## board speed x wind speed

### board speed x wind speed

Hi,

I had an argument with a friend who has a sail boat, and I told him
that a board could run faster than the wind speed, he claims that it
is impossible. Was I correct? I mentioned Bernoulli's(?)
principle...but I'm not sure what is this principle...
Can anyone help with some explanations?

jorge

### board speed x wind speed

Quote:
>Hi,

>I had an argument with a friend who has a sail boat, and I told him
>that a board could run faster than the wind speed, he claims that it
>is impossible. Was I correct? I mentioned Bernoulli's(?)
>principle...but I'm not sure what is this principle...
>Can anyone help with some explanations?
>jorge

Well, you are correct.  I believe the speed record to date (somewhere around
50 or more MPH) was only clocked in about 30 knots of wind.  As soon as the
board picks up speed, it creats an apparent wind.  You then get the apparent
and true winds combining to get a "stronger" wind.  This also explains why
it's easier to stay on a plane than get up on one in marginal wind conditions.
Chris

### board speed x wind speed

Quote:
>>I told him that a board could run faster than the wind speed, he

claims that it is impossible.

An iceboat goes up to five times the wind speed, if you want a really dramatic example for your friend.  It's a wierd experience - novices have great difficulty figuring out what direction the *real* wind is coming from, since a much larger *apparent* wind is always blowing from almost straight ahead of you.  On a light breeze day, you have to run and push to get going, and then magically you go faster and faster until there's a roaring gale in your face.

You don't need to understand Bernoulli.  A simple linear vector diagram and a barn door model of the sail explains the principal just fine.  In the absence of drag, you could go as as fast as you like - there is no fundamental limit until you approach the speed of sound.   `

The practical limit is just that as you go faster, the wind comes from closer to dead ahead of you and the vector component of wind force on the sail which is propelling you forward becomes smaller.  Drag increases with speed.  When the drag equals the forward force, you have reached your equilibrium speed.

/Dave

---
/Dave
David E. Damouth
Xerox Webster Research Center

voice: 716-422-3186
fax: 716-265-7133

### board speed x wind speed

|> I had an argument with a friend who has a sail boat, and I told him
|> that a board could run faster than the wind speed, he claims that it
|> is impossible. Was I correct? I mentioned Bernoulli's(?)
|> principle...but I'm not sure what is this principle...

This issue has been debated to the death in the past.
Let me summarize the major results of the debate.

1. Bernoulli's principle has nothing to do with moving faster
than the wind.  One posting (from an apparently authoritative
source) claimed that Bernoulli has little to do even with the
force on the sail itself.

2. The relative wind does not really help, because it only
adds to the wind component that comes straight down at you.

3. It all boils down to a balance of forces on the board.
The sail generates a strong lateral component, balanced
by the skeg, and a small forward component, balanced by
the resistance to forward motion.  This resistance is
small for a planing board, so it can go fast.  There is
no reason why the speed of the wind should be a limit.

Luigi

### board speed x wind speed

I must point out that the word "run" invalidates your theory. Boards and
boats can certainly REACH faster than windspeed.... On a run, boardspeed
only reduces apparent wind.

-- Clark ick-kyu

|>
|>
|> >Hi,
|> >
|> >I had an argument with a friend who has a sail boat, and I told him
|> >that a board could run faster than the wind speed, he claims that it
|> >is impossible. Was I correct? I mentioned Bernoulli's(?)
|> >principle...but I'm not sure what is this principle...
|> >Can anyone help with some explanations?
|>
|>
|> >jorge
|> Well, you are correct.  I believe the speed record to date (somewhere around
|> 50 or more MPH) was only clocked in about 30 knots of wind.  As soon as the
|> board picks up speed, it creats an apparent wind.  You then get the apparent
|> and true winds combining to get a "stronger" wind.  This also explains why
|> it's easier to stay on a plane than get up on one in marginal wind conditions.
|> Chris

### board speed x wind speed

Quote:

>|> I had an argument with a friend who has a sail boat, and I told him
>|> that a board could run faster than the wind speed, he claims that it
>|> is impossible. Was I correct? I mentioned Bernoulli's(?)
>|> principle...but I'm not sure what is this principle...

>This issue has been debated to the death in the past.
>Let me summarize the major results of the debate.

>1. Bernoulli's principle has nothing to do with moving faster
>than the wind.  One posting (from an apparently authoritative
>source) claimed that Bernoulli has little to do even with the
>force on the sail itself.

>2. The relative wind does not really help, because it only
>adds to the wind component that comes straight down at you.

>3. It all boils down to a balance of forces on the board.
>The sail generates a strong lateral component, balanced
>by the skeg, and a small forward component, balanced by
>the resistance to forward motion.  This resistance is
>small for a planing board, so it can go fast.  There is
>no reason why the speed of the wind should be a limit.

>Luigi

My iceboat can certainly go faster than the wind speed,
and so it wouldn't surprize me that a fast sailboard could
too.  But I saw the word "run" in the original post and
assumed that the writer was asking "can a sail powered
craft go faster than the wind straight downwind (wrt to
the actual, not apparent, wind)?"  When I jibe my iceboat,
I do coast through the straight downwind heading with an
apparent headwind and thus my speed exceeds the windspeed.
But, I remain to be convinced that I could continue to sail
straight downwind for very long with an apparent headwind
question.

..Richard E+17

### board speed x wind speed

Quote:

>Hi,

>I had an argument with a friend who has a sail boat, and I told him
>that a board could run faster than the wind speed, he claims that it
>is impossible. Was I correct?

Absolutely, your buddy is dead wrong. Not only that but even sailboats (some)
can go faster than the wind.

An easy way to visualize it is as follows:

1. When you are going down wind (wind behind you), the faster you go, the less
affective wind you have, so you can never go faster than the wind. A perfect craft
going downwind would go exactly as fast as the wind.

2. When going upwing (into the wind), you are actually making headway into the wind.
The faster you go, the more apparent wind you get. THis is a viscious circle, which
if you did not count drag would allow you to go faster and faster for ever.

3. When on a broad reach, the wind in your sails remains constant, regardless
of you speed at 90 degrees to the wind. The only limiting factor is drag.
Otherwise you could do hundreds of miles per hour in a light breeze.

Ice yachts can do sevaral times the speed of the wind.

Quote:

You're welcome.

Jim Munro