After an opening day of rare harmony Monday, the America's Cup returned to its acrimonious roots today as ultra-aggressive Nippon skipper Peter Gilmour forced penalties by two of the top-rated helmsmen in the world.

Gilmour, the two-time world match-racing champion from Australia who heads a Japanese crew for this event, collided with top U.S. entry Young America in prestart maneuvers in the morning and barely missed hitting AmericaOne in the afternoon, then won redress for both incidents from on-water umpires.

In the process he outfoxed Young America's Ed Baird, a former world match-race champion, and AmericaOne's Paul Cayard, one of the premier big-boat helmsmen in the world. The fierce maneuvering left spectators gasping as Nippon's swinging transom advanced on the other yachts. But winds were light, the boats stayed under control and no damage occurred.

Sparks flew anyway. After judges found him guilty, Cayard declined to do the penalty turn required of him to wipe out the infraction. Instead, he asked for, and received, a delay after the foul occurred in order to fix a problem with the steering on AmericaOne.

When the prestart resumed a half-hour later, he claimed the delay had wiped out his obligation. The Japanese disagreed and the matter seemed destined for an international jury to resolve. Former French Cup skipper Bruno Trouble said, "The America's Cup really started today," as the sort of rules dispute that has plagued Cup racing for decades reappeared, but later the Japanese decided not to lodge their protest.

Unfortunately for Nippon, in both instances Gilmour could not match his prestart successes with speed on the race course and lost both races to faster boats on the water. Charcoal gray Young America remained undefeated with a convincing, 1-minute 32-second, come-from-behind victory and Cayard remained unbeaten with an easy two-minute triumph in the afternoon.

The wins left Young America and AmericaOne tied for the early lead in this first round of Cup trials with a 4-0 record, the same as the speedy Italian entry Prada, which easily beat hapless Young Australia in the morning and Dennis Conner's Stars & Stripes in the afternoon.

Cayard led Nippon from start to finish but Young America had nervous moments catching up and passing the Japanese as 3- to 9-knot breezes veered wildly and the teams' fortunes changed with them. It was the second straight day of light winds in the budding Cup season, which was expected to be largely a test in heavy winds.

"It's like Long Island Sound out there, very shifty," said Young America syndicate chief John Marshall of the Hauraki Gulf. "In these conditions it's really hard to cover the opposition. You split apart and get big spreads, up to a half-mile apart. If you sail into a calm place you're cooked."

In his troublesome prestart brush with Gilmour, Baird approached the line overlapped with just a few feet separating the boats. When Baird nudged closer to force Gilmour over the line before the gun, Gilmour spun away, lightly whacking the Young America hull with his transom, and instantly unfurled a protest flag.

The charge of maneuvering too close was upheld by judges following behind and Baird was in double trouble. He had to steer off to keep clear, giving Nippon the early lead, and was obliged to make a 270-degree penalty turn later in the race.

Boat speed and clever tactics won the day. Young America rounded the first mark behind by 38 seconds but chipped at the lead downwind and by the second turning mark was overlapped with the white Nippon boat. A perfect final jibe toward the mark was matched by an imperfect one from the Japanese, who flogged their spinnaker. Young America shot to a lead it never lost.

In another closely watched matchup, Conner's Stars & Stripes proved unable to keep up with Hawaiian entry Abracadabra. It looked like a case of inferior boat speed as the two started evenly but Abracadabra quickly pushed out to a lead and never was challenged.

Abracadabra skipper John Kolius had an edge of 1:10 around the first mark and stretched it to 2:11 at the second turn. The margin at the finish was 3:50. The result does not bode well for Conner, the eight-time Cup contender who barely beat the low-rated French by nine seconds in his first race. And it was repeated in the afternoon, when Stars & Stripes was handily beaten by Prada by more than five minutes.

Stars & Stripes has a 1-3 record while Abracadabra improved to 3-1, its only loss coming to Nippon by 38 seconds in the opening race.

For all the disappointing lack of wind, it was a glorious day on the water under bright, sunny skies. In other results, the French and Spanish dueled closely in the first all-European race, the Spaniards rallying from an early deficit to win by 55 seconds, and Dawn Riley's mixed male-female team on America True won its first race of the season by 54 seconds over France.