ANNAPOLIS -- The six novices bobbed in and out of the waist-deep water through the afternoon, intent on mastering the graceful sport of windsurfing and the balancing skill it requires.

"Everyone learns balance," instructor Michael A. Murray said with encouragement. "Believe it or not, it's not something you were born with."

Despite the frequent dunkings, Murray's windsurfing classes at his Chesapeake Boardsailing School here, which attracts a large number of Washington residents, and at Fort Smallwood near Baltimore are booming. Last summer, his staff gave about 500 lessons and this summer he estimates the number will double.

Although the experienced teachers made the sport look easy, the students at a recent afternoon class -- who ranged in age from the late teens to their early forties -- gave ample evidence of the difficulty in balancing their bodies on a surfboard, while also trying to steer the attached tall sail with the available breeze.

Betsy Ferris, a nurse from Baltimore, held up two very red and calloused palms. "It's hard," she said. "It takes brute strength. But I saw a jellyfish in the water, and that was incentive to stay on the board."

A 40-year-old Washington woman who asked not to be identified admitted to having a sore back. "If you're doing it properly, your back shouldn't hurt," she said. "You should be using your legs. But you're not on the first day."

Windsurfing, also known as boardsailing, originated in California in the early 1960s as the brainchild Jim Drake, an engineer, and Hoyle Schweitzer, a vice president of a computer software firm. They were seeking to combine the sports of sailing and surfing.

Drake, who helped design ultrasonic aircraft, and Schweitzer, spent nearly seven years trying to perfect the "rig."

According to Bill Clawson, manager of Windsurfing Unlimited in Bethesda, an equipment store, a million people have tried windsurfing in the United States; most are on the West Coast, Hawaii and parts of the East Coast. The sport is more popular in Europe, he said.

The windsurfer stands on a board weighing between 12 and 45 pounds. Windsurfing boards are heavier than conventional surfboards and longer, running up to 11 feet, compared with six feet for a conventional board. On either side of the sail's mast is a horseshoe-shaped "boom" used for steering. Never sailing directly into the wind, the surfer steers the sail first, and the board follows.

W. Mark Weldon, one of the three instructors at the school, has been windsurfing for six years. "Balance is definitely the toughest thing to do," he said. "More women take classes, and they're usually more agile and better listeners. Men tend to be more macho and think they can do it. People who don't have any sailing experience seem to do better, too."

After the first few lessons, Weldon said most people notice sore feet and ankles because their efforts to stay on the board uses muscles not commonly exercised.

As one student said, "We're wind suffering right now. When we're better, we'll be windsurfing."

Christine Olsen, who works for a doctor in the Annapolis area, has taught the sport on weekends for the past three years. She said that students can learn to stay on the board in a four-hour lesson and in the second lesson learn how to steer and how to adapt to wind conditions.

Sure enough, all six students were up and sailing, for at least some time, at the end of the class.

Jim Crabtree of Silver Spring said, "I stayed up for three seconds . . . tomorrow, I'm shooting for a half hour."