Wind powered cart goes faster than the wind, directly downwind

[RAI]

A 2010 thread revived in 2021, congratulations, is this a record?
Just look how fast those AC cats foil downwind.

 

[RichardS]

SDDFTTW has long been a topic amongst experimental sailors that raises rehtoric, abuse, loonies and ire in the same way that flags and anchoring topics do here.
Ther is a world of difference, though, when discussing this topic between the one with two 'D's in it, and the topic with just one. Dead downwind faster than the wind sounds impossible to me but some disagree vehemently, just downwind is another matter as tacking (rather gybing I suppose)allows use of relative wind and thus perhaps - just perhaps, the possibility of beating the wind to the end of the run...

Search for the acronym above to get a flavour of the levels of zealotry involved - your eyes will soon glaze over!

Some sailing boats can sail across the wind faster than the wind and can zig-zag gybe downwind and achieve a faster VMG that the wind speed. This is all well-established.

What no sailing boats can do is sail directly downwind faster than the wind speed as, once they achieve wind speed, there is no additional force available to take them beyond that speed.

However, machines have been built which can travel directly downwind and achieve higher than wind speeds whilst relying solely on the wind for motive force. As far as I know, these machines are land based but I guess that one could be built around a boat. However, it would not have sails but would have a huge propeller where the sails would be.

Richard

 

[fredrussell]

From Wikipedia:

USA-17[a] (formerly known as BMW Oracle Racing 90 or BOR90) is a sloop rigged racing trimaran built by the American sailing team BMW Oracle Racing to challenge for the 2010 America's Cup.[3][4][5] Designed by VPLP Yacht Design with consultation from Franck Cammasand his Groupama multi-hull sailing team, BOR90 is very light for her size being constructed almost entirely out of carbon fiber and epoxy resin, and exhibits very high performance being able to sail at 2.0 to 2.5 times the true wind speed.[6] From the actual performance of the boat during the 2010 America's Cup races, it can be seen that she could achieve a velocity made goodupwind of over twice the wind speed and downwind of over 2.5 times the wind speed.

 

[Slowboat35]

What no sailing boats can do is sail directly downwind faster than the wind speed as, once they achieve wind speed, there is no additional force available to take them beyond that speed.

However, machines have been built which can travel directly downwind and achieve higher than wind speeds whilst relying solely on the wind for motive force. As far as I know, these machines are land based but I guess that one could be built around a boat. However, it would not have sails but would have a huge propeller where the sails would be.

Richard

Clearly there is no "force" available to take a boat dead downwind faster than the wind. it makes no difference what kind of vehicle it is, boat, car, bicycle or railway cariage. A "propellor" (I presume you mean turbine) requires relative windspeed to extract energy exactlt as a sail does. The physics are the same. No relative wind: no energy ego no acceleration.
However that are many who swear blind it is possible.

 

[Slowboat35]

From Wikipedia:

USA-17[a] (formerly known as BMW Oracle Racing 90 or BOR90) .... From the actual performance of the boat during the 2010 America's Cup races, it can be seen that she could achieve a velocity made goodupwind of over twice the wind speed and downwind of over 2.5 times the wind speed.

But not dead downwind!

 

[RichardS]

From Wikipedia:

USA-17[a] (formerly known as BMW Oracle Racing 90 or BOR90) is a sloop rigged racing trimaran built by the American sailing team BMW Oracle Racing to challenge for the 2010 America's Cup.[3][4][5] Designed by VPLP Yacht Design with consultation from Franck Cammasand his Groupama multi-hull sailing team, BOR90 is very light for her size being constructed almost entirely out of carbon fiber and epoxy resin, and exhibits very high performance being able to sail at 2.0 to 2.5 times the true wind speed.[6] From the actual performance of the boat during the 2010 America's Cup races, it can be seen that she could achieve a velocity made goodupwind of over twice the wind speed and downwind of over 2.5 times the wind speed.

Read my post #22 again, paying particular attention to the word "directly".

Richard

 

[RichardS]

Clearly there is no "force" available to take a boat dead downwind faster than the wind. it makes no difference what kind of vehicle it is, boat, car, bicycle or railway cariage. A "propellor" (I presume you mean turbine) requires relative windspeed to extract energy exactlt as a sail does. The physics are the same. No relative wind: no energy ego no acceleration.
However that are many who swear blind it is possible.

Richard

 

[JumbleDuck]

Clearly there is no "force" available to take a boat dead downwind faster than the wind. it makes no difference what kind of vehicle it is, boat, car, bicycle or railway cariage. A "propellor" (I presume you mean turbine) requires relative windspeed to extract energy exactlt as a sail does. The physics are the same. No relative wind: no energy ego no acceleration.

The most common mistake people make is in assuming that power is transmitted from the airscrew (propeller) to the wheels, when in fact the power is transmitted from the wheels to the propeller. In other words, on the Blackbird (for example) the propeller is pulling the car along, not pushing it.

However that are many who swear blind it is possible.

Since it has been done, though only - as far as I know - on land vehicles, it's a bit futile to deny the possibility.

 

[JumbleDuck]

What no sailing boats can do is sail directly downwind faster than the wind speed as, once they achieve wind speed, there is no additional force available to take them beyond that speed.

You can theoretically connect two boats sailing mirror images courses into one which does direct downwind faster than the wind.

 

[Slowboat35]

The most common mistake people make is in assuming that power is transmitted from the airscrew (propeller) to the wheels, when in fact the power is transmitted from the wheels to the propeller. In other words, on the Blackbird (for example) the propeller is pulling the car along, not pushing it.

Since it has been done, though only - as far as I know - on land vehicles, it's a bit futile to deny the possibility.

Then please explain the physics rather than making a dogmatic reply like that. How can "wheels power the propellor" That is a contradiction in terms. A wheel can transmit or obtain energy to drive apropellor, but were does that energy come from? You'd have to tow the vehicle to do that.
Where does the energy come from when close to the point of vehicle speed = wind speed? With no relative wind there can be no energy to harvest. You boat doesn't sail in zero wind! Neither can any other vehicle. And you can't claim that a water turbine (analogy for wheels) could transmit energy to accelerate it by means of a propellor either, can you?

Or maybe you can, please, an explanation!

 

[JumbleDuck]

Then please explain the physics rather than making a dogmatic reply like that. How can "wheels power the propellor" That is a contradiction in terms. A wheel can transmit or obtain energy to drive apropellor, but were does that energy come from? You'd have to tow the vehicle to do that.

I know it sounds counterintuitive but that's how it works. To summarise: power from the wheels (I'll come back to that) drives the propeller round. By pushing air backwards, the propeller generates thrust, just like propeller of an aircraft. That thrust moves the vehicle forward and provides the power which the wheels harvested (told you I'd come back to that).

At which point, people think "Wait a minute. There must be inefficiencies in the system. How can energy from the wheels generate the thrust which produces energy at the wheels?" And in still air, they'd be dead right - it would be a perpetual motion machine.

But this isn't still air, and the added thrust from the tailwind is what makes it work. It means that the wind does more work on the propeller than the propeller does on its slipstream and that excess is what accelerates the cart and drives the wheels.

Where does the energy come from when close to the point of vehicle speed = wind speed? With no relative wind there can be no energy to harvest.

Energy is harvested from the relative velocity of the wind and the land. At the point where vehicle speed = wind speed the propeller is spinning away and pushing back against the wind.

Anyway, it has been done.

 

[mjcoon]

At which point, people think "Wait a minute. There must be inefficiencies in the system. How can energy from the wheels generate the thrust which produces energy at the wheels?" And in still air, they'd be dead right - it would be a perpetual motion machine.

But this isn't still air, and the added thrust from the tailwind is what makes it work. It means that the wind does more work on the propeller than the propeller does on its slipstream and that excess is what accelerates the cart and drives the wheels.
...
Anyway, it has been done.

How many people have claimed to have built a perpetual motion machine?

If the cart goes faster than the wind surely it experiences a headwind not a tailwind?

 

[RJJ]

Then please explain the physics rather than making a dogmatic reply like that. How can "wheels power the propellor" That is a contradiction in terms. A wheel can transmit or obtain energy to drive apropellor, but were does that energy come from? You'd have to tow the vehicle to do that.
Where does the energy come from when close to the point of vehicle speed = wind speed? With no relative wind there can be no energy to harvest. You boat doesn't sail in zero wind! Neither can any other vehicle. And you can't claim that a water turbine (analogy for wheels) could transmit energy to accelerate it by means of a propellor either, can you?

Or maybe you can, please, an explanation!

Epic...

It's definitely the wheels powering the propeller. The kinetic energy to power the propeller comes from the motion of the cart/yacht. the cart/yacht will accelerate as long as the forward force created by the propeller exceeds the total drag of (1) headwind resistance on the static parts of the chassis (2) friction within the drive system (3) rolling resistance of the wheels.

As far as I know, these machines are land based but I guess that one could be built around a boat. However, it would not have sails but would have a huge propeller where the sails would be.

There's no theoretical reason this couldn't work on a boat, only practical efficiency losses. The problem you have is that the (1) capturing energy from an impeller is less efficient than capturing energy from spinning roadwheels and (2) the water resistance on the static parts of the hull is significant, particularly as soon as you have waves to overtake.

So you could build such a watercraft; it would accelerate downwind from stationary up to a certain speed, but increasing drag would soon catch up with the increasing thrust. Until somebody designs a much more efficient impeller and a much lower-drag hull, that equilibrium point will be below the true wind speed.

 

[JumbleDuck]

How many people have claimed to have built a perpetual motion machine?

So many that the US Patent Office supposedly requires to see a working model before accepting an application.

If the cart goes faster than the wind surely it experiences a headwind not a tailwind?

Yup, but the slipstream of the propeller goes faster than the land speed of the cart and so experiences a tail wind, which is what pushes the whole thing along.

 

[JumbleDuck]

So you could build such a watercraft; it would accelerate downwind from stationary up to a certain speed, but increasing drag would soon catch up with the increasing thrust. Until somebody designs a much more efficient impeller and a much lower-drag hull, that equilibrium point will be below the true wind speed.

I think we may see it done on a hydrofoil, but the harvesting of sufficient power from the water may be, as you say, an insuperable problem.

 

[RAI]

If the cart goes faster than the wind surely it experiences a headwind not a tailwind?

If you look at that picture in #28, you can see a telltale on the "bowsprit" that is blowing backwards.

 

[RAI]

DDWFTTW Vehicle Ananlys. Dead down wind faster than the wind.
For a detailed explanation

 

[JumbleDuck]

DDWFTTW Vehicle Ananlys. Dead down wind faster than the wind.
For a detailed explanation

It's exactly the same problem, solution and model as going directly upwind with a coupled airscrew and wheels (or propeller) if you turn it upside down and fix the underneath medium. That article is OK, but makes rather a meal of something which is only counterintuitive, rather than complicated.

 

[RAI]

It's exactly the same problem, solution and model as going directly upwind with a coupled airscrew and wheels (or propeller) if you turn it upside down and fix the underneath medium. That article is OK, but makes rather a meal of something which is only counterintuitive, rather than complicated.

Did you note his reference and link to the boat version?

 

[JumbleDuck]

Did you note his reference and link to the boat version?

Alas

but I have downloaded it through work and had a look. It's a little limited, because it looks at two direct-coupled actuator disks only, but not bad for physicists (I'm an engineer!).

However, my real point was that a boat or cart going directly upwind with coupled airscrew and wheels/waterscrew is something which people generally find easier to accept, and so turning that into a DDWFTTW device is sometimes a useful approach.