They slip from port on secret missions, destination unknown. They ply the oceans more than 60 days at a stretch, occasionally tailed and harassed by the Soviets or battered by huge waves and 95-mph winds.

They make up one of the Navy's smallest and most secretive fleets - 10 ocean surveillance ships in the murky hunt for Soviet submarines.The Navy christens them with names like Indomitable and Audacious and Vindicator, then dispatches them to points ranging from the Alaskan coast to the Caribbean.

Navy officials say the T-AGOS class vessels and the long sled of sophisticated sensors they pull have given the United States a powerful new edge in antisubmarine warfare since they began operation in 1985.

In an era of budget-busting weapons that don't work properly and criticism of outside contractors hired for military jobs, the T-AGOS program is considered a Pentagon success story.

When the Navy ships, which are operated by civilian crews and technicians hired under contract, completed their 100th surveillance mission last week, the chief of naval operations, Adm. Carlisle A.H. Trost, said in a message to top officials throughout the antisubmarine warfare community, "These vessels have made a significant contribution to meeting the Soviet challenge on the high seas."

But Navy officials refuse to say just how significant that contribution has been: They don't discuss specific details of the highly classified missions. They won't say precisely where the ships work or how many submarines they've detected on those 100 missions.

Capt. Ray C. Witter, program director of the Navy's undersea surveillance systems, noted, however, that these submarine hunters are one of those rare projects that have exceeded the military's original expectations.

The 224-foot ships work in tandem with other legs of the military submarine hunting force - P-3 Orion airplanes that drop sonobuoys from the sky to eavesdrop on subs, U.S. submarines that prowl the sea for even closer surveillance, and destroyers that pull their own submarine-seeking sensor devices.

The T-AGOS, which uses the Surveillance Towed Array Sensor System, pull longer antennas with greater ranges than those used by the other submarine hunters, significantly expanding the Navy's ability to target and track enemy submarines, according to Witter.

The ships cost about $20 million each and the towed-array sensors are priced at about $18 million each, Witter said. The Navy's Military Sealift Command spends $40.8 million a year on operating expenses for the 10 ships.

The 19 crew members and seven technicians - men and women - assigned to each ship are civilians; there are no active duty military personnel aboard. When Navy planners came up with the idea for the special submarine hunters, they decided the military didn't have enough skilled technicians to fill the jobs and decided to hire civilians for the sensitive tasks, according to Witter.

When the ships leave port, the crews aren't told where they are headed or how long they will be at sea. Sea missions average 64 days, Witter said. And although five ships are based at Little Creek, Va., and five at Pearl Harbor, a ship can go three years without returning to its home port, he said. Sometimes turnaround time between the two-month-long missions is but a few days.

Navy officials, noting that the Navy spends eight months training a technician, say they first believed the civilian crews would be extremely expensive because of high turnover rates. They predicted as many as 38 percent of all technicians would quit each year. Four years later, the turnover rate is 11 percent.

But there have been some unexpected problems with the program.

"We were beating the daylights out of them in rough weather," said Witter.

So the Navy has contracted to buy a heavier, more stable, more expensive ($30 million each) model called the SWATH - for Small Waterplane Area Twin Hull - a vessel balanced between two underwater submarines, vaguely similar in construction to a catamaran.

While many military programs are facing the prospects of major cutbacks, Navy officials say this program appears safe from the financial hatchet.

"Antisubmarine warfare is my central warfighting concern," Trost told the House Appropriations Committee during budget hearings earlier this year. "The U.S. margin of technology superiority over the expanding and increasing modern Soviet submarine force is eroding. Although we remain superior, we must continue to apply priority to programs which improve our ability to detect . . . a more sophisticated adversary."

The Navy tentatively plans to request that a total of 33 T-AGOS class vessels be in the fleet by 1998, although Witter said the requests will be affected by Soviet submarine construction and operations. He said some officials have estimated the Navy may need as many as 50 of the ships by the end of the century.

In addition to the 10 ships in operation, 12 more vessels are in various stages of construction or have been funded.

The Navy also is planning a joint operation with the Japanese military in which the Japanese will build two of the ships, then will buy the sophisticated sensors from the United States. The U.S. Navy will provide military technicians to help operate the sensors, Witter said.