The last (and most scholarly) of the Vikings is at sea again. As usual, his ship is prehistoric, crude and fragile, theoretically unsuited to its assignment and potentially a death trap. As usual, the voyage is through time (at least mentally) as well as space, into situations undreamed of even by Leif Ericsson.

Since "Kon-Tiki" a generation ago, Thor Heyerdahl has been combining his varied talents as archeologist, sailor, writer and adventurer to plunder the past for its secrets and transform them into best sellers. It is a pleasant, if not exactly a relaxing way to earn a living, and it is not hard to understand why he leaps at new pretexts for repeating yet again what is always essentially the same process. Heyerdahl fans may imagine a subliminal tone of triumph and relief (the highly specialized junkie with a new fix in sight) in the cool paragraph where, after stating his basic theoretical problem, he comes up with his usual tried and true solution:

"Only a practical lifelike test could give the answer. I decided to carry out my project, and the museum staff convinced their ministry that to reconstruct a prehistoric vessel such as I had planned was a sensible thing to do. I was then granted permission to harvest reeds in the marshes, to import equipment free of customs, to assemble the expedition's crew in Iraq irrespective of nationality, and we would all be guests of the country until we sailed away."

Heyerdahl's chief deficiency is also his chief attraction: the de'ja vu quality of his material, which keeps faithful readers coming back (like the author himself) for more. But within the cliche' framework, he has always managed to produce interesting variations of scenery and incident to buttress his recurring thesis.

This time, his five-month, 6,000-kilometer odyssey begins in a fascinating environment: among the Madans or Marsh Arabs of southern Iraq, who live in a watery world unimaginable to their desert-dwelling brethren to the north. It continues through the construction of the ship, Tigris, its launching (sailing under a United Nations flag) and an eventful, often dangerous journey through the Persian Gulf and the Gulf of Oman, across a long stretch of open ocean to the Gulf of Aden and a dead end in Djibouti (a "little nation, not much more than a good port surrounded by a small piece of desert").

Some dangers were expected and did happen, including storms and blankets of oil-based pollution covering the ocean for miles and miles. But they were probably not as serious as the unexpected problems: occasional warships, for example, and enormous supertankers that could have crushed the tiny ship without even noticing it. And, of course, politics. Going through the Gulf of Aden, they had to sail a tightrope, following "the midline . . . 900 miles long, without touching the forbidden lands on either side." Somalia was at war with Ethiopia, so they had to avoid drifting into Somalian territorial waters and thought they would sail close to the Arabian coast; then word came by radio that South Yemen, on the Arabian side of the Gulf, also had closed its borders. "We have had no cooperation from either the South Yemen or Somalian governments," said a radio message from their contacts in London. "Stop." The "Stop" was a punctuation mark, but it must have sounded to the men on the Tigris like an order. Even an appeal by the United Nations produced no guarantee of their safety. Finally, unable to sail from Djibouti, they burned the craft (with the cooperation of the government) as a symbolic gesture of protest against war and the arms race. He mentions in passing that there was no room left for another boat in the Kon-Tiki museum anyway.

The voyage ended with many questions still unanswered (including the origins of civilization, which was part of the quest) but at least a few probabilities were established: That modern Bahrein seems to be the fabled "Dilmun" of Sumerian legend. That a ship built of the reeds which grow at the site of ancient Sumeria, where the Tigris meets the Euphrates, can in fact be sailed from its point of origin to Djibouti. More generally, that flimsy, primitive ships have remarkable, unsuspected capabilities; perhaps, that the limiting factor in what they can do is not their construction but the skill, courage and determination of the men who sail in them. These qualities Thor Heyerdahl has in abundance, and perhaps that is why he goes on proving the same general principles over and over again: "Shipbuilding was old when pyramid building started. Rivers and oceans were open when jungles were closed."

His writing reflects the variety of his other skills; taut adventure segments lie side-by-side with random philosophical and political musings, pure travelogue and hard-core scientific speculations, arguments and proofs. Readers of no matter what temperament will find in "The Tigris Expedition," as in his previous books, some sections that will grip their attention and others that they will want to skip. But the identity of these sections is likely to vary from reader to reader. One segment that should find nearly unanimous acceptance comes near the beginning (right after a banal, quasi-poetic prologue) when he talks about the preliminaries of the building of the Tigris and describes the strange, idyllic world of the Marsh Arabs. In that world, he says, "I sensed that I was traveling back through time, not into savagery and insecurity, but into a culture as remote from barbarism as ours and yet incredibly simple and uncomplicated." Going through his descriptions of the place and the people, most readers will be inclined to agree.

Color photos are abundant in the book and, like the author's style, they range in quality from amateurish to very striking. The ship itself is highly photogenic -- rather similar in shape to ancient Viking vessels, as Heyerdahl notes -- and some of the exotic places visited by the Tigris are captured very well in the photographs. Like the voyage itself, the book has some uneasy moments but, at the end, is worth the trip.