The rules didn't take long to explain.

"No biting, kicking, spitting or throwing sea nettles at each other," said Steve Davis to 40 people in bathing suits, waiting to race around the Chesapeake Bay on surfboards fitted with sails. "Everything else is fair."

Last weekend the beach at Sandy Point State Park in Maryland looked like a burial plot for wrecked sailboats. The sails were fine, gloriously multicolored against the blue water and white sand. But under them, where boats should have been, were narrow boards of fiberglass.

"I thought it looked sort of ridiculous, too, the first time I saw it," said Hayes Harris, a 27-year-old wind surfer from Maryland's eastern shore and one of the competitors in the first board sailing regatta held last weekend in the shadow of the Bay Bridge. "But once you get out on one and catch the wind . . . "

Board sailing, aka wind surfing, advertises itself as "the sport of the '80s." So do most other sports, including shuffleboard. But this sport really has boomed since it was invented in the mid-1960s by two men from -- where else? -- California.

The sport has grown so fast and spread to so many parts of the world that in 1984, if a lawsuit over patent rights is ever settled, wind surfing will be included in the Olympics. It already has a professional tour, with sponsors and enough prize money to support its elite racing class.

"Board sailing is just bursting at the seams," said Tom Babb, manager of a store in Bethesda, one of a dozen in the Washington-Baltimore area that sells boards and offers sailing lessons. At Sandy Point it seemed like half the competitors were involved in some way in the business of board sailing.

"Board sailing is the perfect offseason market for stores like mine," said Gary Breesman, 31, manager of Potomac Ski and Sail. "You can't sell too many snow skis on a summer day in 80-degree heat."

The selling of the sport may make good business sense, but wind surfers with no financial interest in the growth of the sport also praise its virtues.

"It's mobile, it's exhilarating and the air is free," said Janie Brady, 31, a deputy state's attorney for Delaware. Brady jumped from surfing to board sailing last year when she realized she was becoming something of an antique on the surfing beaches. "Everybody kept getting younger and younger except me."

Wind surfers are not exactly a cross section of America. Most are young, trim and athletic. But youth is no requirement. Brady wind surfs regularly with a 69-year-old man near Rehoboth. And Jerry Brown, a 46-year-old Baltimore native with white hair and beard, says he doesn't mind playing the role of the old man of the sea as long as he keeps winning competitions.

Wind surfing enthusiasts claim their sport incorporates the best features of sailing and surfing. You can ride the wind like a leaf, playing air currents like any sailor, on a board that costs as little as $700 new. And when you are done for the day there is no hassle with towing or the expense of a permanent dock berth.

"It's a great sport for women because you can just pick it up and throw it on the roof of your car. You don't need a lot of brawn," said Lynn Hook, a dental hygienist from Alexandria who tried wind surfing while on a vacation in the Caribbean and got hooked. She now teaches classes at Belle Haven Marina on the Virginia side of the Potomac River.

Over the last three years Sandy Point has become a weekend Mecca for wind surfers. It is close to Washington, Baltimore and Philadelphia and enjoys unusually consistent Chesapeake breezes. Sandy Point's beach, which is regularly disparaged by Ocean City-bound bathers as being coarse and unattractive, has finally found an appreciative clientele.