Forty board sailors darted back and forth on the Potomac, their brightly colored sails poised for a strong breeze to drive them into the lead. Several novices, caught off guard by a gust, tumbled over. The more experienced board sailors sprang ahead, but those left behind were not down long. Pulling their sails up out of the river, they dashed off once more.

The Potomac Cup Regatta last weekend brought about 50 people to the river for the chance to race and discuss board sailing, which became an official Olympic event at the 1984 Games in Los Angeles. Said Bob Kozac, the organizer of the three-year-old Regatta, "We really like to . . . show just how good the Potomac is for the sport."

Board sailing, which used to be called windsurfing, has taken off in the Washington area, and enthusiasts say the hot spot is Alexandria. "The best board sailing is out of Washington Sailing Marina in Alexandria," said Bob Redmond, an instructor at the marina. He said the wind there is steadier, there is less current and there isn't any problem with hydrilla.

Last fall, a group of board sailers who use the marina formed the Washington Board Sailing Club, the only such club in the area. It now has about 40 members, some of whom sail all year -- except, of course, when the river is frozen.

Ian Jones, the club's coordinator, said that when the club was founded, it consisted only of a list of people interested in the sport. Each week he would call the members and try to get them to come to the river to race and work on technique.

The member's ages range from 12 to about 60, and they represent a variety of occupations, including law, medicine, consulting, academics and the military.

Joy Yanagida, an Arlington lawyer, is one of the eight women members. Yanagida has been board sailing for two years. "I'm in the club just to learn more," she said. "Board sailing is a demanding sport, and I think competition is a real spur to learning. But they the members are not just competitive."

Nonmembers are encouraged to participate in the club's Sunday races, which usually draw more than a dozen participants. Yanagida described board sailing as "very addictive."

One member, Alexandria resident Aurelio Roca, plans to try to sail the Potomac later this month from Alexandria to the Chesapeake Bay, a distance of more than 100 miles, in four days. He plans to leave Saturday, May 24, at 9 a.m.

Roca, a dentist who practices in Arlington, has been board sailing for five years. He works out at a local gym using a training technique, developed by the U.S. Olympic board sailers, that consists of hanging from a bar similar to the boom on a windsurfer, as the boards are called. The exercise strengthens his grip and builds up protective callouses.

"Board sailing is an intense form of sailing," he said. "Everything happens so fast, and it's quite physically exerting." A flotilla of friends accompanying him will carry supplies and provide a place to sleep.

There now are more than 250,000 board sailors in the United States, up from about 10,000 in 1979. Redmond said he taught and certified 300 people at Washington Sailing Marina last year. "Most of the people are over 30, and my oldest student was 81," he said. "I've never had anyone not sailing after four hours."

Windsurfers cost from $600 to $1,500, and average $1,000. Board sailors on the Potomac and on Chesapeake Bay usually use 14-foot boards, although those who want a racing edge on windy days use 8 1/2- or 9-foot boards. But while short boards may be faster in strong winds, they sink in lighter winds -- and this area is notorious for variable winds.

Board sailing became legal on the Potomac in 1983 when windsurfers were legally designated as boats. Previously, board sailing was considered a water contact sport, and such sports were banned in D.C. in 1972 because of health risks associated with pollution in the Potomac.

The Washington Board Sailing Club holds informal races Sunday mornings and technique clinics on Wednesday nights. The Belle Haven Marina also holds races each Wednesday night.

Jones explained why he likes the sport. "The board fits on top of the car and takes you 20 minutes to rig. Then you're ready to go out and sail for two hours. The equipment is simple, and the sport is progressive. You really get out of it what you put into it."