U.S. Navy rescue planes joined Chinese vessels today in search of an American offshore oil drilling ship with 81 persons on board that disappeared in the stormy South China Sea and may have drifted into Vietnamese waters.

An SOS signal, believed to be from the Glomar Java Sea or its lifeboats, was picked up today by a Japanese freighter, raising hopes that the crewmen, including 42 Americans, were still alive, United Press International reported. The signal, the second from the 5,930-ton vessel since Tuesday, gave a location about 135 miles east of Vietnam, within range of Vietnamese patrol boats.

About half the missing crew are Chinese, who, along with American and European engineers, geologists and roughnecks, had been operating the ship for Atlantic Richfield Co. south of China's Hainan Island before running into a typhoon earlier this week, ARCO sources said.

The vessel sent out its last radio distress signal at 11:30 p.m. Tuesday, according to U.S. Navy sources in the Philippines who monitored the message. The Java Sea then reported its position in the Tonkin Gulf, midway between Hainan and the Vietnam coast--about 100 miles west of its original drilling site.

Much of the Tonkin Gulf is in dispute between Hanoi and Peking, and experts have long predicted it would be a flashpoint once China contracted with foreign firms to drill for oil in nearby waters.

A fleet of 11 Chinese ships has been searching in vain for the Java Sea since it was reported missing, according to Peking sources. They were joined today by U.S. P3 aircraft which also reported finding no sign of the ship, according to the U.S. rescue coordination center at Kadena, Okinawa.

A U.S. Embassy spokesman here said Vietnamese officials in Bangkok were informed today that American planes would be searching the gulf for the Java Sea and might be flying over Hanoi's territorial waters.

It was unknown how Hanoi responded. The Vietnamese Embassy here refused to discuss the matter. Dick Vemeer, vice president of Global Marine Inc. of Houston, the ship's owner, said today he had "tenth-hand reports that Vietnam would not hinder search efforts in its waters."

The Java Sea, a 400-foot ship that looks like a freighter with a steel derrick through midship, is the sister ship of the Glomar Explorer which was owned by Howard Hughes. The Explorer was involved in a CIA-backed operation in the 1970s to raise a sunken Soviet submarine in the Pacific.

Arco China Inc., the first American oil company to win drilling rights in the South China Sea, began operating the Java Sea Jan. 9. The ship had drilled in two other locations within the company's 3,500-square-mile block and has survived several tropical storms and typhoons during the past 10 months, according to company sources.

The sources said the Java Sea had stopped drilling operations last weekend as Typhoon Lex approached, packing winds of 75 miles per hour and churning up 26-foot waves. But the crew maintained radio contact until Tuesday night.

The ship is believed to have pulled up anchor and drifted west to escape the storm.

The South China Sea is thought to be rich in oil and natural gas deposits, and Peking opened bidding with dozens of foreign firms for the right to exploit the area. In exchange for exploration rights, the Chinese government claims for itself a majority share of any discovery.

In addition to ARCO, two other American firms--Occidental Petroleum Corp. and Exxon Corp.--have won contracts to drill in different blocks of the South China Sea.

A potential problem for these business ventures is Vietnam's looming jurisdictional claim to large portions of the sea.