Thursday is "mental day" at Navy, as Coach Gary Tranquill calls it. Preseason practice is done with, so the Midshipmen will devote a light workout to North Carolina, and then they will bow their heads and reflect.

They will touch on the fortuitous development of quarterback Bill Byrne, who has given them a passing threat. They will dwell at length on the great benevolence of the Pentagon, which has delivered to them Napoleon McCallum for one more year. Then they will ask for guidance on defense, without which all the gold braid in the corps won't help them with the Tar Heels, who are sure to "tho it," as they say in Chapel Hill.

The Midshipmen had their last full-scale workout today in preparation for Saturday's opener with North Carolina (Navy-Marine Corps Stadium, 7:30 p.m.), and most pressing preseason questions have been answered. But until the Midshipmen take the field against North Carolina in what will be only the seventh meeting of the schools this century, their defense will be a suspense story.

A fast, experienced unit returns from last year's 4-6-1 team, but whether it is improved remains to be seen. The usually reliable Midshipmen allowed an average of almost 25 points a game last year. That probably won't do against North Carolina, which has junior quarterback Kevin Anthony, who set 10 school passing records last year and was second in Atlantic Coast Conference total offense.

"You have to question our defense a little," Tranquill said. "The positive side of it, though, is that we have a little more speed. Some people might be able to block us, but we can run you down. The other thing is that it's fairly experienced. We may take more chances than we did last year."

At defensive tackle, a prime area of concern, Tranquill initially considered alternating one of his two nose guards, seniors Dave Pimpo and Dirk McFarlane. But five prospects emerged on such an equal footing that Tranquill claims all will see playing time. Juniors Kent Van Horn and Bob Plantz are listed as the starters, but sophomore Mike Musser was an attention getter in scrimmages.

"We'll play them all," Tranquill said. "Maybe we'll find the right combination. We're not big by today's standards, but we're quick. So we'll emphasize that instead of getting into slugging matches."

The Midshipmen return good experience in the secondary, but minor injuries during preseason took away from practice time, which is inconvenient, to say the least, with North Carolina coming up. Cornerbacks Steve Brady and Tom Metzger each had minor ankle ailments. Sophomore free safety Vernon Wallace still is suffering from turf toe.

"We had some nicks and bruises that kept the defense from practicing at full strength, so we lost some consistency and continuity," Tranquill said. "But everybody is back and we had a good week."

Despite the defensive uncertainties, Navy's expectations after last year's injury-filled season are high for the offense. The Midshipmen have McCallum, back for an unprecedented fifth year after breaking his ankle, and they have Byrne, back from a broken leg. Byrne gives them new balance and will keep defenses honest against Heisman Trophy candidate McCallum.

"Overall, we have more depth," Tranquill said. "Some of those injuries last year were unfortunate, but they always end up helping you down the road. Some guys were forced to play and develop quicker."

Navy's only ailment now is a case of nerves. Tranquill is trying to keep the Midshipmen, whom he described as "anxious to get going," from peaking too early in the week for the Tar Heels. Matters aren't helped by the fact that Saturday's contest is a night game, the first ever played at Navy-Marine Corps Memorial Stadium.

"Playing at night doesn't mean anything except lying around all day and thinking about the game," Tranquill said. "I like games that start at 12:15. You just get up, eat and go play."

A little intrigue is in the offing after last year's swap of offensive line coaches. Former North Carolina line coach John Matsko came to Navy this spring while former Navy assistant Bill Stewart went to Chapel Hill. Both teams will disguise sideline signals to prevent the oither side from picking up the calls.