How is it that wind powered land vehicles can travel faster than the speed of the wind? Does the sail act as an aerofoil and “pull” the vehicle? If so, is the difference between low air pressure and high air pressure that great? What kind of forces are at work?
Galyean, You pretty much nailed it when you said the sail acts like an airfoil. Just consider the sail a vertical wing generating “lift” in a horizontal direction.
The other thing to remember is that the wind force acting on the sail is the vector product of the true wind velocity and the sail’s velocity. This “apparent wind speed” can be quite a bit larger than the true wind speed, and can produce some really large boat speeds, especially in the absence of friction, like with an ice boat.
Going downwind, the maximum speed is windspeed. However, if the wind is perpindicular to the direction of travel (reaching), then speeds in excess of wind speed are possible if there is minimal friction with terra firma.
Uh oh, another thread about lift. Someone will be along to dissent.
What you’re positing simply defies logic – when running with the wind, 100% of the force is being applied in the direction of motion. When tacking, the lift supplied is greater than 100%!!! Could you please explain this in terms a non-scientist can understand?
RM Mentock, you were right. 
keno, read this page to get the straight dope on airfoils.
Keno,
It goes back to apparent wind speed. Running downwind, a sailboat can’t go as fast as the wind, because the motive force would drop to zero.
Reaching, or sailing perpendicular to the true wind direction, is slightly different. As the boat gains speed, the apparent wind direction shifts forward, and the apparent wind speed increases. When the boat is traveling at true wind speed, it sees an apparent wind direction of 45 degrees off the bow and an apparent wind speed of 1.414 true wind speed. This extra “lift” can allow the boat to go faster than true wind speed. C.A. Marchaj in “Sailing Theory and Practice” says a decent sloop can go about 1.2 times true wind speed this way, assuming its hull speed allows it.
Of course, without the drag of the water, all bets are off. Watch some ice boats sometime. They are always sailing very close-hauled, because their apparent wind is always in front of them.
Keno,
It goes back to apparent wind speed. Running downwind, a sailboat can’t go as fast as the wind, because the motive force would drop to zero.
Reaching, or sailing perpendicular to the true wind direction, is slightly different. As the boat gains speed, the apparent wind direction shifts forward, and the apparent wind speed increases. When the boat is traveling at true wind speed, it sees an apparent wind direction of 45 degrees off the bow and an apparent wind speed of 1.414 true wind speed. This extra “lift” can allow the boat to go faster than true wind speed. C.A. Marchaj in “Sailing Theory and Practice” says a decent sloop can go about 1.2 times true wind speed this way, assuming its hull speed allows it.
Of course, without the drag of the water, all bets are off. Watch some ice boats sometime. They are always sailing very close-hauled, because their apparent wind is always in front of them.
TNTruth
I hear what you’re saying but it’s not getting thru. Do you have any vector diagrams that illustrate your points?
Vectors? Come on!
Imagine a car on rails with no friction and imagine the air as very dense (or imagine we are doing the experiment at the bottom of the sea).
If the air (fluid) is blowing parallel to the tracks, the best you can do is to move with it, not faster (I hope this is obvious).
But now imagine the air is blowing across the tracks. The angle of the sail (pitch) determines how much the car will advance. Imagine the fluid is so dense there is no slip. The sail has to move forward with a speed that is the speed of the fluid multiplied by the tangent of the pitch angle.
If the angle is zero, the forward speed is zero. As you trim the sail in, the speed slowly increases until at 45 degress, the speed of the car is the same as the speed of the wind. With angles greater than 45 degrees, the speed of the car would be greater than the speed of the wind. Draw a diagram and you will see it is quite obvious.
You can do this experiment: Make wedges of ice with different angles and place them vertically on the kitchen countertop, against the backsplash. Now take a pencil and push against the ice wedge perpendicularly to the backsplash. The Ice wedge moves sideways. Calculate the speed (if the angle of the wedge is less than 45 the wedge moves faster than the pencil pushing it).
Obviously if you are just pushing the ice around the center of the table, then the ice will move at the same speed as the pencil you are pushing it with.
I hope this clarifies the issue.
Keno,
Thanks for staying on this. My scanner only takes loose sheets of paper, and I wasn’t about to start ripping pages out of a book just to scan them. The best I could find on the web is a site called “SAILING FOR NONSAILORS 1.” It’s at:
http://homepages.apci.net/~michalak/15jul00.htm
There are a few diagrams showing how a sail is affected by apparent wind, with force vectors thrown in almost as an afterthought. I hope it helps.
Sailor, I was wondering when you were going to jump into this thread. Come on, don’t you have any good sites with diagrams?
I am trying hard to stay out of this and all other threads as I am very busy these days.
I Could find plenty of sites and books with plenty of diagrams but I think my previous post explains all he needs to know. To understand why a boat can sail faster than the wind you do not need to understand how a sail works, airfoils etc. The concept of apparent wind, vectors, etc, are all true and well but unnecessary to understand the OP.
I think The example I gave explains it in the simplest way.
in any case, I am too busy these days to spend much time here.