How does ice surfing compare to wind surfing? How fast can ice surfers go? How is it possible for ice surfers to travel into the wind?

Insights

Grab your board and hit the beach-it's surfing time again. But instead of baggies and a tank top, you might want to put on a parka and thermal socks, because the kind of surfing we're talking about only happens on solid water. That's right, the topic is ice surfing, and it's the hottest sport on frozen lakes around the world.

Ice surfing is really a hybrid sport, mixing the speed of ice boating with the agility and power of wind surfing. The ice surfing board is similar in size and shape to a big skateboard. Most boards have two blades in the back for stability and either one or two blades in the front to control the steering. Just like the wheels on a skateboard, the blades on an ice surfing board are mounted on flexible "trucks" so the rider can control the steering by leaning forward or backward while standing sideways on the board.

As with ice skates and ice boats, the blades on the ice board are quite sharp on the bottom, putting a great deal of pressure on the ice below. This pressure, combined with the friction between blade and ice, causes some of the ice to melt directly below the blade. A thin film of water lubricates the ice under the blade, which helps it glide smoothly over the ice. This really cuts down on the drag and means that the ice surfer is actually riding on liquid water, not solid ice. Because there is far less friction involved, ice surfers can go much faster than wind surfers, often approaching speeds of 113 kilometers (70 miles) per hour.

Just like wind surfers, ice surfers power up on an ice board by using a sail. In fact, most ice boards use the same type of sail as wind surfing boards. But because there is less friction on the ice, an ice surfer can get away with a much smaller sail. To make the board go, the rider sets her or his back to the wind and sheets in by pulling in on the boom-a bar attached to the bottom of the sail. As the sail catches the wind, the board starts moving forward. The highest speeds are attained when moving nearly perpendicular to the wind. By trimming the sail at the correct angle, the ice surfer can actually move almost three times faster than the wind itself. It's not possible to sail directly into the wind, but by tacking at an angle, surfers can zigzag across the wind, heading upwind little by little.

Connections

1. How do you think an ice surfing board would work if it were fitted with skis instead of blades?
2. Do you think it would be possible to adapt the design of an ice board to make a two-passenger commuter vehicle? How might you do it?

Key Words

boom horizontal post on the bottom of the sail which the rider uses for control
drag force of resistance between the bottom of the blade and the surface it's riding on
luff flapping of the sail when it is pointed directly into the wind or the sail is sheeted out
mast vertical pole, extending up from the board, that supports the sail and boom
power up to lift the sail and capture wind to get the board moving
sheet in to pull in on the boom so that the sail can capture wind and move forward
sheet out to let the wind out of the sail, thereby slowing the board
tack to sail in a zigzag pattern across the wind to move upwind
trim sail to adjust the sail to either capture more wind or let it out

Resources

1. Evans, J. (1992) Windsurfing. New York: Crestwood House.
2. Hewitt, P. (1992) Conceptual physics (2d ed.). New York: Addison-Wesley.
3. Robberson, K. (1988, Jan) The thrill of sailing on ice. Women's Sports and Fitness, p. 82.

Additional resources

1. Newton's Apple Show 1106 (windsurfing).
   GPN: (800) 228-4630. Or call your local PBS station to find out when it will be rerun.
2. 3-2-1 Classroom Contact: Friction: Getting a grip. Videotape. GPN: (800) 228-4630.

Additional sources of information

1. Fiberspar
   West Wareham, MA 02576
   (508) 291-2770
   (ice board manufacturer)
2. U.S. Windsurfing Association
   PO Box 978
   Hood River, OR 97031
   (503) 386-8708

Main Activity

While it is impossible to sail directly into the wind, it is possible to move upwind by sailing at an angle across the wind. This is called tacking. Construct a simple model sail and test it to see what your minimal tack angle is.

Materials

• small lump of clay
  
• protractor
  
• pair of scissors
  
• transparent tape
  
• metric ruler
  
• unsharpened pencil
  
• clear plastic wrap
  
• plastic straw
  
• kite string about 30 cm (1') long
  
• blow dryer

1. To construct your sail, cut a piece of plastic wrap into an equilateral triangle whose sides are each 15 cm (6") long. Cut the straw to
   14 cm and roll the clay into a ball. Insert the pencil into the clay so that it is standing vertically. This will be your mast. Wrap one side of the triangle around the pencil so that it overlaps about 1 cm and tape it in place. Thread the string through the straw and tie one loose end around the base of the pencil where it sticks into the clay. Hold the straw out from the mast and tape the bottom edge of the plastic wrap to it. The straw will be your boom.
   
2. Place your protractor with the zero point directly under the mast and press the clay down to hold it in place. (The boom should line up parallel with the zero line on the protractor.) Place your hair dryer's barrel 30 cm (1') in front of the mast. Turn it on low and aim it directly at the mast. The sail should luff. If the sail and mast blow over, move the dryer farther back.
   
3. Slowly pull the loose end of the string out at right angles across the protractor and watch the sail. As soon as the sail begins to fill with air, measure the angle that the boom makes with the zero line on the protractor. Repeat this so you get an accurate reading, and then average your readings.
   
4. Place the hair dryer directly behind the sail at the same distance that you had it in front of the mast. Start with your sail out perpendic- ular to the barrel of the hair dryer and slowly use the string to pull the sail in until it ends up parallel with the wind direction. Watch the sail and mast and feel the tug of the string as you do this.

Questions
1. At what angle did the sail first fill with air when you were running into the wind? What does this tell you about the minimum angle at which you can sail into the wind?
2. When you were sailing with the wind, at what angle did the sail appear to have the most power?
3. Can you calculate how far you would have to sail to move directly upwind one kilometer?
4. Why do you get maximum power out of a sail when it is at an acute angle to the wind?

How does the amount of water or surface roughness affect how much friction there is between a blade and the ice surface? Freeze water in a rectangular pan. Try dragging a brick across the ice, using a spring scale to measure the amount of force needed. What combination of water and ice produces the least resistance? Which produces the most?

Ever since the Egyptians first built the pyramids, people have been looking for ways to reduce friction. Today, many superlubricants exist for specialized purposes. Research some of these products and explain why each can work under certain conditions.

Research how sail designs for boats have changed over time. What were some of the disadvantages of old-fashioned square riggers compared to modern vessels? Why do racing yachts have three or four different sails? What purpose does each one serve?

Newton's Apple is a production of KTCA Twin Cities Public Television.
Made possible by a grant from 3M.
Educational materials developed with the National Science Teachers Association.