How does the shape of a water ski help keep someone on top of the water? How fast would a boat have to be traveling to pull a barefoot water-skier?

Waterskiing
How do water skis stay up?
SuChin flies over the waves on water skis.
Segment length: 6:15

Insights

Who says you have to be in the sky to fly? Your feet may never leave the ground (well, the water), but the amazing sensation of waterskiing definitely compares to soaring through the air.

The first person to master this sport was an 18-year-old named Ralph Samuelson. In 1922, Samuelson tried skiing over water first on barrel staves and then on snow skis. He finally found success on pine boards 2.5 meters (8 feet) long and 23 centimeters (9 inches) wide. Water-skiing really picked up speed after World War II, when affordable, high-horsepower outboard motors meant more people could own the fast boats needed to tow water-skiers.

But how do you stand on water? The pressure on top of the water skis (including the weight of the ski, the person, and the air above both) remains constant, whether the skier is at rest or moving. However, as the skier picks up speed, the water pushes against the bottom of the skis. The larger or longer the ski, or the faster a boat is traveling, the easier it is for a skier to stay up on the water. The average speed to keep a 68-kilogram (150-pound) adult afloat on water skis would be 32-40 kilometers per hour (20-25 miles per hour). But for barefooting, where all 68 kilograms of weight are concen-trated on the soles of two feet, a speed of 56 kilometers per hour (35 miles per hour) is necessary.

Different lengths and edges of water skis offer different combinations of speed and control. For instance, a beginning skier would want two longer skis for stability, with flat bottoms for riding high and fast on the wake. A more advanced skier could switch to just one ski, called slaloming, and use a beveled bottom for more controlled turns. And highly experienced skiers may choose a ski with a concave bottom, which holds turns by cupping water underneath.

Some expert skiers even add an underwater rudder with wings set at a particular angle to aid turning. These wings create drag and slow down the ski like a brake, making it easier to turn. Then, when the skier leans back to come out of a turn, the wings become parallel to the flow of water and offer minimal resistance.

Connections

1. Why did Ralph Samuelson's first attempts on barrel staves and snow skis fail? What technological advances allow skiers to go completely barefoot or to build pyramids of people on water skis?
2. Trick skis are short and stubby and have beveled edges both on the bottom and top. How would this shape help a skier do side-slides, somersaults, and 180-degree turns?

Key Words

barefooting to water-ski on bare feet
bevel rounded edge on some types of water skis
concave inward-curved bottom on some types of water skis
edge side of a ski, used for acceleration and deceleration
freestyle type of water ski competition using a jump ramp for somersaults and other tricks
kneeboard extrawide water ski which is ridden in a kneeling position
rudder a sort of fin attached to the bottom of a ski, similar to ones used to steer boats
slalom to ski on a single ski, or the ski itself
wake the "V" wave behind a boat

Main Activity

Skimming the Surface
Mold an aerodynamic water ski from clay and see if it stays afloat.

Every time water-skiers cut into a curve, fluid dynamics are at work to keep them upright. In this activity, you'll work with-and against-fluid dynamics to learn how water skis are designed for different purposes.

Materials

1. First, experiment with the buoyancy of your clay Mold different amounts of clay into aerodynamic and nonaerodynamic shapes Drop them into the water from 30 cm (1') above the surface Do some shapes sink faster than others? Does a higher or lower dropping distance affect how quickly they sink?

2. Tie the button onto the end of the thread Shape the clay around the button into your most aerodynamic shape from step 1 Using constant speed, pull the clay along the surface of the water Does it spin or travel straight? Now try your most nonaerodynamic shape, using the same amount of clay, and pulling it along at the same speed as before Is it more difficult to keep the speed constant? Which of the two sinks faster when you stop pulling?

3. Start thinking like a water ski designer First, experiment with different bottom shapes-square, beveled, and concave What happens when you alter the bottom of one side of your clay "ski" but not the other? Can you create a ski that turns right while your "towrope" travels straight? One that dives underwater?

4. Try changing the point where your thread exits the clay Move it up and down in front, as well as back and forth in the body What changes? Add weight with pennies at different places on the clay Can you build a "ski" that still moves straight and stays afloat with five pennies at the tip? How does your design change when you move the pennies to the back?


Dig through some old clothes and select about five of different fabrics, choosing pieces large enough for you to sit on Take them to a park that has a large slide Experiment with the friction level for each fabric by sitting on it as you slide Does the friction level change if you switch to squatting rather than sitting? What happens when you only half-sit on a slippery fabric?


Water-skiers hold onto the towrope using an opposed grip, rather than an overhand This increases power and makes it easier to pull your arms close to your body and control speed and balance Make a tug-of-war towrope using 30-cm (1') lengths of plastic pipe as handles on each end Try pulling against a friend, once with your hands gripping with all your fingers the same way, and once with the opposed "baseball bat" grip Do you notice a difference in your pulling power?


Water-skiers always wear a personal flotation device made of closed-cell foam covered with vinyl or nylon fabric Using a blunt table knife, carve different shapes out of Styrofoam® Fill a sink with water and experiment with the different sizes and shapes Do different shapes seem to offer more resistance to sinking when you weigh the pieces down with coins? Can you create a shape that sinks?


Powerboat engines are measured according to their horsepower, with one horsepower being equal to 550 foot-pounds per second You can measure your own horsepower and compare it to a boat engine First measure the vertical height of a flight of stairs Have a friend time how quickly you can run up the stairs Your horsepower equals:

your weight in pounds x stairway height in feet
number of seconds it took you to run up the stairs


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