Emil (Bus) Mosbacher Jr., who skippered two winning teams in the America's Cup races and shepherded visiting diplomats through official Washington as the State Department's chief of protocol in the Nixon Administration, died yesterday at his home in Greenwich, Conn. He was 75.

The cause was cancer, his family said.

Mr. Mosbacher was the chairman of the first Operation Sail, which brought to New York Harbor in 1976 a procession of tall ships not seen since rough-talking deckhands dominated the docks and men-of-war dominated the seas -- square riggers trailed by an armada of more than 200 schooners, yawls, ketches and sloops. He also helped organize two other Operation Sail events, in 1986 to coincide with the 100th anniversary of the Statue of Liberty, and in 1992 to mark the 500th anniversary of Christopher Columbus's first voyage to the New World.

The son of a Wall Street stockbroker who pulled out of the market just before the 1929 crash, Mr. Mosbacher oversaw his family's oil, natural gas and real estate business after serving on a Navy minesweeper in the Pacific in World War II. It was one of the few engine-powered craft he spent much time on. His father, who belonged to the Knickerbocker Yacht Club and helped work out the specifications for the Interclub class of racing sloops, had introduced him to sailing on a tiny flat-bottomed catboat on Long Island Sound as a child.

Developing skill in tacking and maneuvering, Mr. Mosbacher progressed from one class of sailing vessel to the next, winning prizes along the way and ending up at the helm of the 33-foot-long sloops that represent the major contenders in sailing on the Sound -- the International One-Designs. He won eight consecutive season championships in the International Class in the 1950's.

But he also won world attention for his handling of the aging 12-meter yacht Vim in the America's Cup selection trials in 1958, gaining greater fame in a lost cause than any yachtsman since Sir Thomas Lipton. By 1962, Henry Mercer, a shipping magnate who owned the sloop Weatherly, invited him to take command and race in the America's Cup trials.

Single-masted sloops like the Weatherly -- more than 65 feet long, with a mainsail and spinnaker -- are designed to an international mathematical formula that governs length, girth and sail area. But Mr. Mosbacher did some calculations and removed what he considered to be excess weight, from a two-pound wind gauge that had perched atop the 90-foot mast to the previous skipper's pipe rack. This ended up saving more than half a ton, which let Mr. Mosbacher put lead in her keel for greater stability in strong winds.

Weatherly won its elimination trials against Columbia, Easterner and Nefertiti and was selected to defend the America's Cup. Mr. Mosbacher heard the news from two of his sons -- Emil 3d, who was 10, and R. Bruce, who was a year younger -- who ran into his bedroom as he was dressing after taking a shower.

''You remember when you wanted a red bicycle for Christmas real bad,'' Mr. Mosbacher said, ''and then Christmas morning finally came, and you crept downstairs and there it was?''

Weatherly went on to win the America's Cup after taking a crucial fourth race by 26 seconds, the smallest margin in the race's history. But the months of trials, plus the pressure of the four-of-seven series, had left him exhausted, and he said ''Never again'' when asked about a second try at the America's Cup.

But in 1968, he was back at the helm, in the brand-new 12-meter Intrepid. And, after fending off a team of Australian challengers, the America's Cup remained bolted in its glass case in the New York Yacht Club in Manhattan.

To his hand-picked crews, he was a sharp-eyed perfectionist. ''Most of yacht racing is study, work and development,'' Mr. Mosbacher said. ''There are no radical breakthroughs, like suddenly discovering you can hit a golf ball farther if you use the back of your club. It is a combination of chess and bridge, with a certain amount of physical ability thrown in. It is tactics, strategy, organization and a happy association with nice people.''

Mr. Mosbacher was appointed chief of protocol shortly before Richard M. Nixon's inauguration in 1969 and served until 1972. In 1973, Gov. Nelson A. Rockefeller named him to head the New York State Racing and Wagering Board. Mr. Mosbacher resigned after six months on the $75,000 annual salary, which was second among public servants only to that of the President and the Governor. The money became an issue in the 1974 Governor's campaign, and his inexperience with thoroughbreds was criticized in horse-racing circles. He resigned early in 1974, after Malcolm Wilson had replaced Rockefeller in Albany.

He is survived by his wife, Patricia; three sons, Emil 3d of San Diego, R. Bruce of Woodside, Calif., and John of Greenwich, Conn.; a brother, Robert, of Houston, who was United States Commerce Secretary in the Bush Administration; a sister, Barbara, of Manhattan, and two grandchildren.

Emil Mosbacher (1922)