Six months after Thailand launched a new antipiracy program funded by the United States and 11 other countries, Thai pirates are continuing to prey on Vietnamese refugee boats with frequency and near impunity, according to statistics based on survivors' accounts.

The victims offer a litany of plunder, murder, rape and abduction as seemingly endless as the flow of refugees across the South China Sea. Since the Communist takeover of South Vietnam in April 1975, about 550,000 "boat people" have made the perilous escape from their homeland. An untold number of others never reached their destination, often because of pirate attacks.

"I hate to say it, but at this point the antipiracy program has not been very effective," said a western relief official. Despite claims to the contrary by Thai officials, he said, the percentage of arriving refugee boats that have been attacked has not appreciably declined.

More than 60 percent of the Vietnamese refugee boats arriving in Thailand continue to be attacked by pirates, according to statistics based on refugee accounts. Many of the boats are attacked two or three times, and the usual robberies are often accompanied by rapes, abductions and murders.

In November, for example, 16 refugee boats arrived loaded with 476 Vietnamese. Ten of the boats, or 63 percent, were attacked a total of 28 times. There were at least three murders, 26 abductions and nine known rapes. Generally, the abductees are women who are also rape victims.

Behind the statistics are accounts of shocking, relentless cruelty. The case of Le Quynh Loan, 15, is an example. A student from Rach Gia, she was among 19 boat people who left Vietnam March 24 last year.

On their second day at sea, Vietnamese fishermen stopped them and stole their valuables, she later told refugee officials. Two days later, the crew of a Thai fishing boat took what was left. The next day, she said, another Thai boat took her and five other women aboard. The men were beaten and thrown into the sea, where they are presumed to have drowned. The women were held for nine days and raped repeatedly. Then they were thrown overboard.

Loan is the only known survivor. Among those who drowned were her three older sisters and two older brothers. After being rescued by a Thai fishing boat and taken to the port of Pattani, she was employed as a maid by a Thai family that she said treated her well. She came to the attention of refugee officials in October after she wrote a letter to a sister-in-law living in the United States. She then was taken to the Songkhla refugee camp.

Another victim was Tranthi Thu Nga, 21, from My Tho, Vietnam. She was among 25 Vietnamese who set out Oct. 2. Five days later, four Thai fishing boats surrounded them. The crewmen robbed the Vietnamese, seized six women and divided them among three of the boats.

During the next several days, Nga was transferred to two other boats, ending up as the only woman on the third Thai fishing vessel. Its seven-man crew raped her day and night for the next three days, she said, and she finally jumped overboard in a suicide attempt. But the crew pulled her out of the water.

The same day, she said, the crewmen abducted three more Vietnamese women from another refugee boat and held the four of them for the next 12 days. The pirates invited the crewmen from another Thai boat to come on board and rape the women. Afterward, the four were beaten and thrown overboard, Nga said.

By dawn the next day, her three companions had disappeared and presumably drowned, she said. But after swimming for more than seven hours, Nga was rescued by a Thai fishing boat. She landed in southern Thailand Oct. 28 and later related her story to refugee officials.

One problem in combating piracy is that survivors like Nga are rarely willing to testify in criminal proceedings against the pirates. In November 1981, for example, a Vietnamese refugee, Nguyen Tien Hoa, 31, turned up in Malaysia as the sole survivor of a Thai pirate attack in which he said at least 67 refugees were killed. He was able to identify the Thai boat, but refused to testify because he felt it was dangerous and might delay his resettlement in the United States, refugee officials said. He left in June last year.

To deal with the problem, Thailand waged a U.S.-funded antipiracy campaign from March to September 1981. Before the money for the program ran out, 13 Thais were caught, tried and sentenced to jail terms of 15 to 20 years.

Subsequent negotiations for a new effort funded by a dozen western countries dragged on for several months before agreement was reached and the current $3.67 million antipiracy program began in mid-July.

So far, however, the new effort has not resulted in any piracy convictions. Refugee officials credit the Thai Navy's antipiracy task force with enthusiasm but concede that its three new fast patrol boats, two spotter planes and three decoy boats are probably not enough to do the job.

Two other recipients of the antipiracy funds, the provincial police in southern Thailand and the Harbor Department, have not yet done anything with the money, western diplomats said.

The task force covers 18,000 square miles of the Gulf of Thailand, which is less than half the area in which pirates have operated. In any event, the zone of coverage is still so vast that the patrol boats often cannot reach the scene of a pirate attack in time after it has been spotted by a plane.

This was the case in November when a spotter plane came upon a refugee boat surrounded by four Thai fishing boats. They backed off when the plane flew over but attacked when it left after dark and abducted 12 women, officials said.

The plane took photographs of the boats, but authorities hold out little hope that they can be identified and caught. As many as 60,000 Thai fishing boats operate in the gulf, only about half of them registered.