To cross the ocean alone is challenge enough for the boldest of sailors. To make that crossing in a 10-foot sailboat built at home is to call into question the mariner's sanity. An American named Gerry Spiess, with all faculties apparently intact, did it anyway, and he spanned not one ocean but two. In 1979 he piloted his fiberglass-plywood sloop, Yankee Girl, 3,800 miles from Virginia Beach, Va. to Falmouth, England in 54 days. Last year he topped himself with a five-month transpacific solo voyage that took him 7,800 miles from Long Beach, Calif. to Sydney, Australia.

Spiess, 42, is currently a star attraction on the winter boat show circuit, and his book Alone Against the Atlantic (Control Data Publishing, $12.95) is selling briskly, with a 55,000-copy first printing. Yet in spite of his daring, and the fame it has brought him, Spiess is not obsessed with heroics. "I'm very cautious, very careful," he insists. "I don't take any chances." If there is a philosophical underpinning to his adventure, it lies in his conviction that ordinary mortals, when properly motivated, can achieve almost anything. "Human capabilities," he exclaims, "are just amazing!"

So in her own way is Yankee Girl, the tiny vessel that Spiess admits looks like "a sawed-off pumpkin seed." When he built her in his garage in 1977, he wanted a boat that could not be capsized, that would bob on the waves rather than plow through them, and that would set the Guinness record for the smallest sailing craft ever to cross the North Atlantic. Yankee Girl is only five and a half feet abeam, but for his Pacific journey Spiess managed to fill her with 500 cans of food, 24 gallons of drinking water, 54 gallons of gas for an outboard motor (to be used when becalmed), plus radio and navigational gear, clothing and spare parts. The room left over for the skipper, says the 5'10" Spiess, was "comparable to the space you'd have under a card table. It's like packing a suitcase and then getting into the suitcase itself."

Conditions aboard Yankee Girl on the high seas were often "utterly miserable," he says. Because of the constant motion everything had to be strapped down, and in foul weather the closed cabin became suffocatingly stuffy and humid. Spiess had a small butane-fueled stove for cooking (his favorite meal: Dinty Moore beef stew, for which he later filmed a TV commercial), but only salt water for washing.

The discomfort was less of a problem than the need to be constantly vigilant. "There was always a certain amount of fear," Spiess says. "You can't afford mistakes and, above all, you can't afford to hurt yourself. I'd sleep only an hour at a time. You get to the point sometimes," he admits, "where you scream and cry for release from the frustration." On his Atlantic voyage, Spiess braved terrifying storms that churned up 17-foot waves. In the Pacific, he reports, "The dangers are the reefs and shoals. You can get caught up in those breakers and nothing can save you." Loneliness heightened all other difficulties; when he made radio contact with a ham radio operator in Honolulu after 18 days of solitude, he broke into tears. "It was such a relief to be able to talk to someone," he explains.

A native Minnesotan and the son of a plant manager for the 3M company, Spiess is a Phi Beta Kappa graduate of the University of Minnesota with degrees in psychology and education. After serving as an Air Force missile launch officer, he never found a civilian job that fully satisfied him. Prone to stomach ailments("I get ulcers and things"), he sought relief in sailing and home boat building. But a 1974 attempt to circle the world in a self-designed trimaran aborted after only 53 hours at sea, leaving him "exhausted, hallucinating and close to complete collapse."

Still he pursued his quest. Spiess gives generous credit to the understanding of his parents and of Sally, his wife of 20 years. The couple have no children, and waiting and worrying at home in White Bear Lake, Minn. hasn't been easy on Sally Spiess, a data programming consultant. "But," she says, "I decided I'd rather have a husband who's happy, healthy and following his dreams."

At this point Spiess is under little pressure to do anything else. Royalties from his book and lecture fees "allow me to do virtually anything I want," he says. Is he thinking of another voyage in Yankee Girl? "No, it's too hard to live on the edge 24 hours a day," he says. Then, after a pause, he adds, "I've taken up flying."