You skim across the surface of the water at incredible speeds, drive towards a whitecapped wave, and then take flight like a skateboarder in a half-pipe, each muscle straining against the force of the wind. Then you smash into the trough of the wave, spring up from near disaster, and look quickly for the next wave so you can do it all over again. This is the exciting sport of windsurfing. Windsurfing has its roots in the '60s when it began to take shape from conversations between an aeronautical engineer and a scientist. In 1969, the engineer presented a symposium paper entitled "Wind Surfing: A New Concept in Sailing." This new concept entailed releasing the mast from its traditional fixed vertical position and allowing it to pivot around its base. The sailor then can both steer and balance the board through appropriate movements of the mast and sail. From these beginnings, windsurfing has grown to become a popular sport attracting 100,000 newcomers a year. How does a windsurfer manage to move the sailboard across the water? The force that moves a sailboard (or any sailboat) is not produced simply by the wind pushing on the sail. The process is closely related to the lift
on an airplane caused by the air moving over a specially-shaped wing. Bernoulli's principle tells us that in a fluid (such as air), regions of higher velocity have lower pressure
than surrounding regions. A sail may be thought of as a vertical wing. The wind moves faster (in order to travel the greater distance) over the convex
curve of the forward part of the sail than it does when it goes straight over the back part of the sail. According to Bernoulli's principle, there is lower pressure on the forward part of the sail (shaded region in the figure at right) and thus a net force perpendicular to the sail. This is the force that propels the windsurfer, but you may have noticed that part of this force is to the side of the sailboard. What keeps the board from moving sideways? The answer is the "daggerboard" which extends from the sailboard down into the water. This daggerboard not only keeps the board from drifting sideways, but it also acts as a fulcrum
about which the sailor pivots the board for steering.
The lift on the sail of a sailboard is similar to that on a wing of an airplane. What forces and pressures generate lift? Construct a model to demonstrate the force of lift. Materials
- two 8 1/2" x 11" sheets of paper
- transparent tape
- sharpened pencil
- 60-cm (24") piece of monofilament fishing line
- one 15-cm (6") straw
- two 7.5-cm (3") straws
- Fold one sheet of paper in half, but do not crease the fold.
- Tape the long opened edge of paper with three small pieces of tape to keep it closed. This taped side will be known as your "trailing edge," while the folded side will be known as the "leading edge."
- With the pencil, mark an "X" on the center line of the paper about one inch from the leading edge.
- Punch a hole through both the top and bottom of the paper at the "X." (Be careful not to crease the paper at the fold.)
- Place the 15-cm straw through the hole you just punched. Use tape, if necessary, to hold the straw in place.
- Tie one end of the fishing line to the middle of a 7.5-cm straw.
- Pass the other end of the fishing line through the 15-cm straw which is attached to the paper.
- Pull the fishing line through and tie this end to the other 7.5-cm straw. The 7.5-cm straws will be your handles.
- On the other piece of paper, trace two copies of the airfoil shape below and cut out the shapes. Tape the shapes to the open ends of the "wing." The flat edge of the shapes should be on the bottom of the wing (see illustration).
- Taking the 7.5-cm straw handles, one in each hand, draw the fishing line tight and position it so the line is perpendicular to the floor. Make sure the flatter surface of the wing faces down.
- With your arms out in front of you, make a quick sweeping motion through the air. Be certain that the leading edge of the wing is in front. Questions
- What did your piece of paper do?
- What caused the wing to move upward?
- Explain how this demonstration relates to a sail.
- Explain the forces that move the paper upward. Activity adapted with permission of AIMS, Fresno, CA.