Georgetown University of Washington, D.C., and T.C. Williams High School of Alexandria, Va., will find out how good they are Saturday after posting victories today to advance to the semifinals of their classes in the 129th Henley Royal Regatta on the river Thames.

Georgetown easily left Exeter University of Devon, England, in its wake, winning by more than three lengths in the Visitors' Challenge Cup quarterfinal for straight fours.

Williams used an early burst before the half-mile mark to gain a lead it never relinquished in narrowly defeating Shawnigan Lake School of Canada in competition of the Princess Elizabeth Challenge Cup for schoolboy eights.

Despite their most solid performances to date, both crews will have to row faster still if they hope to survive Saturday.

Georgetown draws Lady Margarets of Cambridge, England, which defeated defending champion Durham College of England by two lengths, and Williams must race its nemesis, Ridley College High School of St. Catherines, Ontario, when defeated the Titans by 1 1/2 lengths for the Scholastic Rowing Association of America regatta crown in May.

"Tomorrow the fun and games are over," Georgetown Coach Greg Carroll said after his crew's second successive easy victory. "Tomorrow we will be able to hear them breathing next to us, and that's not a good feeling . The sign of a good crew is looking back and seeing their faces. Now, that's good feeling."

Exeter took the lead for the first strokes, but Georgetown, stroking 41 times in the first minute, went in front quickly. Settling to a rating of 34-36 for the remander of the race, the Hyoa four of bowman Mark Wessell, Peter Radell, David McAneny and stroke Jim Costello moved out to a length read after a quarter mile, nearly two lengths after a quarter of the 1-mile 550-yards course and three lengths in open water halfway through the course.

When a strong side wind sent Georgetwon veering toward the center of the corse at about that juncture of the race, the shell was too far ahead to cause any interference. The winning time was 7:49.

"It was strong at the beginning, which is what we wanted," McAneny said. "After that it was just to them to catch up."

Castello, who has been plagued with a lung infection, needed first aid treatment immediately after the race for exhaustion and dehydration, but the regetta doctor said he would row Saturday.

Williams' coxswain, Sean McGowan, called a power-10 [10 strokes at maxium effort] and his strategy helped the Titans burst from a quarter-length deficit to a similar lead. The Titans, rowing smoothly with a rating between 34-36, never led by as much as a length. But they never gave the western Canadian crew an opportunity to take the race from them. A clock malfunction prevented an official time.

"They did like they were asked. They settled down after the start," Titan Coach Jon Butler said. "They were behind and they still relaxed, settled down and didn't panic. The tendency of this crew is to row too high (of a rating). This crew is fastest at 34-35. When they row 36-37, they start losing efficiency."

The Titans will need a better start against Ridley, which averages 6 feet 3 and nearly 180 pounds per man. Ridley, which was awesome in defeating a decent Kent (Conn.) School by more than three lengths in a blazing 7:08, has won the Princess Elizabeth Challenge Cup four times since 1970.

Yale continued its domination by posting three victories today. The Eli junior varsity, stroked by Dave Potter of Vienna, Va., drowned GRS Aegir of the Netherlands in a Quarterfinal for the Ladies Challenge Plate for eights; the lighweight eight eliminated the City Orient Rowing Club of London by three lengths, clocking 7:03 in Thames Challenge Cup action, and the pair of Ed Chandler and Hal Evans advanced to the semifinals, winning in open water over G. W. Canning and C. W. Hunter of Queens University of Belfast.

The Yale varsity starts its quest for the Grand Challange Cup Saturday with a match against Oxford University, and the University of California-Berkeley takes on the combined Thames Tradesmen and London Rowing clubs eight in the other semifinal.

Two other American crews -- California-Berkeley in the Ladies Challenge Plate and the Belmont (Mass.) Rowing Club in the Wyfold Cup for straight fours -- were eliminated today by narrow margins.