The United States Olympic coxed pair of brothers Mark and Fred Borchelt of Arlington, Va. and coxswain Chris Wells of Alexandria, Va., set a Bosbaan racing course record of 7:03.54 today. Aided by a stiff stern wind, Americans set four course records in the second day of the Open Dutch rowing championships.

The coxed pair, all members of the Potomac Boat Club of Washington, D.C., won its Holland Cup race easily Saturday, but had a much stiffer test today, finishing less than three seconds ahead of Magdenburg of East Germany over the 2,000 meters.

The American straight four, which included Dan Sayner of the U.S. Naval Academy of Annapolis, won for the second straight day, nipping the Soviet Union's only entry in the regatta by slightly more than 2 1/2 seconds. s

Walter (Chip) Lubsen of Arlington, Va., who won in both the straight pair and coxed four events Saturday, raced only in a four with coxswain today that finished fourth in a competition won by another American four.

The American men's eight recorded its second straight victory, finishing one length ahead of Australia in a course record 5:36.82. The American women's eight, rowing 1,000 meters, recorded the first sub-three minute clocking on this course at 2:57.85, three lengths better than Canada. Their second straight victory was nearly five seconds better than the old record of 3:02.64 set by the East Germans in 1977.

In all, the Americans won 19 of the 27 events they entered, with the men winning 10 times. Americans also finished second five times and third twice.