Board speed faster than wind speed!!?

Board speed faster than wind speed!!?

Post by Martin Alle » Wed, 11 Oct 2000 04:00:00


I remember reading that windsurfers often travel faster than the wind speed.
I went land yachting at the weekend and our instructor told us that land
yachts could travel at 3 times the wind speed. Can someone explain to me how
that is possible in language that someone who only did physics till the age
of 16 can understand.
Cheers
Martin

--

--
http://www.***.net/

 
 
 

Board speed faster than wind speed!!?

Post by B Kenne » Wed, 11 Oct 2000 04:00:00

Simple answer:
air flowing over the sail generates the force to make the board move
air flow over the sail is the sum of the wind and the board speed
(actually it is the vector sum which means you have to consider directions if
you want actual values, but you can forget this for the simple answer)
Therefore, the faster the board speed, the more force available to go faster

What limits your board speed to something below the speed of sound  is drag
windsurfers have less drag than sailboats - and go faster
land yachts have less drag than windsurfers - and go faster yet
iceboats have less drag than land yachts - and go fastest

If you like going fast, check out:
http://SportToday.org/

Hope this helps

Regards

Barney

Quote:

> I remember reading that windsurfers often travel faster than the wind speed.
> I went land yachting at the weekend and our instructor told us that land
> yachts could travel at 3 times the wind speed. Can someone explain to me how
> that is possible in language that someone who only did physics till the age
> of 16 can understand.
> Cheers
> Martin

> --

> --
> http://www.***.net/


 
 
 

Board speed faster than wind speed!!?

Post by Mike » Wed, 11 Oct 2000 04:00:00

Da faster ya goes, da more wind yer sail sees. Da more wind yer sail sees,
da more power it generates. Da more power it generates, da faster ya goes.
Da faster ya goes, da more wind yer sail sees. Da more wind yer sail sees,
da more power it generates. Da more power it generates, da faster ya goes.
Da faster ya goes, da more wind yer sail sees. Da more wind yer sail sees,
da more power it generates. Da more power it generates, da faster ya goes.
Da faster ya goes ...

Are you beginning to see a pattern? The only thing that limits this
snowball, besides the size of the lake, the size of your gonads, and
Albert's Theory of Relativity, is drag ... on the ice, in the water, in the
wheel bearings, around your body and around your sail. At some point every
additional bit (almost said erg, but that's physics) of energy gazinta an
extra bit of drag rather than more speed, so there's no more speed to add to
the snowball. At that point the speedometer sticks until we find a way to
decrease the drag and start over. Ice sailors claim it can stick at up to
five (that's fivISH, as in V, not 5.000 ... this ain't physics) times the
windspeed Lars measures sittin' on his ice-fishin' stool.

There ... no physics, no grammar, no spelling, no math, no home ec, no gym
class, no shop, no history.

No mas?

Mike \m/
To reply directly, remove the SpamDam.

Quote:
> I remember reading that windsurfers often travel faster than the wind
speed.
> I went land yachting at the weekend and our instructor told us that land
> yachts could travel at 3 times the wind speed. Can someone explain to me
how
> that is possible in language that someone who only did physics till the
age
> of 16 can understand.
> Cheers
> Martin

> --

> --
> http://www.***.net/


 
 
 

Board speed faster than wind speed!!?

Post by j.. » Wed, 11 Oct 2000 04:00:00


Quote:

>I remember reading that windsurfers often travel faster than the wind speed.
>I went land yachting at the weekend and our instructor told us that land
>yachts could travel at 3 times the wind speed. Can someone explain to me how
>that is possible in language that someone who only did physics till the age
>of 16 can understand.
>Cheers
>Martin

Hi Martin,

Maybe this will make sense to you.  The wind is pushing you from one side.
Your fin and board are resisting that force from pushing you directly down
wind.  Because the force of the sail is ahead of the resistance from the
board and fin, the only way to absorb the sail's force is to go forward.
Like squeezing a watermelon seed between wet fingers, it pops out sideways
faster than the fingers are actually closing in one each other.

Then there's the apparent wind thing too.  I'll let someone else go over that
one.

Sail fast,

Jon

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Board speed faster than wind speed!!?

Post by Tom McClellan » Thu, 12 Oct 2000 04:00:00

Windsurf boards are not blown directly by the wind like toy paper boats
driven downwind across a pond.

They are driven by suction that results from air moving across the sail,
which you will notice is shaped like an aircraft wing when the wind fills
it.

To Elaborate:

The air on the far side of the sail is moving more rapidly because it has
further to go than the air on the near side because of the foiled shape of
the sail. Science tells us that faster = lower-pressure because the
molecules spread out more to travel the greater distance (this is similar to
the pull you feel towards a moving subway train, or that your car feels when
you pass a big truck at speed), so the air on the far side of the sail is at
lower-pressure than the air on the near side. So the sail moves forward into
the void (nature abhors a vacuum). The faster you go, the greater this
pressure differential because the speed at which the sail moves through the
air is added to the airs own windspeed. Drag ( the tendency of the air to
stick to the sail and the water to stick to the board) stops things from
getting out of hand before you reach escape velocity. (Perhaps in a dragless
parallel universe we would run into severe problems as we approached mach
1. )

This all applies only when sailing *across* the wind. Clearly when sailing
close to downwind for any length of time the board *is* being blown along
like a toy boat, and is limited to below the windspeed.

I knew that fluid dynamics course I did for my Civil Engineering degree
would come in useful sometime.Its only taken me 20 years to find a use for
that knowledge.

 
 
 

Board speed faster than wind speed!!?

Post by Martin Alle » Thu, 12 Oct 2000 04:00:00

Thanks Tom,
Just the right level of dumming down for me.
Would I be right in thinking that a board would need to be planning for the
drag to be small enough to allow faster than wind travel?
 
 
 

Board speed faster than wind speed!!?

Post by Peter Som » Fri, 13 Oct 2000 15:35:39

Quote:

>I remember reading that windsurfers often travel faster than the wind speed.
>I went land yachting at the weekend and our instructor told us that land
>yachts could travel at 3 times the wind speed. Can someone explain to me how
>that is possible in language that someone who only did physics till the age
>of 16 can understand.
>Cheers
>Martin

>--

>--
>http://www.***.net/

Let me put my oar in too:
If you sail dead downwind (square running), at best, (with no
friction=drag) you would sail at the windspeed - in practice a bit
under. However if you sail right angles to the wind (reaching) your
speed CAN be higher than the wind-speed, but if you insist on no
physics, that's all I will tell.
Peter
 
 
 

Board speed faster than wind speed!!?

Post by Tom McClellan » Fri, 13 Oct 2000 04:00:00

Quote:
>> Would I be right in thinking that a board would need to be planing for
the
>> drag to be small enough to allow faster than wind travel?

This is likely, board drag decreases dramatically as it jumps onto the plane
because at sub-planing speeds the board is constantly climbing over its own
bow-wave and the wetted area when not planing is huge. However other craft
may give different results. I think that I've seen catamarans go very fast
in light winds where they don't seem to be planing yet.

Regards

Tom

 
 
 

Board speed faster than wind speed!!?

Post by Martin Alle » Fri, 13 Oct 2000 04:00:00

I didn't want no physics. Just not too much.
Thanks for everyone's contributions anyway. It makes more sense to me now.
Best wishes
Martin
 
 
 

Board speed faster than wind speed!!?

Post by Stergios Papadaki » Sat, 14 Oct 2000 04:00:00

Quote:

> Thanks Tom,
> Just the right level of dumming down for me.
> Would I be right in thinking that a board would need to be planning for the
> drag to be small enough to allow faster than wind travel?

No, if the wind speed is less than hull speed you
can sail faster than the wind without planing.
This applies to any sailboat.

Stergios
--


Dept. of Physics and Astronomy          
http://www.physics.unc.edu/~papadaks
Univ. of North Carolina at Chapel Hill,   (o) (919) 962 7298
Chapel Hill, NC  27599-3255               (f) (919) 962 0480

 
 
 

Board speed faster than wind speed!!?

Post by B Kenne » Sat, 14 Oct 2000 04:00:00

Quote:


> > Thanks Tom,
> > Just the right level of dumming down for me.
> > Would I be right in thinking that a board would need to be planning for the
> > drag to be small enough to allow faster than wind travel?

> No, if the wind speed is less than hull speed you
> can sail faster than the wind without planing.
> This applies to any sailboat.

And particularly America's cup yachts.
Quote:
> Stergios
> --


> Dept. of Physics and Astronomy
> http://www.physics.unc.edu/~papadaks
> Univ. of North Carolina at Chapel Hill,   (o) (919) 962 7298
> Chapel Hill, NC  27599-3255               (f) (919) 962 0480

 
 
 

Board speed faster than wind speed!!?

Post by Ryan » Sun, 15 Oct 2000 04:00:00

America's Cup yachts aren't particularly fast for their size.  Look at
the Open 60's or Whitbread 60's used in around the world races, or the
average beach catameran.

Ryan

Quote:

> And particularly America's cup yachts.

> > Stergios
> > --


> > Dept. of Physics and Astronomy
> > http://www.physics.unc.edu/~papadaks
> > Univ. of North Carolina at Chapel Hill,   (o) (919) 962 7298
> > Chapel Hill, NC  27599-3255               (f) (919) 962 0480

--
Ryan
http://members.tripod.com/~ryan-psu

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Board speed faster than wind speed!!?

Post by Ryan » Sun, 15 Oct 2000 04:00:00

Because the acceleration is proportional to the amount of force your
sail creates, not wind speed.  Secondly because of apparent wind.

Apparent wind is what your sail feels.

Think of it this way.   If you are standing still and the wind is
blowing 30mph then you feel like you are standing in a 30mph wind.

On a calm day stand up in the back of your friends convertible going
30mph.  The wind to you feels the same as it was when you were standing
still on a day it was blowing 30mph.

Now on a day when the wind is blowing 30mph get in your friends
convertible and tell him to drive into the wind going 30mph and the
wind will feel to you like it is blowing 60mph.  The 60mph is the
apparent wind.

Ryan



Quote:
> I remember reading that windsurfers often travel faster than the wind
speed.
> I went land yachting at the weekend and our instructor told us that
land
> yachts could travel at 3 times the wind speed. Can someone explain to
me how
> that is possible in language that someone who only did physics till
the age
> of 16 can understand.
> Cheers
> Martin

> --

> --
> http://www.***.net/

--
Ryan
http://SportToday.org/~ryan-psu

Sent via Deja.com http://SportToday.org/
Before you buy.

 
 
 

Board speed faster than wind speed!!?

Post by Mark Phillip » Thu, 19 Oct 2000 04:00:00

Quote:

> Drag ( the tendency of the air to
> stick to the sail and the water to stick to the board) stops things from
> getting out of hand before you reach escape velocity. (Perhaps in a dragless
> parallel universe we would run into severe problems as we approached mach
> 1. )

The other parameter is attack angle (how close to the wind can you sail).  We
can't simply push-start a sailboard in a frictionless environment and expect it
to go faster and faster.  We do sail the apparent wind  -  speed and
direction.   As the component induced by our movement through the air becomes
more significant, we must turn more down wind to keep the angle of attack
constant.  It becomes apparent that we approach a finite limit to how fast we
can go in a given wind if we are to sail a reasonably straight coarse.  Push
start a sailboard  in a frictionless, windless place and the resultant path will
be circular.
mark
--
 
 
 

Board speed faster than wind speed!!?

Post by Tom McClellan » Thu, 19 Oct 2000 04:00:00

Mark,

Thank you. These fantasy subjunctives are tricky things. I assumed that my
jocular example would not stand up to rigorous examination and wondered what
corrections would come my way.....

Regards

Tom


Quote:


> > Drag ( the tendency of the air to
> > stick to the sail and the water to stick to the board) stops things from
> > getting out of hand before you reach escape velocity. (Perhaps in a
dragless
> > parallel universe we would run into severe problems as we approached
mach
> > 1. )

> The other parameter is attack angle (how close to the wind can you sail).
We
> can't simply push-start a sailboard in a frictionless environment and
expect it
> to go faster and faster.  We do sail the apparent wind  -  speed and
> direction.   As the component induced by our movement through the air
becomes
> more significant, we must turn more down wind to keep the angle of attack
> constant.  It becomes apparent that we approach a finite limit to how fast
we
> can go in a given wind if we are to sail a reasonably straight coarse.
Push
> start a sailboard  in a frictionless, windless place and the resultant
path will
> be circular.
> mark
> --