Gerry Spiess, the American who sailed across the Atlantic in a boat not much longer than a ping pong table, is a self-styled adventurer who left home without telling most of his neighbors what he was up to.

"Gerry is a real humble type man," said a friend, Norman Schwietz. "When he left, he only told a handful of close friends. He likes to do things for his own achievement, not for a lot of attention."

Spiess, 39, built his 10-foot sailboat "Yankee Girl" in his garage in White Bear Lake, Minn., a suburb of St. Paul, and quietly launched it from Virginia Beach, Va., 54 days ago. Tacked to a galley wall was a picture of his wife, Sally, and a small prayer which read, "Oh Lord, thy sea is so big and my boat so small."

Neither his former bosses at 3M Co., where he taught service technicians how to repair business machines, nor his hometown newspaper, the White Bear Press, knew he had left.

"It was all very low key. The fellow lives about a block away from me. I knew he was building a boat, but I didn't have the slightest idea he had actually taken off until about a week ago," said White Bear Lake Mayor Brad Stanius, who was frantically trying to organize a welcome home parade and celebration yesterday.

Spiess' friends described him as a soft-spoken, "gutsy guy," who looked and acted more like a school teacher than a man willing to brave the dangerous Atlantic seas for weeks at a time. "I've met him at company parties and he always seemed like a very average guy," said Yvonne Johnson, who worked with his wife.

Behind the quiet exterior, however was an intensely driven adventurer, a man who formerly raced motorcycles and once tried to build a single-engine airplane. His wife, the assistant manager of a computer firm, shared and supported his wanderlust, both financially and emotionally.

"People ask why he does it, and the only answer I have is that it's much like climbing the mountain," she said in an interview with the White Bear Press before she left to meet him in Falmouth, England. "If you ask the question, you can't understand the answer.

"It's a challenge. It's a thrill that a lot of people don't put into their lives anymore."

Spiess has been interested in sailing for more than 20 years. Although he once sailed down the Mississippi River and across the Gulf of Mexico to South America, he was basically a weekend sailor, who didn't even belong to the local yacht club in his hometown.

The Yankee Girl is the smallest sailboat ever to complete the Atlantic crossing in modern times. The fiberglass and wood hull is 10 feet long and is shaped like a wedge of pie. It has a 14-foot mast, a radiotelephone and a 4-horsepower motor. Beneath its check is a narrow bunk, a small chart table and a tiny galley.

For company on his voyage, Spiess took a collection of Mark Twain books and a series of recordings of old radio programs. For food, he carried canned soup and fruits, along with homemade beef jerky.

His wife's biggest concern during the trip was that he might be run over by a large cargo ship while traveling in shipping lanes.

But she said she was never really worried that he wouldn't complete the voyage. "I knew the man that built that boat and the years of effort and study that he put into its construction," she said in one interview. "I never considered myself a potential widow."

"The voyage," she added, "has captured the imagination of a lot of people. I think a lot of people picture it as a carefree jaunt and they don't realize how much personal effort went into both the planning and the execution. I don't think it is the feat that's important. The determination to make it happen is realy what this is all about."

Spiess began building his boat more than three years ago, and first tested it in While Bear Lake, a body of water that is about two miles wide at its widest point, in the fall of 1977. Friends would come over to watch him work, but by their accounts the boat was entirely his undertaking.

"A lot of people laughed at him. They thought he was crazy," said Schwietz."But I knew he would make it. His the type of person who has a calm head about him.'" CAPTION: Picture, On reaching Falmouth, Speiss hoists the U.S. flag.