For nine months of the year, he is a public personality, a trusted and familiar face to virtually all Americans who watch television. During the other three months, when he is at leisure, he is an avid sportsman, enjoying the one activity he cherishes more than any other, sailing. Walter Cronkite, the CBS newscaster and confidant of presidents, took a personal hand in the design and construction of his 42-foot yawl, the Wyntje, and dreams of the day when he and his wife Betsy will be able to sail it on a leisurely trip around the world. Until then, Captain Cronkite enjoys sailing the Wyntje up the coast of Maine during the summer, and back to Fort Lauderdale, Fla., for the fall.

Although Cronkite isn't involved in competition, he drove racing cars at one time, giving that up for boats, an activity he was able to share with his family. The Wyntje, named for a Dutch ancestor of Cronkite, was the sixth sailboat acquired by the newscaster and has a fiberglass hull and a 55-foot mast.

The soft, soothing delights of sailing provide a perfect antidote for Cronkite's other world of turmoil and trouble, one that has infatuated him since youth when he was a combat correspondent for United Press. During World War II, the young and reckless wire service reporter flew on the first B-17 bombing raids over Germany, parachuted into the Netherlands and landed with the Allied forces in Normandy. Cronkite later was chief correspondent at the Nuremberg war crimes trials and was UP bureau manager in Moscow before joining CBS in 1950.

As a sailor, one of Cronkite's biggest thrills came when he got to sail one day with the Swedish 12-meter candidate for the America's Cup in 1977. The America's Cup is the most prestigious prize in sailing, and has been retained by the United States ever since the competition was initiated in 1851. Making the occasion perfect for Cronkite was the fact the Swedes allowed him to take a turn at the wheel. A year before that, Cronkite sailed from Bermuda on the Danish tall ship Danmark to mark the Bicentennial Operation Sail when some of the world's most elegant craft came to New York Harbor.

WALTER CRONKITE Born Nov. 4, 1916, St. Joseph, Mo.

Photo: Joy at the tiller