Billy XXI, the Naval Academy's mascot, was goatnapped over the weekend. The lines were six deep at the ticket windows in Preble Hall most of yesterday. The Yard, as this campus is called, is going bananas over the 4-0 Midshipmen.

Capt. Jack Darby, commandant of midshipmen, stood at the door before yesterday's Quarterback Club meeting, accepting kudos from the team's boosters, and said to a visitor:

"It's different here. It's electric, We're basking in the light of four wins . . . We have depth, size and speed, and the midshipmen on the team are representative of what the academy is about.

"The team turns out for inspection before going on the road and they look like they just came off a recruiting poster."

Darby, the commandant since January, has been stationed at Annapolis for little more than a year. This is Navy's best start in 18 years and only one Navy team has had a winning record in the past decade, after a span in which the Mids were national powers in the '60s and early '60s.

"It is like the past," said John Probert, chairman of the political science department and a longtime member of the Quarterback Club. "In the '50s, when George Welsh was the quarterback (and was taught American Government by Probert), winning like this is something we expected . . . There is electricity on the campus."

Tickets for Saturday's home-opener against Duke are going briskly and the Pittsburgh game Oct. 28 is expected to be sold out by the end of the week. Pep rallies have been scheduled by the brigade throughout the week.

"The brigade is making a lot more of 4-0 than the team is," said Lt. Tom Rehrig, a June graduate of the academy who served as senior football manager last season. "The coach always says to look at one game at a time. He has always preached, "Never look two games ahead or the games behind you."

"It's a good philosophy. As a result, 4-0 means more to the brigade."

For the players, whose remodeled locker room was carpeted over the weekend, 4-0 and having the nations's No. 1 team defense means higher morale and harder work to prove the Mids can keep this kind of football up against more competitive opposition.

"Practices go smoother-full speed all the time," said defensive end Mark Stephens. "There's no reason to be down. People come by my room, pating me on the back and asking me how I'm doing. This (Duke) is going to be our best game ever at home. It'll be great."

Sophomore Steve Callahan, the team's leading rusher, said he attempts to block out what others - classmates and media - says about the Mids.

"No, it's not easier to practice," he said. "You do the same things, go out and work hard at it. I knew we could win, but I didn't think the team would come together this fast. We just say we want to keep winning. If we want to stay on top, you have to pay the pr the more you want to stay on top, you have to pay the price to stay on top."

Offensive guard Frank McAllister, who earned ECAC honors for his play in last Saturday's 37-8 victory over Air Force, has seen both sides of the major college football world. He is a Naval Academy rarity: a player who transferred in, not out.

He says the football players here are just as good as at the University of North Carolina, where he started his freshman year in 1975. Except for the academic workload, McAllister sees only subtle differences between the way the two football programs are run.

McAllister said he had little self-discipline in high school and when he went to UNC, he found himself frittering away time.

After his freshman year at Carolina where he started five games, McAllister was in danger of flunking out.

Navy earlier recruited McAllister, who needed a year of prep school to quality for the academy. So, McAllister left college, enlisted in the Navy and enrolled at Navy's prep school in Bainbridge, Md. Then, last year, he sat out his plebe year as a transfer student. Because his year at Bainbridge is considered military duty, he has three years eligibility here.

McAllister sees Duke as the first "good, all-round team" that the Mids play this season. "Everybody's taling about this (being 4-0)," he said. "But the meat of our schedule is coming up now. We have the potential to be a good football team."

Confidence is high.

"If we're ever going to be ready to play a good football game against a quality opponent, this should be it," Welsh told the Quarterback Clubbers. "But I don't know if we will win. You never do."

Later, a friend of Welsh's teased him, "We could be 10-0 going into the Army game, and you'll get up there . . ."

"And I'll still be nervous," Walsh interjected.