Waving an American flag and giving the thumbs-up sign, Phil Weld crossed the finish line of the Observer Single-handed Transatlantic Race at 8:12 a.m. today to set a course record of 17 days 23 hours 12 minutes for the 3,000-mile passage from Plymouth, England.

Nick Keig of England followed at 3:03 p.m. in his 53 foot trimaran Three Legs of Man III, edging American sailmaker Philip Steggall by 47 minutes. Michael Birch, a Canadian who finished third in the 1976 race, was fourth at 4:16 p.m.

Walter Greene, an American yacht designer, and Poland's Kazimierz Jaworski were expected tonight. Jaworski's 56-foot sloop would be the first monohull to finish.

Weld, at 65 the oldest of 88 starters, shaved nearly three days off the record of 20 days 12 hours 15 minutes set in 1972 by Frenchman Eric Tabarly. r

Weld, a retired newspaper publisher from Gloucester, Mass., stood on the bow of his sleek 51-foot trimaran, Ms. Moxie, as she finished at 8 knots in a light haze under automatic pilot.

At a press conference, Weld said he had had an "easy" crossing, sometimes attaining speeds of 20 knots. His best day's run was 265 miles.

"My lordy. I've been preparing fro this for 10 years," beamed Weld. "I've had a couple of bad shots at it, and you know, the third time never fails."

Asked if he encountered any problems, Weld said a halyard gave way but was easily replaced.

"I think I was covered in every department except an unmerciful thing; I didn't have the right size dry-cell batteries for my tape recorder. So posterity is spared that."

He gave credit for victory to his boat, a Richard Newick-designed three-hulled speedster, and said his fast passage suggests "a breakthrough in showing how science can use wind to drive vessels." He said he was able to pass several "gas guzzler" oil tankers at sea, and called for a return to the use of sailpower commerce.

Annapolis sailors: Francis Stokes was reported in 18th place by satellite yesterday. The positions of William Homewood and Judy Lawson have not been updated in several days.