An obsession with a medieval saint can lead a man almost anywhere.

This is what Timothy Severin learned after he sold all his earthly possessions and sailed the North Atlantic for Ireland to Newfoundland in a 36-foot leather boal, a banana-shaped curragh with a skeletal frame of oak and ash covered by 42 oxhides.

Storms and furious waves threatened to blow him away. A floating ice pack punctured the bat's skin. Whales regularly came up for a close inspection. When killer whales approached. Severin said he was "afraid they might mistake the boal for another whale and take a nip out of the leather. But they decided it wasn't on the menu."

A 36-year-old Englishman an dOxford scholar in the history of exploration. Severin ahd become intrigued with St. Brendan, an Irish monk and seatarer of the 6the century. Some scholars theorize that the fist European discover of North America was St. Brendan - or, more likely, one of the many contemporary Irish seafarers, many of whose exploits came to be attributed to Brendan as legends grew.

So 1,400 years later. Severin followed th saint's supposed route - and made it. "It proves." he said yesterday, "that there's no earthy reasen it couldn't have been done" by St. Brendan and his monk friends, or some other Irish monks centuries before Christopher Columbus and Lief Erikson.

The voyages of St. Brendan and the early Irish missionaries were described in a n early Celtic book, the "Navisatio Sancti Brendani Abbatis," a popular adventure story that first appeared in the 10th century. No archeological trace has been found of Irish explorers in North America but Severin says. "Now there is every reason to look."

Severia said his voyage, besides proving it could have been done all those years ago, also throws a bit of light on the original story. For example, the early missionaries were supposed to have landed on the back of a whale, built a fire, and had lunch. Then the whale swam away, came back, and was friendly. A nice Irish tale, you say.

"Well, it turns out that whales and leather boats have quite an affinity for each other. Every calm day a whole would come by for a close look, anli I mean a close look. But the only concern about whales we had were the killers, a hunting group of killer whales that we fonfronted for a couple of hours."

Severin's first concern - after talking over the plan with his wife, herself a medievalist he had met at Harvard - was scraping together enough money to build his boat. "We sold everything we had - the yacht, our car," he said. But help came in the form of an advance from National Geographic, where he was visiting yesterday and which will publish an article later this year on his trip, and Reader's Digest Press, which will publish a book on the adventure. Besides being an historian and adventurer, he also, handily enough, is the author of six books.

After three years' preparation, on May 17, 1976, Severin set sail in the aptly named "Brendan" with four other experienced hands. A wiry, well-onditioned man, Severin would experience no difficulty, but one man promptly injured the muscles around his heart while rowing and trying to keep the boat on course in a storm, and had to be let off at an Irish prot.

If one can overlook the killer whales, a couple of patches or "really bad weather" when winds hit 75 miles an hour and threatened to swamp the boat, and the seasickness of one crewman, the 1,500-mile, two-month crossing to Iceland proved nothing less than a joy ride. As St. Brendan supposedly did, Severin stopped at the Hebrides and the Faeroes.

But from Iceland on, the going got rougher, so much so that the boat could go nowhere because of contrary high winds. "We had planned to make the crossing all at once, but that was a mistake." Severin said. "So we did what the old Irish seafarers did. We stopped (in Iceland). It's supposed to have taken St. Brendan seven years."

Severin resumed his voyage, a 2,000-mile leg, on May 7 of this year. He had just as rough a time as did Brendan, who supposdly was blown far north among icebergs, "a great pillar of transparent crystal floating in the sea," according to the "Navigatio."

Severin had to contend with The Wave, which he differentiated on the spot from all the other waves that washed over Brendan's replica. This particular wave hit the windblown speck with such intensity that Severin marvels at how the boat stayed upright.

"It just engulfed the boat," he said. "Standing in the helmsman position, I could only see tha mast, nothing more." He was up to his armpits in water. "But there was air trapped underneath us and we bobbed up like a lightbulb," he said. "If we had capsized then, we would have been done with, for sure."

Across to the east coast of Greenland they went, then south for miles along a mammoth edge of packed ice, then southwest toward the coast of Labrador. Nearing there, a piece of floating ice pierced the leather - like a "stabbing," Severin said. It was night; they couldn't find the hole. They bailed until daybreak.

Luckily, the hole was at the waterline, not below. They could sew their boat, and they did, as onewould sew a pair of jeans. Only this was a trifle more difficult: One crewman had to ean over the side and shove the needle through the hull while a second man shoved the neele out. It was a cold job for the first man, hanging in the water.

Of course they brought the proper attire for a possible dip in the frigid waters, as well as alife raft, radio and flares. "The purpose," said Severin, "was to test the boat, not drown ourselves. Although I shoudl qualify that by saying that th eleather boat was very difficult to see, that it does not give a good picture on radar, and that off Greenland there was a very big gap in which there would have ben no way of finding us."

Luckily, the hole was at the waterline, not below. They could sew their boat, and they did, as one would sew a pair of jeans. Only this was trifle more difficult: One crewman had to lean over the side and shove the needle through the hull while a second man shoved the needle out. It was a cold job for the first man, hanging in the water.

Of course they brought the proper attire for a possible dip in the frigid waters, as well as a life raft, radio and flares. "The purpose," said Severin, "was to test the boat, not drown ourselves. Although I should qualify that by saying that the leather boat was very difficult to see, that it does not give a good picture on radar, and that off Greenland there was very big gap in which there would have been no way of finding us."

Less than two weeks ago, on June 26, the "Brendan" landed at Peckford Island, six miles off the Newfoundland coast. If he could have done it. Severin believes St. Brendan, or some Irish seafarer, could have done it. In the "Navigati," which is "really a number of voyages by a number of monks rolled into one seafaring tale." said Severin, St. Brendan arrives at "a country of autumn sunshine, well-wooded, and with a great river gliding into the interior."

North America perhaps?