Unbeaten Navy turned what was supposed to be its first test of the football season into a 31-8 rout of Duke yesterday, impressing almost everyone except Coach George Welsh.

Not that Welsh didn't enjoy his club's fifth straign triumph. But until the Midshipmen either fall behind get involved in a close contest. Welsh remains low key about his surprising teams.

"We played a good football team that had won some games, and we did some things that have to be good," Welsh said. "But until we are behind, wel, I have to wait. We aren't supermen."

His players, however, are starting to flex their muscles. They handled Duke better than any previous Blue Devil foe save for mighty Michigan. And just as important, they made it look easy.

Their defense, ranked No. 1 nationally, throttled Duke quarterback Mike Dunn, whose most spectular moves were in retreat, trying to avoid onrushing Navy linemen. Their offense shook off a slow start and wound up befuddling the Blue Devils with a mixture of passing and running.

About the only thing Navy didn't handle was the kickoff returns of Dennis Tabron, who almost broke two for touchdowns and set up his club's only score.

"There is no excuse for letting them break off runs like that," said Welsh, trying hard to find something - anything - to criticize. "I don't know what happened, but I figured they would be able to move the ball and score on us, anyway."

He proved a lousy prognosticator. Duke finished with a paltry 122 yards, 10 below Navy's already stingy defensive average. Only 40 of those yards came through the air as the Midshipmen picked off four passes to wipe out any of Welsh's early worries about his secondary.

Dunn, the flashy option-operator, was hindered by the absence of leading Duke rusher Greg Rhett, who sat out with a sprained ankle. That left the Blue Devils without any outside speed until coach Mike McGee went in the second half to a freshman, Keith Crenshaw, whose end runs came too late to make much of a difference.

The best quarterback in the game was Navy's fast-maturing Bob Leszezynski. Welsh, searching for the proper adjective to describe his signal-caller, finally settled on "superb." Steady might have been just as accurate.

He finished with three touchdown passes, completing 10 of 12 for 128 yards. He was cool under pressure and made his old quarterback head coach smile with his audible calls at the line of scrimmage.

"I called about 60 percent audibles on running plays," Leszezynski. "But I also called my first two audibles for passes this season."

The first of those passing audibles came midway through the third quarter after Duke's only touchdown. Leszezynski cought the Blue Devils in a rover-back blitz and fired a pass to end Phil McConker, who made a spectacular 34-yard diving catch.

Five plays later, the same two teamed up again. This time, Leszezynski noticed Duke had shifted into man-to-man coverage on McConkey, a dangerous tactic against the speedy receiver. So he called for another post pattern and connected with McConkey at the goal line with a fine bullet pass. That score gave Navy a 28-8 lead and put the game out of Duke's reach.

Two other plays contributed heavily to the Blue Devils' demise. The first was a roughing-the-kicker penalty in the opening quarter that kept alive an eventual Navy scoring march. McGee protested long and loud that one of his players had deflected the ball before hanging into punter Art Chanian, but his plea was rejected.

The second play came late in the first half. Duke went for a first down on fourth and one at its 40 and failed with Navy recovering a fumble at the 42. A 13-yard pass in McConkey, who had four receptions in the game, a roughing penalty and a five-yard toss to tailback Chuck Callahan earned Navy a 14-0 margin at intermission.

Callahan also rushed for 96 yards before sitting out the fourth quarter with what was diagnosed as a bruised wrist.

McConkey and Leszezynski combined for another score in the third quarter. McConkey caught a 29-yard pass on the Duke 14, sidestepped two defenders and raced into the end zone.

At this point, Duke seemed completely befuddled. But Tabron took the kickoff and bolted up the field 52 yards before being stopped by kicker Roland Ellis. It was only the second time in the game that the Blue Devils had been able to penetrate into Navy territory.

Dunn took advantage of the opportunity by completing a four-yard pass to tailback Mike Stopper for Duke's only touchdown. A 77-yard return by Tabron later in the quarter was quickly spoiled by an interception. It also was the final time the Blue Devils crossed the 50.

Duke couldn't handle the interior of Navy's defensive line. Once McGee found it was useless to attack John Merrill, Terry Huxal and Steve Chambers, he began working on the ends. However, that idea didn't strike him until he was trailing, 21-0.

"I think the sky is the limit for this team," said McConkey. "I could see last spring that we had the size, speed and talent I had never seen here before. But we aren't winning on potential. We've been really performing."

Leszezynski was just an optimistic, especially because he can see the offense taking on a much-welcomed balance.

"We are mixing up our running and passing better than last year," he said. "It makes us harder to stop. We already know what our defense can do. So it's up to us to score enough points."