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Can You go Faster than Downwind Speed Directly Downwind

05/29/2021 10:39 PM

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#1

Re: Can You go Faster than Downwind Speed Directly Downwind

05/30/2021 7:10 AM

Yes. I think it has to do with wind gradient. The wind blows faster higher up from the ground.

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#2
In reply to #1

Re: Can You go Faster than Downwind Speed Directly Downwind

05/30/2021 9:24 AM

OK, that may be part of it, but it's really a gear ratio thing. It's like a bicycle can go faster than you push on the pedal.

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#3
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Re: Can You go Faster than Downwind Speed Directly Downwind

05/30/2021 2:08 PM

I think it works by having the propeller directly geared to the rear wheels, the prop acts as a sail, and when it reaches wind speed, the pitch on the prop can be adjusted, as the pitch is adjusted to blow air backwards, this increases the push forward artificially increasing the wind speed and increases the vehicle speed, as the speed increases this drives the wheels faster, so theoretically your speed is only limited by drag...

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#4
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Re: Can You go Faster than Downwind Speed Directly Downwind

05/30/2021 4:00 PM

That's a good explanation. Instead of the wind blowing against a sail stationary to the cart, it's blowing against the moving blades, adding to the speed that the cart already has.

I suspect you could make a boat version, with a catamaran hull for stability and low drag. The propeller in the air would be coupled to the propeller in the water. However, it probably would not be as efficient as the wheeled version.

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#5
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Re: Can You go Faster than Downwind Speed Directly Downwind

05/30/2021 5:54 PM

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#10
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Re: Can You go Faster than Downwind Speed Directly Downwind

05/31/2021 4:02 AM

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#6

Re: Can You go Faster than Downwind Speed Directly Downwind

05/30/2021 6:21 PM

Haven't watched this all yet, but it minds me of one of the Challenge Questions - a sailboat is in a wide river and there's no wind. How should sails be set to get best speed. The given answer was that the boat can go faster by tacking against the airflow caused by the river pushing the boat along. If I can unscramble my brain from being reminded of that I'll try understand this.

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#7
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Re: Can You go Faster than Downwind Speed Directly Downwind

05/30/2021 7:04 PM

If the boat isn't moving in the water you can't steer it....If there is no wind you need to fire up the motor, or get out the paddles or will just be drifting along with the current...Now if the current has sufficient speed you can tack into that wind and gain some speed...

https://physics.stackexchange.com/questions/410484/can-you-sail-up-down-a-moving-river-on-a-windless-day

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#8
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Re: Can You go Faster than Downwind Speed Directly Downwind

05/30/2021 7:30 PM

"If the boat isn't moving in the water you can't steer it"

Lack of clarity by me. The boat is being carried along by the waterflow, and that effectively creates an oncoming wind. No motor or paddles. I think your last sentence hits the point. It's very counter-intuitive and as said, I'm not sure it relates to this question. Slippers and cocoa time here so I'll have ponder overnight.

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#9
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Re: Can You go Faster than Downwind Speed Directly Downwind

05/30/2021 7:52 PM

The relative motion between water and air is what drives a sailboat. In this case, it's the water that is moving rather than the usual case of the air moving, but it makes no difference to the sailboat whether it's in a lake or drifting down a river.

If he lowers the sail, he will just drift down the river, but if he tacks into the relative wind, he can zig-zag down the river faster than the river current.

Now, here's the bonus question: If you have a DWFTTW (Downwind faster than the wind) boat (see #4) on a still day, could you go up the river?

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#12
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Re: Can You go Faster than Downwind Speed Directly Downwind

06/02/2021 4:43 PM

First parts are pretty much what I was trying to explain with " The given answer was that the boat can go faster by tacking against the airflow caused by the river pushing the boat along".

I'll ponder the last bit, but am groaning at the current repeat question about light and switches. The comment about LED lights was funny.

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#14
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Re: Can You go Faster than Downwind Speed Directly Downwind

06/03/2021 12:02 PM

Here's a bit more interesting chat.

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#11
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Re: Can You go Faster than Downwind Speed Directly Downwind

05/31/2021 9:05 AM

Years ago I worked for a First Nations community in central British Columbia. Their lands straddled the North Thompson river. To get from one side of the river to the other there was a ferry which could carry about four cars. The ferry was unmotorized. It was connected fore and aft to a cable that was stretched across the river, and the connections to the cross-river cable could be adjusted to angle the ferry at any direction into the current, enabling it to be pushed by the current back and forth across the river. Very resourceful. Your post reminded me of those happy times.

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#13
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Re: Can You go Faster than Downwind Speed Directly Downwind

06/02/2021 4:47 PM

Sounds like Jose Welles and a Missouri boat ride

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#15

Re: Can You go Faster than Downwind Speed Directly Downwind

06/05/2021 6:33 PM

Even in the case of geared wheels, I think there is a flaw in the argument. Once the vehicle's speed reaches wind speed, there is no net force on the prop/sail to add any further speed.

The various answers involving cross-wind sailing do not apply.

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#16
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Re: Can You go Faster than Downwind Speed Directly Downwind

06/06/2021 3:12 AM

Wind usually blows at different speeds, with wind gusts...now picture the vehicle pushing wind back opposing the oncoming wind, the faster you go the more opposing wind force is generated....eventually drag limits the speed of course...The propeller can be tuned by changing the pitch, so it can be used as a flat round sail, or by increasing the pitch an opposing wind force...

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#17
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Re: Can You go Faster than Downwind Speed Directly Downwind

06/06/2021 12:39 PM

It's trivial that a momentary gust could give extra speed to the vehicle, and then the wind slows down. However, that violates the spirit of the question, and it isn't sustainable.

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#18
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Re: Can You go Faster than Downwind Speed Directly Downwind

06/06/2021 1:13 PM

No the question stands, can you go faster than downwind speed directly downwind?

The variation of the windspeed plays a part in stepping up the vehicle speed, but the vehicle in the end exceeds the top speed of the wind, including the gust...remember the pitch of the propeller blades can be constantly varied...

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#19
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Re: Can You go Faster than Downwind Speed Directly Downwind

06/06/2021 4:54 PM

I don't think that helps anything at all. Once the vehicle reaches wind speed, there is no net blowing in the prop, no matter what the pitch is.

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#20
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Re: Can You go Faster than Downwind Speed Directly Downwind

06/06/2021 7:29 PM

Assuming the device is legitimate (no concealed propulsion system), IMHO I don't think gusts would be necessary to cause the vehicle to exceed wind speed. Once it gets moving - blades initially acting as sails - the rotation of the blades via gearing to the wheels forces air rearward, which causes, in combination with the tail wind, a high pressure zone aft of the rotating blades, and a low pressure zone forward of the rotating blades, both of which magnify the effect of the tail wind, enabling speeds in excess of the tail wind, and without gusts being necessary to achieve this.

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#21
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Re: Can You go Faster than Downwind Speed Directly Downwind

06/06/2021 8:35 PM

I do not believe that. When the vehicle approaches wind speed, the net velocity of the wind w.r.t. to the vehicle becomes zero, effectively not blowing on anything, no matter what configuration or gearing. Once the net force becomes zero, so does the acceleration of the vehicle.

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#22
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Re: Can You go Faster than Downwind Speed Directly Downwind

06/06/2021 10:33 PM

Then how do you explain this?

..."This is the Baltic Thunder, a land yacht that competed in 2019 in the Netherlands where they hold an event each year for students to pit their wits against each other and nature trying to get more speed from their wagons than the prevailing wind.

On July 2, 2010, the Blackbird land yacht set the world's first certified record for going directly downwind, faster than the wind, using only power from the available wind during its run. The yacht achieved a dead downwind speed of about 2.8 times the speed of the wind, sponsored by Google."...

..."On June 16, 2012, Blackbird set the world's first certified record for going directly upwind, without tacking, using only power from the wind. The yacht achieved a dead upwind speed of about 2.1 times the speed of the wind."....

https://www.bluebird-electric.net/wind_powered_ships_marine_renewable_energy_research.htm

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#23
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Re: Can You go Faster than Downwind Speed Directly Downwind

06/06/2021 11:07 PM

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#24
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Re: Can You go Faster than Downwind Speed Directly Downwind

06/07/2021 12:44 AM

The upwind version makes sense: The faster it goes, the harder the oncoming wind blows to power it.

In a quick reading, I did not find an explanation of the downwind version. As of yet, I can't explain it, but it might be nice to see their explanation.

Cousteau's Alcyone, which I once saw, is an example of a VAWT-driven vessel. No problem understanding its operation, but I don't recall if any claims were made about faster-than-downwind speed.

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#25
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Re: Can You go Faster than Downwind Speed Directly Downwind

06/07/2021 10:56 AM

To scientists of yesteryear, the sight of a bumblebee in flight was an affront to their theories of why it couldn't fly. I am reasonably convinced that there is no trickery involved with the windwagon in the video, so I am attempting to hypothesise how the faster-than-windspeed is accomplished. As you know, a sailboat can sail across the wind faster than windspeed due to the airfoil effect of the sails and the resulting pressure differential between the windward and the leeward sail surfaces. My hypothesis for the windwagon is also one of pressure differential, but achieved not via an airfoil effect. If, in say a 20km/hr. wind, the blades of vehicle as it approaches windspeed are producing a 40km/hr. rearward airflow, I suggest that the collision of these opposing moving air masses will result in a high pressure zone immediately aft of the blades, and consequently the blades will be pulled forward into the low pressure zone forward of the blades - just as the sail of a sailboat is pulled forward into the low pressure zone when sailing across the wind. The resulting speed increase of the vehicle will not cause a dissipation of the pressure differential because the blades will then be turning at an even higher rpm and driving a higher speed airflow rearward to encounter the 20km/hr. tailwind, thereby maintaining the fore to aft pressure differential. The vehicle will continue to accelerate until a stable condition between drag and motive force is reached. That is my hypothesis.

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