It was the last race of the 6 Metre world championships in Seattle and, as usual, Pelle Petterson, the flying Swede, was out in front. Tack by Tack, he covered the leaders of the 25-boat fleet, knifing to windward against a 15 knot breeze. Puget Sound glittered in the late afternoon sun, framed on one side by the sawtoothed, basalt-gray Olympic Mountains and on the other by the magnificient snow covered heights of Mount Rainier.

All week Petterson had seemed immune to a major mistake: He started fast at the leeward end of the line, then tacked to cross the fleet. He sailed fast, conservative windward legs. He didn't foul out and he didn't foul up. And now, with the lead in hand again, surrounded by a posse of spectator boats, his mainsail fell unaccountably to the deck.

A universal moan went up from the bobbing onlookers. Rotten luck. From behind, Ted Turner closed the gap and passed Petterson. Tom Blackaller, the former Star and 6 Metre world champion, roared up, also, as Petterson labored under jib alone. And the rest of the fleet closed in.

But by the time those on the spectator boats had freshened their drinks, a blond-headed young man was already halfway up Petterson's 37-foot mast, pawing his way up like a squirrel on a sapling, then clinging for a moment at the very top.

In a moment he was down, repair completed. In another moment, the mainsail was back up, and by the end of the race Petterson was in third place and the undisputed new world champion of the old 6 Metre class.

It was a remarkable performance in a remarkable season of racing in a class that was all but extinct 10 years ago. The world championships had been preceded by the North American championships in San Francisco the week before, won by Tom Blackaller with Turner second.

The skippers and crews hardly had time to catch their breath before this week's Australian-American Cup match-race eliminations back in San Francisco, again won by Blackaller Thursday after a last-minute challenge by Turner. The Australians and the Swedes are, this weekend, completing their elimination series and tomorrow their winner will begin the final joust against Blackaller.

For a class that celebrates its 75th anniversary in 1981, and which has virtually no fleets in this country outside the Pacific Northwest, that's a lot of action. For Petterson and Turner, the lure is obvious: a chance to sharpen skills for the 1980 America's Cup campaign, for which each skipper hopes to advance to the final showdown off Newport next fall.

But as fleet racers, the Sixes are anomalies indeed, determined survivors of the old "flank on edge" school of yacht design. About 37 feet long and only six feet wide, the modern Sixes weight in at more than 8,000 pounds. They are financial heavyweights, too -- one syndicate in Fort Worth expects its campaign to cost about $175,000.

The Sixes are the elite of the formula classes, with the newest Gary Mull designs featuring Courageous-style stepped bow profiles and long, delicate reverse transoms. They take a lot of water going into a chop, and are so deep in the hull that the five-man crew is frequently invisible in the cockpit.

It hardly matters to the proud owners of these lovely and complicated thoroughbreds that a stock Etchells 22 can sail through their lee without effort.

There were more than 100 spectator boats at the San Francisco match races, and 6 Metre social life is clearly a cut above the bring-your-macaroni-salad-over-to-our-place postregatta parties of the usual club circuit. This may be inevitable given their insistence on spelling metre the English way. Perhaps the Laser class, in a attempt to change from a beer to a wine image, should henceforth regard its boat as the lasre.

At Seattle, after the crews had spent as much as 12 hours trying to fit two 15-mile races into one day, the gang gathered alternately at a wine tasting party at a nearby vineyard, a lawn party of barbecued ribs presided over by the boggling heights of Mount Rainer and a highly-sauced midnight dinner with Werner Erhard.

Erhard, the motivation magnate who founded est, dropped by the racing here to find out what makes super competitors such as Blackaller and Turner tick. He just happened to be in Portland the day before, himself winning the North American championship in Formula Super Vee racing cars.

Plying a dozen racers with an eight course meal that didn't end until 2 a.m., Edhard got his money's worth. Turner and Blackaller, both of whom are talkative in a manner of speaking, analyzed each other's performance record like presidential candidates the week before an election.

The long triple series of 6 Metre events was considered a success, with the winners winning fair and square the class healthy, and the race organization excellent. How the owners and skippers will explain to their stockholders, employes, bosses and failies their long absence from office and home is another question.

Anyway, the Sixes are boats from an earlier time, a golden era when an ounce didn't cost $375. In this class, if you have to ask somebody for extra vacation days to race, you can't afford the time.