A capsized lifeboat and what appears to be part of the missing American oil drilling ship have been discovered off China's southern coast, but searchers today found no trace of the 79 crew members three days after they disappeared in the stormy South China Sea.

In an unusual diplomatic twist, Vietnam sent two naval vessels to help its two bitterest foes--the United States and China--in their search for the Glomar Java Sea. The ship was last heard from Tuesday night while battling a typhoon.

The discovery of a white lifeboat and a huge submerged object off China's Hainan Island provides the first indication that the Java Sea was disabled near its drilling site instead of drifting into Vietnamese waters where, it had been feared, it could have run afoul of Hanoi's gunboats.

Nevertheless, the official New China News Agency said tonight the still unidentified object--about 100 yards long, 50 yards wide and 20 yards high--"has not been proved to belong to the American ship."

The agency said a Chinese tugboat, using depth measuring equipment, located the metal hunk yesterday near the well site where Atlantic Richfield Co. officials had been operating the ship. Larger vessels with more sophisticated devices have been sent to identify the object.

An ARCO official said earlier today, "It's a mere possibility" that the object is part of the Java Sea, adding, "but we don't know."

The lifeboat spotted this morning by a Chinese helicopter belonged to the Java Sea, according to Lt. Col. Jack Gregory, spokesman for the U.S. rescue center in Okinawa. "There was no one in it " he said.

Foreign sources said the lifeboat had been badly battered, deepening fears that few, if any, of the crew members--42 Americans, 35 Chinese and the rest unkown--could have survived in the choppy sea and squalls.

Searching U.S. Navy patrol planes reported picking up a distress signal last night, but the sound was too faint to trace and could have come from craft unrelated to the Java Sea.

[The Singapore salvage company Selco reported Friday sighting a lifeboat with flashing lights near the Vietnamese coast, according to Reuter. Selco said one of its tugs was speeding to the area. A search plane from the U.S. rescue center on Okinawa also reported seeing the lights Friday night but then lost sight of them.]

Washington initially worried that its two P3 Orion patrol planes might stray over nearby Vietnamese waters, prompting some retaliation by Hanoi. Vietnam, a close Soviet ally, has no diplomatic relations with the United States and regards China as its archenemy.

Parts of the South China Sea that Peking claims and plans to open to foreign oil company drilling are disputed by Hanoi, which has warned against such activities near its coast.

To avoid problems, Washington informed Hanoi in advance of its plans to search the Tonkin Gulf for the Java Sea.

Hanoi consented, said U.S. Embassy officials in Peking. But, in a surprise move, Vietnam sent two of its own ships to join the search, according to a New China News Agency report from Hanoi. A Vietnamese Foreign Ministry official, the agency said, informed a Chinese diplomat in Hanoi that the ships were sent to the area yesterday at the request of Washington and Peking.

Peking has 14 ships and three helicopters searching for the ship, according to the agency.

The Java Sea, a $29 million, 11,000-ton vessel leased to ARCO by Global Marine Drilling Co. of Houston, had been exploring for oil off Hainan Island since Jan. 9. ARCO officials said the 400-foot ship has survived rough tropical storms and typhoons over the past 10 months.

ARCO is in a joint venture with the Chinese government to drill in a 3,500-square-mile zone. If it discovers oil, it is obligated to return a majority share to its partner.