Standing before the gleaming 12-meter sloop Courageous, Olin J. Stephens II has the furrowed look of a man whose reputation, once again, is on the line. Courageous is Stephens' latest design, and as the United States yachting community this week musters in defense of the America's Cup, it is largely his responsibility to make sure that the 123-year-old cup remains securely bolted to a table in the trophy room of the New York Yacht Club.

For Olin Stephens, of course, the burden is hardly new. The introspective 66-year-old MIT dropout has designed all but one of the postwar cup defenders. Indeed, his domination of 12-meter design is so complete that this year the fiercest competition to see which boat would represent the United States was between two Stephens designs, the Intrepid, built in 1967, and the newer, aluminum-hulled Courageous. After a seesaw battle all summer, Courageous was finally chosen last week.

The Stephens legend began in 1930 when Olin designed and sailed Dorade to a class victory in the Bermuda race. He went on the next year to win the transatlantic race and the Fastnet race, soundly defeating the pride of the British fleet. For that he became the only yacht designer ever honored with a ticker-tape parade upon his return to New York. Since then an armada of swift Stephens designs have brought home the first place silver from every ocean of the globe. But racing isn't all of it. Over the years his firm of Sparkman and Stephens has become the General Motors of boating by designing everything from dinghies to sea-going 100-footers.

Despite his considerable success, Olin Stephens retains a bedrock modesty. His office in midtown Manhattan, furnished with an old wooden desk, two chairs and a plywood drawing table, reflects his Spartan taste. His clients, including notables like the Aga Khan and former British Prime Minister Edward Heath, quickly learn that lunch with Olin often means a Dutch-treat hamburger at a nearby hotel. About his only indulgences are expensive Porsche sports cars and a farm in rural Massachusetts, where he likes to putter in the garden and paint landscapes.

By all accounts, this year's best-of-seven races against the swift Australian challenger Southern Cross will be the toughest defense in years. But even this enormous pressure is not enough to ruffle the composure of a man who once greeted the sight of two $40,000 masts toppling overboard in a single day with a mild "my goodness gracious." Asked if he was worried about the stigma of losing the cup to the Aussies, Olin pauses only momentarily. "It's a boat race," he says matter-of-factly. "If they have the best boat, let them win."