of the wheels.... so the only thing that needs to be overcome, is the much smaller drag forces of the vehicle." Is not even wrong, it is just nonsense. The drag of the rotor is much greater than the drag of the rest of the vehicle. You really don't know what you are talking about.
          Joeviocoe
          • 1 Day Ago
          "You can NEVER get more power out than you put in! Otherwise, please explain where the additional power comes from?" The Wind power obviously. It is not a closed system. "You also seem to be mixing up the upwind and downwind cases. Please be more clear about which case you are talking about." Yes, sorry, but I was not clear... I was speaking of a constant velocity case at approximately max speed. Real wind is a tail wind, but the apparent wind is a head wind in this case. "Regardless of how fast the wind is moving, the sweep area converts that power into forward motion of the wheels.... so the only thing that needs to be overcome, is the much smaller drag forces of the vehicle." Not exactly true, I've built several wind turbines. There is the drag of the vehicle, but also, the the blades and hub are not transparent to the air. Whatever air is not either converted to rotary motion or allowed to pass through is considered drag. Even efficient blades have drag, both in the vector parallel to the wind, and in its own rotational plane. It is not nonsense, just difficult for non-engineers to understand sometimes. So when I say the 'drag area' in includes the vehicle plus the sum of the drag for the turbine assembly too with blades. --------- What I wrote is not the complete story, but a simplification to help make intuitive. So you can call the parts you still don't understand, as 'nonsense' if you like. But I do know what I am talking about. What are your calculations? Do you claim that the vehicle does NOT work as shown?
      Letstakeawalk
      • 2 Years Ago
      Congrats to the DDWFTTW Team! This is sure to be another epic thread as commentators try to prove why they theoretically shouldn't be able to do this - even though it works on well-known principles.
        Joeviocoe
        • 2 Years Ago
        @Letstakeawalk
        Perhaps not so epic... most here should know by now how it works. Unless this article gets reposted on Autoblog regular.
        Joeviocoe
        • 2 Years Ago
        @Letstakeawalk
        Perhaps not so epic... most here should know by now how it works. Unless this article gets reposted on Autoblog regular.
      Joeviocoe
      • 2 Years Ago
      No, don't discuss! Nothing spectacular here. The principles are well known to all sailors and aircraft designers.
        • 2 Years Ago
        @Joeviocoe
        I assume you are refering t
          Joeviocoe
          • 1 Day Ago
          Not really referring tacking. Although tacking does use some of the principles of generating faster than wind motion. More referring to the general concept that wind velocity is not really a direct correlation to vehicle speed, since the wind is merely a vector force and can be manipulated from there to produce thrust that is independent of wind velocity.
          • 1 Day Ago
          I assume you are referring to tacking which is not what this vehicle does.
      • 2 Years Ago
      Joe, What makes you mistakenly believe, I wanted people to think, that this demonstration is mine? But, nevermind Joe. We already went through recently (pointlessly) a Tolstoy's War and Peace length discussion, where you sufficiently demonstrated, that arguing with you simply makes no sense (as well as, no reason in doing it). #justgotbored
      Joeviocoe
      • 2 Years Ago
      You are calling me " emotion driven "... yet you have a history of resorting to name calling and vulgarity. Sure, I am annoyingly persistent in not letting you get away with gross errors or plagiarism. But I don't call you names and curse at you. That would be immature, and is the common resort of someone who cannot continue the debate because of a lack of reasonable argument.
      • 2 Years Ago
      Rick Cavallaro : "Perhaps it should be mentioned that "theengineeroverthere" has been on the attack since well before we built and tested the Blackbird. He was quite certain it could never do as we claimed." That explains a lot. It was quite obvious from his first posts that he doesn't understand basics physics, which prevents him to understand how this works. Apparently "theengineeroverthere" just cannot admit being wrong about something, and develops paranoid conspiracy theories to explain reality away.
        • 1 Day Ago
        Well before the trial, actually, because the "physics" is nonsense, and many including Dr's Allain, Whiteman, Camp and Kammen agree. 1) The small cart, which was also said to be proof, never once achieved >WS. The video produced as evidence, shows the cart starting with the rotor turning the wrong way, then slides as the wheels lock, until finally being blown along at less than 1/3 of the speed of a passing leaf. 2) Absent from Cavallaro's reply, is specific denial that he had access to the data, selected the record window, performed all calculations, prepared the chart of the official NALSA report, calibrated the instruments and data logger, and the means of testing was changed on the day of the trial to accommodate winds that were so variable, that there was no possibility of doing anything else. The trial is as reliable as the word of the competitor.
      BipDBo
      • 2 Years Ago
      I'm an engineer and an avid sailor, and it took me a while to figure it out in my head. I assure you though, it's a sound concept. From a conservation of energy standpoint it made sense, and it seemed similar to the refrigeration cycle. I just had a hard time at first, figuring out all of the force vectors in my head. I windsurf and kiteboard, which gets me going faster than the wind. What held me up a first was that when windsurfing, you can't go straight downwind and go very fast, certainly not faster than the wind. You need to go across the wind to go fast. The I realized, that since the sails on this thing rotate, they are moving across the wind, even if the vehicle is traveling straight downwind.
        lne937s
        • 1 Day Ago
        @BipDBo
        Go one step further. As the propeller is attached to the car, moving in the direction of the wind, if the vehicle hits the same velocity as the wind speed, then the wind is not putting any force on the propeller. When going in the sames direction, the speed wind (and force it places on the blades) is relative to the speed of the car. When they reach the same speed, the relative wind speed is 0. The propeller can spin faster than the wind, but when the car reaches the same speed as the wind, there is no force to spin the propeller.
          Joeviocoe
          • 1 Day Ago
          @lne937s
          yes, but if the acceleration as it crosses this threshold is great enough... then it should be able to reach a speed slightly greater than the tail wind... and thus have an apparent head wind. Which will cause the blades to spin again and produce wheel thrust. I believe the blades and gearing are designed to spin in one direction regardless of whether a tail or head wind. The same is true for tacking sail boats... as BipDBo (or any of us who sail) can tell you... you need to change direction quickly enough so that momentum carries the boat through the angles that are too close to directly facing into the wind. *in irons*.... and into a proper tack on the other side. The acceleration of the vehicle must carry it through this period as well, enough speed must be attained to achieve a good apparent head wind. And as long as the drag is low, just look at this car.... it works. See my calculations below to show how drag is still less than the forward power achieved by such a large sweep area compared to a low coefficient of drag.
        Rotation
        • 1 Day Ago
        @BipDBo
        No, that's not it at all. Watch the video below http://www.youtube.com/watch?v=k-trDF8Yldc That vehicle goes downwind faster than the wind and doesn't have any cross-vector component. The key is that the element (in this case sail) receiving the input force (wind) is not moving downwind faster than the wind, even though the vehicle is. So you could make a cart where it had wind catching sails that are on top of the vehicle but are geared to move rearward as the vehicle moves forward. Their net movement rate would be less than the wind speed in the direction of the speed, but the force exerted by the wind is still captured and geared to push the vehicle forward. Mechanically it would be difficult, but it should be possible, at least before efficiencies are accounted for. It isn't about moving crosswind, the key is just that the system is designed such that as the vehicle approaches the speed of the wind, the apparent force (wind speed minus vehicle speed) doesn't go to 0.
          BipDBo
          • 1 Day Ago
          @Rotation
          My point was that the blades have a cross vector because they are spinning, even when the vehicle is traveling at the same direction as the wind. The cross vector cause by the rotation, which is forced by the gearing to the wheels, is what gives the blades apparent wind speed, even at the point where the velocity of the vehicle matches that of the wind. To put it into a newtonian or energy conservation perspective, wind that is moving has kinetic energy that came ultimately from the sun. When the wind hits the blades of the proeller, the wind slows down, loosing some of its kinetic energy. With every action, there is an opposite reaction, so the kinetic energy lost by the wind, is transferred to propelling the vehicle forward.
          • 1 Day Ago
          @Rotation
          > "I wonder if there would be any way do to it without that, i.e. to make a hovercraft go downwind faster than the wind." Perhaps with a strong wind gradient. But you have to lever one medium against another. It can be wind against the ground or water, or wind against the wind (through a gradient).
          Rotation
          • 1 Day Ago
          @Rotation
          BipDBo: Yes, I understand your point. But you seem to have missed mine. The sails don't have to have a cross-vector component (be moving crosswind). They do in this case, but it isn't strictly necessary. The vehicle does have to "generate its own wind" so that the sails don't luff as the vehicle reaches windspeed. But there's no reason it has to move the sails crosswind to do it. It does in this case, because that's mechanically simplest. But it isn't necessary. The reason why this matters is because crosswind sailing has the force of some keel (skeg, etc.) which works to convert the directional force of the wind into the direction you want to go. This doesn't have that, it requires no lateral movement resistance. It does require grippy tires though to drive the transmission. I wonder if there would be any way do to it without that, i.e. to make a hovercraft go downwind faster than the wind. I admit I can't think of a way.
      • 2 Years Ago
      NALSA REGULATIONS FOR RECORD ATTEMPTS Revision 3 July 2010...The second day of the trial. The original claim which violates the Conservation of Energy, was ddwftw, steady state. That was reflected in the claim that the cart would travel ddwfftw in a wind tunnel. At no time were critics informed that the trial would allow the cart to run as long as it pleased, not ddw, and steady state would be replaced with transient acceleration in variable winds. The winds often change by up to 150 degrees in less than three minutes. With that energy available, the C of E is not a problem, and many carts can reproduce the same result, if not better it. What the rules say, and what was done, are not the same thing. The vane has a NIST certificate, and that allow 4 degrees of error, as is. A certification, is not calibration. Acceleration is determined by badly employed consumer hand-held GPS devices, having no traceability. The rules may perhaps be suitable for a casual speed record, but not for supporting a claim that contradicts established physical laws, assuredly promoted to the point where the competitors refer to some academics and Nobel Laureates as idiots. From the general rules: ADDITIONAL INFORMATION AND COMMENTARY "No specific rules or techniques have been adopted by NALSA for the setup or calibration of speed measuring devices. It shall be the contestants' responsibility to convince the NALSA Board of Directors that the measurements are scientifically valid" Convince, as in persuade, or provide evidence of the instruments validity? That latter is definitely missing. (Perhaps you can explain the cart's magical inverse relation to wind speed. The slower the wind, the faster the cart..) A point that both you and the competitors are avoiding, is that the competitor handled and processed the data as he pleased. That is not acceptable, even if NALSA think so, or sometimes do. It is only the dubious acceleration that separates this cart, having a rotor area of 20m^2, from a sail cart. The means of determining that acceleration was not verified, and is beyond the GT31's capacity in any case. As for data handling: From Greenbird: The Regulations require an evaluation of the risk of deception in the measurement process.... I was the first person to have access to the data files on these units from the 3/26 runs. It is theoretically possible to create a fake GPS log file and place them on the GPS data card surreptitiously.
      • 2 Years Ago
      >with respect to the comments of theengineeroverthere Actually, the post below contains factual errors. NALSA formulated the rules under which the cart ran. They also used independently placed measurement instruments in addition to the instruments that the contestant carried. Verification and calibration of the instruments was assured validated, and cross checked. The "magical" cart does indeed seem a bit "magic" given what it achieved. But upon closer study, those who understand basic physics understand that the only "magic" was the everyday "magic of standard physics" which can produce some counter intuitive results when exploited properly. What made the Downwind accomplishment so surprising is that most people (including theengineeroverthere) have the mistaken belief that when the cart is at windspeed, that no more energy exists to push it past this point. This is false, the energy of the wind exists as long as it is blowing. The cart exploits a very clever mechanism to accomplish in a rotary fashion with its propeller (in the downwind direction) what sailors have been doing for ages (just not directly downwind). The cleverness comes from the idea that the propeller "sails" (blades) can be forced to run on a continuous circular "tacking" path that allows the blade itself to always be moving with respect to the wind so there is never a point where the blades are actually at windspeed. This clever trick was to gear the cart such that the rolling of the wheels over the ground maintained this relationship of the prop blades to the wind at all times. Then, the propeller could exploit the "power of the wind" even when the cart is experiencing zero wind on the cart's body! The poster below cannot understand that the energy in the wind does not disappear when the cart is at windspeed for the propeller. The cart works a bit like a rear-end differential in reverse, it doesn't care if the cart is moving over the ground or the air is moving over the prop, as long as the air is moving with respect to the ground which is known as wind! This unique method of harvesting the wind energy in spite of the speed of the cart with respect to the air is what is so very clever! Congratulations to Cavallaro's team for this very wonderful accomplishment!
        • 1 Day Ago
        1) The rules were written in conjunction with the competitors, but were never published. However, Cavallaro is on record as saying that after the first day, when the winds were found not to be suitable, NALSA allowed the cart to maintain a fixed heading, and not to follow the wind. The rules are dated to reflect the changes made on that second day. 2) The idea that acceleration could be used as a discriminator, came from Cavallaro's claim that the cart is "best ddw". That claim has no theoretical support, and NALSA are not physicists, but amateur sailors, who are not qualified to accept without recourse to a suitably qualified authority, any claims the contestant may make. 2) The Metone vane is a heavy single-axis unit, intended for fixed general use, and not mobile applications, nor capable of measuring the vertical winds that may also drive the cart, as well as the lateral. The competitors claimed to have " previously identified a roughly 1 mph favorable bias in wind speed measured at the front of the craft, when the vane is on the cart", but it there is no evidence to support that claim, or how they could have determined that without the use of even more sophisticated instruments....Anyway, that forced the vane onto the chase vehicle. A consequence, is the there are GPS speed units on the cart, while the other instruments, and a second GPS unit, are on the CV. The error margin produced by that lack of cohesion of cart/CV speeds, and the other instruments, is enough of itself to dismiss the claimed acceleration readings. There are good reason why medical trials are double-blinded; not only to remove the possibility of fraud, but personal bias, intended or not. The cart has 3GPS units to choose from, and there is one on the CV....which is correct? Which data will the competitor, who has loudly proclaimed success to all and sundry, or admit failure, choose, to determine that all-important acceleration? If is a fact, that the competitors were directly involved in the calibration of the instruments, data logging. The data was processed by Cavallaro, to produce the chart published in the NALSA report. It is not of NASLA' s hand. It is also true, that the cart followed a fixed heading, as the wind changed about it. The only metric that may identify this cart from a sail cart capable of providing the same result, is the claim of acceleration; and that is not only technically dubious, but in the hands of the competitor. 3) It is claimed that the cart extracts energy from the wind's speed relative to the ground, and because that is always present, the cart can extract energy regardless of its speed relative to the wind. That is nonsense, but excludes the claim that the cart can accelerate when the wind speed relative to the ground falls, because, of course, that represents a fall in the avialable energy source.
        • 1 Day Ago
        > "Actually, the post below contains factual errors. NALSA formulated the rules under which the cart ran..." Thank you. As you know, I have LONG ago given up on explaining to "theengineeroverthere" what has become clear and even obvious to most people. Like the moon landings, there will always be a few dedicated deniers. Unlike the moon landing, this can be easily repeated by anyone that chooses to build their own small working model from my detailed build videos.
          • 1 Day Ago
          There is a post, where you made the claim that on the second day, the cart could follow a fixed heading. Determined that way or not, the result is a cart that starts off wind and remains that way, until by chance, the wind and cart direction coincide. Only the dubious claim, of improved acceleration when ddw, separates this cart, from any other sail cart. It is contrary to your own claims, for the cart to accelerate when the wind speed falls. That not withstanding, you were involved in the calibration of the instruments, acquisition of the data, and all calculations performed on that data. The ten-second record window was chosen by you, from that data, after the trials were completed. It that correct or not? The only instruments capable of supplying the cart's acceleration, are the 3 Lycos GT31's fitted to the cart, and only Doppler can provide the necessary resolution. However, the 3 units are spaced apart, and will not receive identical carrier signals, while even 1cm of vibration will produce a 5% change in the 20cm carrier. The GT31 employs FFT analysis. How much data is in the internal memory buffer, before it is output to be read? Unless you can prove that is insignificant, (and it isn't) the data used to derive the acceleration comes from the past. NALSA are responsible for the record, and it is obviously totally inadequate and impractical, for them to rely upon the competitor's technical ability.
          • 1 Day Ago
          I find it especially disingenuous to say that the rules were never published. A trivial look at the NALSA website reveals this link which is the rules that govern the events including DDW record events: http://www.nalsa.org/BlackBirdDDWSR/NALSA%20REGULATIONS%20FOR%20SPEED%20RECORD%20ATTEMPTS%20%20Rev%203.pdf Some of the rules that are a direct contradiction of what is stated by theengineeroverthere from this document are: The primary speed measurement method must have a measurement uncertainty of 0.25 mph (0.16 kph) or less. In addition to this primary measurement, at least two secondary measurements shall be made by separate systems. All measurement systems will be calibrated. A Measurement Plan that addresses the measurement uncertainty of all measurement methods as well as the identification and elimination of spurious data will be given to the observers prior to attempting to establish a record. and: ADDITIONAL RULES FOR ‘DEAD DOWN WIND FASTER THAN THE WIND’ CRAFT Overview: The following are additional rules for craft that are designed for dead downwind faster than the wind (DDW) record attempts. A DDW record will consist of the ratio of craft speed to true wind speed over a 10 second measurement period. DDW 10) On the in-run the pilot will steer the craft as close as possible to dead down wind. During the 10 second measurement period, the craft’s travel direction, averaged over 10 seconds, will stay within 10 degrees of the 10 second average down wind direction DDW 11) The craft speed and ratio of craft speed to wind speed will be calculated over the 10 second measurement period. A competitor, after reviewing the run data, may choose which 10 second period to use as the measurement period. The craft speed must be at least 0.2 mph faster in the tenth second of the measurement period than in the first second. DDW 12) The primary wind speed measurement device must have a measurement uncertainty of 3% or less at the wind speed the craft attempts to set a record in. Wind speed will be recorded at 1/2 hz or faster with no averaging by the instrument electronics beyond two seconds. All primary wind speed measurements will be made between propeller hub height and 5 feet above hub height (or the hub height equivalent for non propeller designs). Primary wind direction measurements will have an uncertainty of 4 degrees or less. DDW 13) At least one off board, wind direction and wind velocity instrument that meets the primary instrument requirements must be within 200 feet of the position of the craft during some part of the measurement period. DDW 14) All recorded primary instrument data will be time stamped within 1 second of UTC to allow correlating measurements. DDW 15) To be a new record, the reported value of ratio of craft speed to wind speed must be at least 6% higher than the previous record. These certainly are a comprehensive set of rules and disprove what is stated by theengineer.
      • 2 Years Ago
      Perhaps it should be mentioned that "theengineeroverthere" has been on the attack since well before we built and tested the Blackbird. He was quite certain it could never do as we claimed. He was similarly certain that NALSA would see us for the fools we are, and would never let us operate at one of their events. When I suggested a conversation by Skype, "theengineeroverthere" declined, admitting to me that he had a condition known as alexia - http://en.wikipedia.org/wiki/Alexia_(condition) He has continued on this tear for several years now, having made nearly 24000 posts on the current forum. His objective has been to discredit the Blackbird team and have the records rescinded. It hasn't worked out to his liking. Instead he has proven to be a source of endless entertainment to the folks that come there just to see what he'll say next.
      2 Wheeled Menace
      • 2 Years Ago
      OK.. i get it.. but the unanswered question is... why? Are you trying to make the most efficient/best wind turbine?
        • 1 Day Ago
        @2 Wheeled Menace
        > " why?" To settle an internet pi$$ing match.
        • 1 Day Ago
        @2 Wheeled Menace
        2WM It's kinda like the same thing as in the history people stubbornly tried to create heavier than air flying machines, despite most scientist warned them: it's impossible and simply futile. Without those people we would still be flying on hot air balloons (or something like these). This thing is an interesting scientific (and maybe practicable) puzzle, which must be cracked even just for the sake of cracking it. #thatswhy
      • 1 Day Ago
      Initially, the whole vehicle is simply pushed by the wind as a bluff-body. Obviously this causes the wheels to turn. This in turn causes the prop to rotate, which produces thrust. The reason the thrust can be greater than the drag caused by the wheels is sort of indirectly due to the wind. The propeller is operating in one medium (the air) while the wheels are operating on another medium (the ground). Because of the tailwind, the cart is moving more slowly through the air than it is over the ground. Just like any lever, we can trade a small force over a long distance for a greater force over a smaller distance. "He says the toughest part was designing a transmission system to transfer power from the wheels to the propeller." I don't know where that came from. I don't recall speaking with the author. I suspect it's related to something I've said or written, but I don't really know what to make of it. " Answering this question comfortingly would give you way less headache too, due to the tidal wave of skepticism." I'm happy to answer all questions to the best of my ability. But I don't think a tidal wave of skepticism remains. Perhaps a tidal wave of curiosity. The fact that we've demonstrated it to disinterested critical observers under controlled conditions, documented it, and posted detailed build plans so people can make their own working models seems to have convinced most would be skeptics. " is it possible actually [since the wind on the available videos are clearly changing in direction as the weather vane shows it], that it was simply a well known tacking, as usual in common sailing?)" No. The principle of operation is clearly understood. It works best when going directly upwind or directly downwind .
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