The start was hardly pretty, but by the time they reached the grandstands near the finish of the Henley Royal Regatta course today, Mark and Fred Borchelt were ready to do some showing off.

The victory by the brothers from Arlington, Va., in the Silver Goblet and Nickalls' Challenge Plate final, was just one part of a triumphant day for American Olympians, whose eight won the Grand Challenge Cup and whose coxed and uncoxed four also captured titles.

In a rain that varied from steady to drizzling, Americans won six of 11 open events, their best showing in the 130 years of this regatta.

The Borchelts led the British pair of James Palmer and Hugh laurie by two lengths with a mile gone in the River Thames course that measured 1 mile 550 yards.

But instead of lowering their rating to less than 30 strokes per minute as they had in three easy preliminary victories, they increased their pace to 35, then 36 strokes per minute to win by the umpire's designation of "easily" which means more than three lengths.

"I wanted to go up to 40, but Fredie said no. I've always wanted to win a final by 'easily," Mark Borchelt said. "I've waited seven years to race a pair with my brother. . .and we won."

Mark's Borchelt's victory for the Goblet with Terry Adams in 1973 and the Stewards' Challenge Cup with Fred and fellow 1980 Olympian Chip Lubsen of Arlington, Va., in a straight four in 1975 were the only previous American victories in those events.

The Borchelts did have their tense monents. Their exceptionally fast start was negated when they veered toward the buoys on their port side and, in compensating, headed toward the opposing pair, before Mark, who steers from the bow, righted the course.

They were still even and led by a third length at the Barrier (700 yards elapsed) and by 1 1/2 lengths at Fawley, 445 yards later.

"We got off to a great start. We had a couple of seats on them, but then we veered toward the buoys," said Fred. "I was just trying to avoid the buoys (with his oar). If I had hit one of those suckers, we could have crabbed and they would have had a length on us."

In addition to the featured Grand Challenge Cup, the U.S. Olympics straight four took the second most prestigious sweep rowing event -- the Stewards' Challenge Cup -- by two lengths over the New Zealand Olympic four.

Despite minor steering problems at the outset, the American four led after 10 strokes. The lead was one length at the Barrier, where the Amercians then gradually reduced their rating from 36 to 33 in clocking 7:04.

"We just wanted to row hard from the start. In a Henley race, you want to get ahead because it's demoralizing to get behind," said Dan Sayner of Annapplis, who manned the bow.

For New Zealand as well as the U.S., this was the end of their "Olympics" tour. The U.S. eight jumped off at 43 1/2 strokes per minute to gain a quick three-quarter length advantage over New Zealand, extending it to two full lengths after a mile before New Zeland closed to within 1 1/4 lengths at the finish. The winning time was 6:31.

In the Prince Philip Challenge Cup, which some U.S. Olympians called "the great American seat race," the U.S. first coxed four broke ahead by one-third length at the Barrier and won eventually by more than 1 1/2 lengths over the second U.S. four, which had Lubsen striking from the bow. The two American crews had switched personnel amongst themselves on the first two stops of the tour in Lucerne Switzerland and Amsterdam, making this their final championship.

Yale University's junior varsity eight successfully defended its Ladies' Challenge Plate and St. Paul's of Concord, N.H., defeated St. Joseph's of Philadelphia, for the St. Elizabeth's Challenge Cup for high school eights, to complete the record American showing.