You glide across the surface of the water at unbelievable speeds, steer towards a white capped wave, and then lift off like a bird, each muscle resisting against the force of the wind. Then you smash into the trough of the wave, leap 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 began in the ’60s when an aeronautical engineer and a scientist had a discussion. In 1969, the engineer presented an idea entitled “Wind Surfing: A New Concept in Sailing.” This new concept involved releasing the mast from its fixed vertical position and allowing it to turn around its base (Now a days the vertical positioning is not fixed) The sailor then can both steer and balance the board through correct movements of the mast and sail. The early Windsurfer boards measured 12 feet (3.5 m) long and weighed 60 pounds (27 kg).
WHAT IS A SAILBOARD?
A sailboard is composed of a board and a rig. There is variation in modern sailboards; they generally range from 8 to 12 ft (2 to 4 m) and weigh between 7 to 18 kg; some have attained speeds of over 40 knots
CONTROL AND MOVEMENT
There is lower pressure on the forward part of the sail and a net force perpendicular to the sail. The net force propels the windsurfer, but part of this force is to the side of the sailboard. The dagger board keeps the board from moving sideways. The dagger board extends from the sailboard down into the water. The dagger board not only keeps the board from drifting sideways, but it also a device used to turn the board so that one may steer. You steer by tipping it forward and back.
The sailboard doesn’t move across the water by simple a force of wind that pushes on the sail. The process is related to the lift (the net force caused by different pressures on opposite surfaces of a wing or sail)
A sail may be seen as a vertical wing. The wind moves faster over the convex (surface that is curved or rounded outwards) Curve of the front part of the sail than it does when it goes straight over the back part of the sail. If the wind hits the front part of the sail then the windsurfer will travel a greater distance.
I have constructed a model to demonstrate the force of lift.
* 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
1. Fold one sheet of paper in half, but do not crease the fold.
2. 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.”
3. With the pencil, mark an “X” on the center line of the paper about one inch from the leading edge.
4. 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.)
5. Place the 15-cm straw through the hole you just punched. Use tape, if necessary, to hold the straw in place.
6. Tie one end of the fishing line to the middle of a 7.5-cm straw.
7. Pass the other end of the fishing line through the 15-cm straw which is attached to the paper.
8. 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.
9. 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).
10. 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.
11. 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.
FORCES ACTING ON A SAIL
There two forces acting on the sail. Lift and Drag. Lift acts in a perpendicular direction to the Chord Line and we will be discussing it first.
The diagram above shows the sectional profile of a sail with the mast on the right and the leech on the left. The Chord Line is an imaginary line drawn through the mast and leech of the sail profile. The arrow AW represents the apparent wind. The angle ^aa between AW and the chord line is referred to as the Angle of Attack. The way a sail works is very similar to the way an aircraft wing works. The action of the wind on the sail generates lift in the same way as lift is generated when air flows over an aircraft’s wing.
If the angle ^aa is zero, very little lift is generated and there is no power in the sail. As the angle ^aa increases, the lift in the sail increases and one should feel an increased pull in the sail. The increase in lift is in direct proportion to the increase in the angle ^aa, but if this angle is increased beyond a certain point (about 15 degrees), the sail rapidly loses power and stalls. This is referred to as over-sheeting.
Drag on the sail is caused by the wind flowing over it. This force acts in the same direction as the apparent wind. Both lift and drag are proportional to the square of the wind speed. Example: a wind speed of 10 knots will give a lift and drag of 100.
In the diagram, a-b represents the magnitude and direction of the lift generated in the sail. b-c represents the magnitude and direction of the drag on the sail. These two forces added up will result in a combined force represented by a-c. There is an angle between a-b and a-c, shown as ^da which I will call the drag angle. The higher the drag, the larger this angle will be..
It is the forward component that drives the board forward. The other tries to push the board sideways. The angle at which the sail is sheeted in, i.e. the angle between the Chord Line and the board line of travel, is shown as ^sa in the diagram. As the sail is sheeted out, the forward component becomes larger and the lateral component becomes smaller. As the sail is sheeted in, the forward component becomes smaller and the lateral component becomes larger. When angle ^sa is equal to angle ^da, the forward component is zero and there is no force on the sail to drive the board forward. When the angle ^sa is smaller than angle ^da, the forward component is negative and the board will move backwards.
This lift and drag also explains the fact that in strong winds a small sail will go faster than a large one. What happens is that a sailor using a large sail becomes overpowered and he cannot fully sheet the sail in. By switching to a smaller sail which the sailor can fully sheet in, the angle of attack will be at its best ,therefore producing a lift which will be about the same as with the larger sail, but the drag on the smaller sail will be a lot less.
TRUE WIND AND APPARENT WIND
If an instrument for measuring the speed and direction of the wind is mounted on shore then the readings obtained will be the speed and direction of the true wind. If we take this same instrument, and mount it on a boat that is moving through the water, then the readings will be show the speed and direction of the apparent wind relative to the boat
If the true wind is blowing at 20 knots from the south and the boat is traveling at 15 knots in a southerly direction, then the wind speed measured on the instrument will be the sum of the two speeds, i.e. 35 knots and the direction of the wind will be from the south. Similarly, if the boat is traveling at 15 knots in a northerly direction, then the wind speed measured will be the difference of the two speeds, i.e. 5 knots. if you travel at 15 knots in an easterly direction there will be complications and we will have to use something called a vector. A vector is a line with an arrowhead. It can be used to represent the speed and direction of anything you like, whether it is the wind, a boat or a windsurfing board. The length of the line represents the speed and the direction of the arrowhead shows the direction in which the wind or boat is moving.
In the diagram, the vector TW represents the speed and direction of the true wind, BV represents the speed and direction of the boat and AW represents the speed and direction of the apparent wind. TW has a length representing 20 knots coming from the south. BV has a length representing 15 knots going to the east. AW is the result and represents the apparent wind relative to the boat. Its length represents 25 knots. The angle between AW and BV, shown as ^aw, is 53 degrees. The apparent wind therefore has a speed of 25 knots coming from a direction 53 degrees.
It is the apparent wind that acts on the sail, not the true wind.