Author Topic: Windmills  (Read 17349 times)

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Offline Will_Lee

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Windmills
« on: March 25, 2010, 10:00:47 AM »
Can boats go downwind faster than the wind (ie could you outrun a puff of smoke as it travels downwind).

The answer is yes. First, because in suitable conditions Cherubs and other high performance boats do that, second there are some v complicated equations to prove it, and third because it is quite easy to see that when the apparent wind on you when you are sending it downhill passed 90 degs to the true wind, then that is what you are doing.

However, there are many people who think it is impossible, so people have set out to demonstrate it. 

http://www.youtube.com/watch?v=7SYvg40NHtc

It is less obvious to me how it starts from a standstill, but there it is!

Offline MK

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Re: Windmills
« Reply #1 on: March 25, 2010, 10:43:38 AM »

Offline Banshee Ambulance

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Re: Windmills
« Reply #2 on: March 25, 2010, 12:27:25 PM »
I dont think the issue is that boats can't go downwind faster than the wind, the real question is can you build a device that will do DEAD downwind faster than the wind? As to how that would work I dont have a clue. You would have to put it in reverse when you got over the windspeed as the apparent would be in the opposite direction.

Offline Phil Alderson

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Re: Windmills
« Reply #3 on: March 25, 2010, 03:08:35 PM »
I think the key point with these windmills is that while the hub may be travelling directly down wind the blade tips are not.

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Offline JimC

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Re: Windmills
« Reply #4 on: March 25, 2010, 07:24:11 PM »
while the hub may be travelling directly down wind the blade tips are not.

'xactly so Phil.

It is less obvious to me how it starts from a standstill, but there it is!
Plain ordinary wind drag on the structure with the blades stalled out. Then as it starts moving the blades start rotating in the opposite way to the way wind pressure tries to rotate them.

They are extraordinarily counter-intuitive contraptions: it took me a long long time to get the concept straight in my head.
« Last Edit: March 25, 2010, 07:32:24 PM by JimC »

Offline Phil Alderson

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Re: Windmills
« Reply #5 on: March 25, 2010, 08:05:13 PM »
while the hub may be travelling directly down wind the blade tips are not.

'xactly so Phil.

It is less obvious to me how it starts from a standstill, but there it is!
Plain ordinary wind drag on the structure with the blades stalled out. Then as it starts moving the blades start rotating in the opposite way to the way wind pressure tries to rotate them.

They are extraordinarily counter-intuitive contraptions: it took me a long long time to get the concept straight in my head.

I find the aerodynamics of windmills in general is quite counter intiuative when compared to propellers

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Offline andy_paterson

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Re: Windmills
« Reply #6 on: March 30, 2010, 12:40:36 PM »
Can boats go downwind faster than the wind (ie could you outrun a puff of smoke as it travels downwind).

The answer is yes. First, because in suitable conditions Cherubs and other high performance boats do that, ....

Imagine there were two cherubs, tied together with a long bit of very stretchy bungee - this makes one 'boat'.

Then both sail downwind at normal gybing angle but on opposite gybes.  ( and gybe back together before the bungee gets too tight ), gybe again before they crash, and repeat.

So the effect is that each cherub is going downwind faster than the wind, but since they are connected as one 'boat', the middle of the bungee or the  middle of the boat  IS going dead downwind faster than the wind.

The windmilly thing does the same, the middle goes dead downwind, but with the blades sweeping in a circle instead of back and forth. The sweeping or sailing at an angle, + apparent wind , is the thing that makes the difference, by extracting energy from a bigger amount of 'wind'.  Which is why it's faster to gybe downwind rather than dead running.

Interestingly, the AC boats both sailed vmg upwind and downwind at more than twice the windspeed, so that if some smoke was let off at the top mark at the start time, the boats would have sailed to the top mark and back again to the finish before the smoke got there. That's fast!



Offline Tim Noyce

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Re: Windmills
« Reply #7 on: March 30, 2010, 01:07:20 PM »
Interestingly, the AC boats both sailed vmg upwind and downwind at more than twice the windspeed, so that if some smoke was let off at the top mark at the start time, the boats would have sailed to the top mark and back again to the finish before the smoke got there. That's fast!

I must say I had never thought of it like that before... but that is amazing!