Sailboat Drag

SCIENCE CONCEPT:

A sail on a boat causes increased drag. When the wind blows, the increased drag caused by the wide surface of the cloth or plastic pushes the boat and makes it move.

STUDENT OBJECTIVE:

The student will explore how drag effects the sailing of a boat when the wind blows.

OVERVIEW:

In this activity, the student will make a small boat with a cloth sail. After making the boat the student will use a fan to create wind to make the sailboat move to see how drag works. The boat will be made out of a piece of Styrofoam, the mask of the sailboat out of a wooden skewer (used for cooking shishkebob), and a small piece of sheet for the sail.

PREPARATION TIME:

15 - 20 minutes.

LESSON TIME:

35 - 45 minutes.

TEACHER PREP:

Gather the materials that you need for the experiment. You will need clean, empty Styrofoam trays (that meats are sold on in the grocery store), wooden skewers (that are used for shishkabobs), a small piece of a clean sheet, fan, large container of water, and pen.

WORDS TO KNOW:

drag

windward

square sail

triangular sail

pushing force

pulling force

TEACHER TEXT:

When a ship has many sails, like a windjammer clipper ship, the wind can make the ship move through the water quickly. When the wind blows, increased drag caused by the wide surface of the cloth pushes the boat and makes it move. At night at sea, the sails of the ship will be lowered so that there will not be a lot of movement. Sometimes, there will be a need for some movement at sea so some of the sails of the ship will be left at half mast.

Although flight is a recent human achievement, the wind was harnessed for transportation long ago. Although refinements continued, the shape of a sail remained the same for nearly 4000 years. Its biggest aerodynamic drawback was that, although it could be angled to achieve maximum thrust, the square sail received air only from the rear - the boat was pushed by the wind.

Sailing was revolutionized in the 9th century A.D. by the lateen or triangular sail, probably invented by Arab seamen. Hung fore and aft (front and back) of the mast and easily shifted, the lateen sail received wind on either side - in effect, the boat was pulled as well as pushed. A triangular sail allows a boat to move toward the wind (windward). A boat is moved in a windward direction by using forces that are created on each side of the sail. This total force is a combination of a positive (pushing) force on the windward side and a negative (pulling) force on the leeward side, both acting in the same direction.

At sea level air pressure is 2,116 pounds per square foot. When the air flow on the leeward side of the sail is increased, you recall that air pressure decreases. Suppose if decreases by 4 pounds per square foot. Likewise, air pressure on the windward side increases - let's say by 2 pounds per square foot. And even though the leeward pressure is negative and the windward is positive, they both work in the same direction. So now we have a total of 6 pounds of pressure per square foot. Multiply that by a 500 square foot sail and we've created a total force of 3000 pounds of pressure on the sail.

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Last modified: Sat Dec 20 15:10:43 PST 1997