Potomac Boat Club of Washington, D.C., boasting two rowers in their 40s, easily defeated Aberdeen University yesterday in the opening heat of the Henley Royal Regatta's Thames Challenge Cup.

Even before their eight-oared shell hit the water, Potomac's Tom Charlton and Townsend Swayze became object of considerable interest. There are those who find their mere entry in the regatta at the respective ages of 43 and 40 quite remarkable.

True, Charlton won an Olympic gold medal as a member of the U.S. eight at Melbourne in 1956, before many of this year's Henley competitors were born. And Swayze was in the Harvard crew that won Henley's top event, the Grand Challenge Cup, in 1959.

But few age-conscious Henley spectators and competitors know that only the good health and fitness of Swayze, Charlton and other members of the Potomac stands between them and an even bigger shock.

"I'm the 53-year-old spare," announced Bill Middendorf. The former U.S. Secretary of the Navy on Wednesday rowed 18 miles on the Thames.

Potomac drew away from Aberdeen to lead by perhaps 25 lengths at the finish of the 7-mile-550-yard course.

It was not as tough a test as Potomac would have preferred. From 41 strokes in the first minute, the eight reduced their rate to 31 at the finish.

Their time of 7:03 was 14 seconds faster than the posted by their next opponent, Molesey Boat Club of England.

Last year, Charlton fulfilled a life-long ambition to row at Henley. His coxless four, including Swayze, reached the quarterfinals of the Wyfold Cup.

"We came back after we had the prospect of putting together a good crew," he said.

The eight have been practicing together only since the beginning of June when Potomac stroke Fred Borchelt, a Rutgers graduate student who coaches the varsity heavyweights there, returned to Washington. One of his Rutgers charges, Rich Schmidtt, is the youngest member of the Potomac crew at half Charlton's age.

The others in the eight are Kevin Sauer, J. Forster, Bob Espeseth and paul Knight. The cox is Coit Liles.

The luck of the draw could not conceivably have been worse for Potomac's other two Henley entries.

Dave Keefe and Jamie Ferrara have never rowed together in a competitive race. As Georgetown Medical students, they have been able to train properly only in the last few weeks. During the academic year, their training was wedged into their few spare moments, whether at 5:30 a.m. or 7 p.m.

Their debut today in the double sculls is against the reigning world champions, Britian's Mike Hart and Chris Baillieu.

Keefe's luck was no better in the draw for the Prince Philip Cup. The Potomac coxed four rows today against the Bulgarian National crew, which placed third in the even in last year's world championships.

The Bulgarians, rowing under the colors of Trakia Club, are overwhelming favorites in the event.

The Potomac four, includes, in addition to Keefe, two other Georgetown Medical students, Oscar Ellison and Stephen Mawn. The fourth man is Carl Anderson, a Geologist. He won at Henley 14 years ago with a Washington Lee High School crew from Arlington, Va. Liles also coxes this crew.

Since their arrival here Monday afternoon, the eight has rowed twice a day, at 6:30 a.m. and 4:30 p.m. They've been at a disadvantage, Charlton said, because they've had no coaching.

Their coach, Ken Hoffman, was called back to the States after only one day in England because of a death in the family.