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

--- Quote from: IanB on December 19, 2021, 02:27:58 am ---
--- Quote from: electrodacus on December 19, 2021, 02:22:52 am ---...as boat is still slower in the direction of the wind.
--- End quote ---

No, the boat is faster than the wind in the direction of the wind. That is the whole point of how the boat can travel 20 mph downwind in less time than the wind takes to cover the same distance.

(If you disagree with this, you will have to tell the sailors that what they are doing is impossible. They will be very interested to hear that.)

--- End quote ---

Let me try this again. In ideal case a boat can get to 2x even more wind speed at an angle and then since there is no friction it can just turn directly down wind and travel forever at 2x. In real world there is friction so sailor will get the boat to speed means increased kinetic energy (stored kinetic energy) and then it can turn straight and for a few minutes stay above wind speed as kinetic energy and boat speed reduces then he can turn again to an angle to get to speed and repeat. If friction is sufficient low average may be higher than wind speed but even if it is not above wind speed it is still faster as most sail boats direct downwind can do just 0.5x wind speed.
So in this case kinetic energy is used as energy storage.  In case of blackbird driving at an angle is not allowed so kinetic energy can not be used as energy storage in the same way it is used in the example above.
electrodacus:

--- Quote from: bdunham7 on December 19, 2021, 02:42:00 am ---
OK, you're right.  It's turtles all the way down.

--- End quote ---

It is.
Brumby:

--- Quote from: electrodacus on December 19, 2021, 02:26:00 am ---
--- Quote from: Brumby on December 19, 2021, 01:38:39 am ---
Using this, you have 5.5W spinning a propeller.  This propeller creates a thrust force against an air mass which is stationary with respect to the vehicle.
How can a non-zero force NOT create a change in velocity?

--- End quote ---

Yes but those 5.5W will not create more than 5.5W at the propeller output. So the kinetic energy / speed reduction to get those 5.5W will require the same 5.5W just to be where you started from in ideal case.

--- End quote ---
I know exactly what you are saying here ... and it is what you would naturally expect from applying intuitive thinking.

However, the mechanism utilises a mechanical advantage (if you will) with the power of the wind augmenting the effectiveness of that 5.5W.
IanB:

--- Quote from: electrodacus on December 19, 2021, 03:07:42 am ---
--- Quote from: IanB on December 19, 2021, 02:27:58 am ---
--- Quote from: electrodacus on December 19, 2021, 02:22:52 am ---...as boat is still slower in the direction of the wind.
--- End quote ---

No, the boat is faster than the wind in the direction of the wind. That is the whole point of how the boat can travel 20 mph downwind in less time than the wind takes to cover the same distance.

(If you disagree with this, you will have to tell the sailors that what they are doing is impossible. They will be very interested to hear that.)

--- End quote ---

Let me try this again. In ideal case a boat can get to 2x even more wind speed at an angle and then since there is no friction it can just turn directly down wind and travel forever at 2x. In real world there is friction so sailor will get the boat to speed means increased kinetic energy (stored kinetic energy) and then it can turn straight and for a few minutes stay above wind speed as kinetic energy and boat speed reduces then he can turn again to an angle to get to speed and repeat. If friction is sufficient low average may be higher than wind speed but even if it is not above wind speed it is still faster as most sail boats direct downwind can do just 0.5x wind speed.
So in this case kinetic energy is used as energy storage.  In case of blackbird driving at an angle is not allowed so kinetic energy can not be used as energy storage in the same way it is used in the example above.

--- End quote ---

Here's a picture. This is with friction, traveling at a steady speed, with no speeding up or slowing down, and no kinetic energy. At no time is the boat traveling downwind slower than the wind speed. The boat can tack like this forever, always going faster than the wind, and never going below the wind speed. This is what real boats can do, in the real world, with lots of evidence to prove it.

If you think this is not possible, you will have to argue with the people who sail these boats.

IanB:
This is how Blackbird can do the same thing:
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