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Mess with your minds: A wind powered craft going faster than a tail wind speed.
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PlainName:

--- Quote ---Not quite sure I understand your infinitely long vehicle
--- End quote ---

OK, just very long then. Let's say it is 30m long. The sail is mounted on the vehicle just like Domagoj T's affair, but a single vertical sail. The sail starts at the front and stays there.

Now:

1. the body of the vehicle is streamlined so the only power transferred to it by the wind is via the sail. With the sail fixed in place and the wind blowing at 2m/s, are we agreed that the vehicle will move at 2m/s (or close enough)?

If not, why not?

2.  There is no wind. A motor turns the pulley moving the sail to the back at 1m/s. The power for that motor comes from a battery - we are not concerned with that at the moment, but it has nothing to do with the wind, the road, anything. Are we agreed that the vehicle moves forward at around 1m/s?

If not, why not?

3. There is a 2m/s wind, and the sail is moving backwards at 1m/2. Why cannot the vehicle be moving at ~3m/s when we know the sail will be pushed at 2m/s and the vehicle is pushed by the sail at 1m/s?
fourfathom:

--- Quote from: electrodacus on December 24, 2021, 09:23:50 pm ---
--- Quote from: dunkemhigh on December 24, 2021, 09:16:18 pm ---
OK, so if we ignore the prop and have an infinitely long vehicle with a sail moving from the front to back, don't you agree that the SAIL could be moved at wind speed and the vehicle would then move faster? Ignore for the moment the power needed to move the sail - we are just interested in if we are together at this point.

If you don't agree, please say exactly why not.

--- End quote ---

Not quite sure I understand your infinitely long vehicle but you seems to say the sail will move directly down wind relative to vehicle and then that movement will be used to power the vehicle.
If that is correct then no the vehicle can not move faster than wind since at the point vehicle speed and wind speed are equal the sail will no longer move relative to vehicle body and no movement means no power.
But also when vehicle is above wind speed the sail will move in the opposite direction relative to vehicle thus will slow the vehicle down.

--- End quote ---

No! No! No!

The sail is on a carrier that rides on a track bolted onto an infinitely long vehicle.  The sail carrier is pushed by a worm-gear, turned by the motion of the wheels.  Or attached to a belt-drive conveyer belt, powered by the rotation of the wheels -- those details don't matter.  As the vehicle moves forward, the sail moves back (towards the rear, and the source of the wind).  When the sail is at windspeed, the vehicle is traveling faster than the wind.  How much faster is set by the gear ratios.

The vehicle doesn't need to be infinitely long, just long enough that you are satisfied that any stored pressure-differential energy has been used well before the sail hits the end of the vehicle.

You will then want to consider other ways that a sail can be moved through the wind.  For example, a spinning propeller.
electrodacus:

--- Quote from: dunkemhigh on December 24, 2021, 10:14:15 pm ---
OK, just very long then. Let's say it is 30m long. The sail is mounted on the vehicle just like Domagoj T's affair, but a single vertical sail. The sail starts at the front and stays there.

Now:

1. the body of the vehicle is streamlined so the only power transferred to it by the wind is via the sail. With the sail fixed in place and the wind blowing at 2m/s, are we agreed that the vehicle will move at 2m/s (or close enough)?

If not, why not?

--- End quote ---

Yes if ideal case no friction vehicle will move close enough to 2m/s


--- Quote from: dunkemhigh on December 24, 2021, 10:14:15 pm ---2.  There is no wind. A motor turns the pulley moving the sail to the back at 1m/s. The power for that motor comes from a battery - we are not concerned with that at the moment, but it has nothing to do with the wind, the road, anything. Are we agreed that the vehicle moves forward at around 1m/s?

If not, why not?

--- End quote ---

If vehicle has any weight then that will need to be accelerated thus vehicle will not be at 1m/s relative to ground. If vehicle is light enough it may get very close to 1m/s before the sail gets to the back of the vehicle.


--- Quote from: dunkemhigh on December 24, 2021, 10:14:15 pm ---3. There is a 2m/s wind, and the sail is moving backwards at 1m/2. Why cannot the vehicle be moving at ~3m/s when we know the sail will be pushed at 2m/s and the vehicle is pushed by the sail at 1m/s?

--- End quote ---

I will assume you now removed that battery from the vehicle. So what moves the sail backwards ?
PlainName:

--- Quote ---I will assume you now removed that battery from the vehicle. So what moves the sail backwards ?
--- End quote ---

No, the battery is still driving the motor. It is simply a combination of 1 and 2: wind blowing in the sail, sail being moved along the vehicle.

Do you agree the vehicle ground speed would be close to 3m/s?
electrodacus:

--- Quote from: dunkemhigh on December 25, 2021, 12:11:34 am ---No, the battery is still driving the motor. It is simply a combination of 1 and 2: wind blowing in the sail, sail being moved along the vehicle.

Do you agree the vehicle ground speed would be close to 3m/s?

--- End quote ---

Yes but this just proves my point that an energy storage device is needed to exceed wind speed.
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