For the 24th time in 110 years, foreign boats are in America trying to wrest the America's Cup from its perch in the New York Yacht Club.

Early Friday in the dark before dawn, Lionheart, the British 12-meter, slipped into town under tow from New York. Now the circle is complete.

Seven racers are in Rhode Island, three American and four from across the sea. The U.S. boats began preliminary trials Saturday to determine who will defend international yacht racing's most coveted prize when the cup races begin Sept. 16.

Not until August will the foreign boats go head to head to pick a challenger.

To a man, the American sailors say they don't really care who the foreign challenger turns out to be. They say the toughest test is going to be picking a defender from among the three American boats. They maintain that for the 24th straight time the foreign fleet will go home without the cup.

"There are three reasons for that," said Jeff Spranger, coeditor of the weekly newsletter "America's Cup/1980 Report" and a veteran watcher of cup racing.

"First, logistics. The challengers are thousands of miles from where they can get parts for their boats. Second, they are rivals in separate camps; they won't race together as a cooperative thing." The Americans, conversely, are all under the aegis of the New York Yacht Club, which mounts the defense, and are required to test extensively against each other.

Third, Spranger said, the foreigners' trials are compressed into a month, while the Americans have the entire summer to battle formally and improve.

Tony Hagar, bowman on the newest American boat, Clipper, put it more simply: "Nobody's worrying about them (the foreign entries). If they decide a new jib, they've got to make it. They can't call Hodd or North or Ratsey and get it made."

If the Americans are not concerned about the threat from abroad, they are worried about each other.

The three U.S. boats are in two camps. The camps keep their distance.

Ted Turner, the loquacious, belligerent Atlantan with his fingers in a hundred pies, is the recognized favorite with his yacht Courageous. He piloted her to victory in 1977 and she won in 1974 under sailmaker Ted Hood's band.

If she triumphs this year, it will be the first hat trick ever for an America's Cup boat.

Courageous has spent the spring practicing here against Clipper, a boat Turner calls his "stablemate." Clipper and Courageous both get donations for the Cup's defense through the same tax-exempt foundation; they share dock space at Bannister's Wharf.

Turner and Russell Long, Clipper's 24-year-old skipper, know plenty about each other and most of it points to Turner's superiority, at least for now. But they know almost nothing about the progress of the third American, Dennis Conner, who will campaign the year-old 12-meter Freedom.

On the weekend that the America's Cup summer begins, here is a look at the boats and the people who will come together for three months in yachting's greatest international event.

THE DEFENDERS: "Turner's the boat to beat," Conner said flatly. Courageous has been modified slightly, her bow snubbed and her stern raked for less weight and more speed, but no one in either American camp expects the defense trials to be decided on boat "speed."

As Conner put it, "it's like a one-design race," where all the boats are exactly the same. There is latitude in the construction requirements for 12-meters, but this year Clipper, Courageous and Freedom have hull designs so similar they seem perfectly matched.

What it will come down to, according to people who know, is sails, tactics, and the efficiency of the crew.

Turner has the edge almost all the way round.

His 11-man crew is the same as it was in 1977, and many sail with him regularly on his ocean racer Tenacious.

They know each other, they know the boat inside out, and they know match racing, boat against boat, America's Cup style.

Turner's tactician. Gary Jobson of Annapolis, is regarded as the best in the trade.

Nonetheless, it will be no cadewalk.

Conner, a California drapery and carpet manufacturer, is formidable. His boat, Freedom, is a little over a year old, making it five years newer than Courageous. He has sailed it hard on both coasts the full year. He's gone through 120 applicants, selecting the best crew.

He has campaigned Freedom in near secrecy with Enterprise, which the Freedom syndicate also owns. Conner decided only this month that Freedom was the better boat. Until then, no one knew which boat he would race. "We used Enterprise as a measure, assuming she is as fast as Courageous," he said. "Our boats are really about equal. We picked Freedom basically because we liked the deck layout."

Conner and Turner have stayed well apart in practice sessions here. Both concede they have no idea how fast the other's boat is and won't until they have met in formal races this week.

Clipper is the longshot. She is only 2 months old; her crew is new and her skipper new to the game.

One day last week, Clipper and Courageous went head to head in five four-mile practice races off Newport. Clipper lost every one, including one on a bungled tack by the crew.

"She was ahead," said Marty O'Mears, Courageous' operations manager, "but the crew blew it."

Says Clipper navigator Tom O'Brien, who sailed on Courageous in 1974, "Turner's crew is a decided advantage at this stage. But you really ought to get it together in two or three months, and we're starting together.

"Right now, our feeling is good. We've got a good horse. We have to be the decided underdogs, but our learning curve is going to be the best of anyone's."

THE CHALLENGERS: Practically nothing is known of Lionheart, the latest and final arrival, yet the British boat is already being discounted. "She's just too heavy," Long said. "She won't accelerate in a chop. She isn't built for our kind of wind."

Long figures the English and the Swedes will be the first eliminated in the challenger trials, and the final contest for the challenger spot will be between Australia and France III.

Sverige, the Swedish boat, is the only radical hull design among the seven competitors. "She looks," said one observer when the Swedish 12 was hoisted out of the water for cleaning last week, "like she's pregnant."

Radical hull designs can be fast and they can be hopelessly not fast. Sverige (pronounced Sver-ee-ya) brushed briefly with Courageous a few days ago when both were out on practice sessions. Courageous blew past her, according to those on board, and since then Sverige's stock has fallen. She has not faced well in informal races with France III, either.

The serious challengers -- Australia and France III -- are of proven hull design.

Local sentiment rides with Baron Marcel Bich, France III's owner, who has been scratching for the America's Cup for a decade. The elegant scion of the Bic pen dynasty has spent millions, built three boats, mounted three campaigns and never won a race.

Close observers say France III is the best design he has ever had and maintain that if he keeps his hands off the helm and lets his professional skipper, Bruno Trouble, drive the boat, he could be the challenger.

Australia won the right to challenge last time round, in 1977, but lost in four straight races to Turner.

THE SCHEDULE: The American preliminary trials will run until Saturday, eight days in all.

The preliminaries will be the first official races during which the New York Yacht Club selection committee formally observes the boats and begins the selection process by which the defender is chosen.

The next U.S. trials will be the observation trials, which run along the same lines, July 19 to 30. The final trials will begin Aug. 19, and sometime between then and midnight Sept. 11, the seven-man selection committee will notify two of the three crews that their boats and services are no longer needed.

The U.S. selection process is subjective, left in the hands of the committee and not tied to individual race results.

The foreign boats operate on a round-robin schedule with the winners of four of seven race series advancing.

Their preliminary trials will be Aug. 5-10. The semifinal series to eliminate two boats will be Aug. 14-25. The final series between the two remaining boats will begin Aug. 29. The challenger must be named by Sept. 11.

THE FIREWORKS: Turner is working hard not to let the reputation he built here in 1977 as the "mouth from the South" expire.

At a press conference last week, he called Conner "unsportsmanlike, short-sighted and selfish." And they hadn't even raced yet.

At issue was Conner's decision not to keep Enterprise, his No. 2 yacht, in the running. He elected instead to use the best of Enterprise's equipment and people as backup for Freedom.

Turner contends that Conner should have taken everything he wanted off Enterprise, then given the helm to some young crew to round out the defense with four boats.

"It denies another group of youngsters a chance to learn to sail a 12-meter," Turner said, implying something up-American about the move.

Said Conner, "Ted and I have different ideas about how to mount the best defense. He's trying to stir up trouble for me, and if he can upset me, it's to his advantage. Well, he didn't.

"What can I say? He doesn't like me. It's too bad. I like him . . ."

THE COURSE: All races will be run on a triangular, 24.3-mile course in the ocean off Newport. There will be six legs to each race, three windward, two reaching and one downwind. Some trials will be run on shorter courses for convenience.

THE SPOILS: The winner of the America's Cup races, assuming it is the U.S. defender, will have his boat's name and the year of its victory inscribed upon the cup, which will remain at the New York Yacht Club.

Should a foreign challenger win, the cup would go home to his yacht club.

For this potential honor, seven groups have raised and spent the $1 million to $2 million apiece that is required to build and campaign a 12-meter yacht.