A severe thunderstorm with winds gusting to 50 miles an hour battered the Washington area last night, knocking down power lines and trees, capsizing sailboats on the Potomac and persuading some airline pilots to delay their takeoffs from National Airport.

The storm dumped almost an inch of rain in some places in the area during a three-hour period and pelted parts of Montgomery County with pea-sized hail. Lightning and wind knocked out electric service to more than 24,500 customers in the area, including at least one hospital.

National Airport reported that takeoffs were delayed for 42 minutes, with at least 18 planes waiting on the runways, during the most severe part of the storm. The decision to delay was made by the individual pilots.

"It's really routine this time of year," said a Federal Aviation Administration official. "Pilots have always been very respectful of thunderstorms."

A spokesman for the Airline Pilots Association said, "this collective individual decision by pilots not to fly is not a common thing . . . If the pilot decides that the conditions don't suit him for whatever reasons, he can say he is not going to do it."

Weather conditions have been factors in three airplane disasters this year -- in Washington, Boston and New Orleans.

D. C. Harbor Police were busy between 6:30 and 7:30 p.m. They rescued six sailing class participants whose six 14-foot boats capsized in strong winds in the Washington Channel near the Gangplank Restaurant. Fifteen minutes later, they fished out a wind surfer whose craft overturned in the Potomac near the Thompson Boat Center. A 16-foot catamaran capsized near the Washington Sailboat Marina. Harbor Police said its occupant swam ashore.

No injuries were reported in any of the incidents, police said.

Virginia Electric and Power Co. reported outages for more than 17,500 customers in Northern Virginia, including Northern Virginia Doctors Hospital in Arlington, which switched to emergency generators for 2 1/2 hours. Almost 7,000 Potomac Electric Power Co. customers in Maryland, Washington and Rosslyn lost power during the storm, power company officials said.

D. C. police said that during one 10-minute period they answered 60 burglar alarms that apparently were set off by lightning or winds.