The Coast Guard cutter's loudspeaker boomed angrily over Boston Harbor: "Move clear of that ship! Get back! Get back!"

A 32-foot launch had chugged to within one-quarter of a mile of the huge tanker Descartes and its cargo of liquefied natural gas. That was too close for the Coast Guard's liking.

Liquefied natural gas (LNG) makes people nervous, although the industry says such fear is ridiculous.

The debate centers on whether the tankers are floating bombs capable of nuclear-style devastation or a crucial part of the nation's energy supply picture that careful handling renders as safe as any other fuel.

The controversy is being made real here at the Distrigas Corp. LNG import terminal, the nation's only one so far, and is sure to intensify as imports increase and more terminals are built, including one at Cove Point, Md.

The $350 million Cove Point terminal, scheduled to become operational early next year, will be considerably larger than the Distrigas terminal, which cost about $28 million and receives a tanker from Algerian gas wells about once a month. Cove Point will unload once a month. Cove Point will unload a ship every 2 1/2 days.

Cove Point will be the nation's first "base load" LNG terminal, importing 1 per cent of the country's entire natural gas need. The Distrigas Corp. terminal here is a "peak load" terminal, pumping gas into the mains only when demand is high.

Both plants use the newest technology in handling supercooled material, called cyrogenics. LNG is ordinary cooking and heating gas chilled to minus-260 degrees Fanrenheit so it becomes a liquid. At that temperature, LNG takes up 1/600th of the space it does as a gas and shipping it becomes economical, if not necessarily easy.

Gas coming here first is cooled in Algerian plants that cost as much as $850 million each and then sent over in sophisticated, highly insulated ships that, with the exception of aircraft carriers, are the most expensive vessels on earth. With their network of pipes and pumps and tanks, the ships may cost $180 million each.

THe Descartes, a 722-foot, French-built ship and one of 32 such tankers in existence, was carrying 50,000 cubic meters of LNG when it appeared out of the fog to enter Boston Harbor. That amount is enough to supply a town of 30,000 persons for a full year.

The Descartes' arrival was several days later than originally scheduled, and the fog fouled the schedule further. With one ship a month, such delays are small problems, but Cove Point scheduling will have to be much tighter to keep traffic flowing, according to Lt. Steve Poole, Coast Guard port safety watch officer who oversaw unloading of the Descartes.

The Coast Guard's main worry is the possibility of a collision bad enough to pierce an LNG tanker's double hull and insulated tanks. Special running lights are required at sea to warn other vessels away, and all traffic was halted two miles in front and one mile behind the Descartes as it entered Boston Harbor.

Every 15 minutes, the Coast Guard broadcast a stay-clear warning on marine radio, pertaining to all but small boats that can navigate outside the main channel. When the 32-foot launch ignored the warnings, the Coast Guard cutter accompanying the Descartes intercepted the small boat and ordered it back.

"A lot of what is done is unnecessary but done so no one can say 'What if?'" said Distrigas terminal manager George B. Auchy.

"There's a lot of politics involved . . . a lot of stuff goes in and out of here every day that makes LNG look like olive oil," Auch said. Shipments of propane gas, chlorine and naphtha are as dangerous as LNG but do not receive the same amount of publicity, he said.

"The public is really paying (through utility costs) for lots of insurance no pragmatic man would pay for," he said.

That view is challenged by industry critics who call LNG tankers "floating bombs." Since natural gas is lighter than air, they say, any LNG spill would produce a gas vapor cloud that the slightest spark could convert into a devastating fireball.

The only known spill-related disaster took place in Cleveland in 1943 when two storage tanks ruptured and 128 persons died in the resulting fire. Experts still debate the lessons of that event and argue about wheter an empty storage tank rook collapse that killed 40 persons in Staten Island, N.Y., in 1973 was related to LNG.

"We've had no trouble at all" with the Everett terminal or LNG imports, said Henry Lee, director of the Massachusetts Energy Policy Office, "but you only need trouble once to last you for a long time." He added that more attention probably should have been paid to the terminal's location, now in the industrial heart of greater Boston and within hailing distance of the mammoth Mystic River bridge. Any major spill there could be disastrous.

The Cove Point terminal, by contrast, is on the shoulder of Chesapeake Bay about 40 miles south of Annapolis and 60 miles southeast of Washington, a bucolic spot about 10 miles south of the Clavert Cliffs nuclear Power Plant.

"It's just an excellent design, an excellent spot "where God would have built it if God could afford it," Auchy said. President Carter's energy program calls for location rules that would prohibit future LNG terminals in densely populated areas.

If the Coast Guard was edgy about the Descartes, the men aboard the tugboats guiding it were not. "We dock it like any other ship," said Bill Cohen, dispatcher for the Boston Tow Boat Company, which handles 90 per cent of all harbor traffic.

Capt. Sal Vernaci of the tugboat Cabot was clearly bored on the slow six-mile journey up the deserted channel behind the big tanker. At one point, he edged the Cabot up to the tanker's stern just over the propeller and nudged its, as if urging the vessel to move faster. Tanker crewmen grinned down on the tug from above. "Ah, just fooling around," Vernaci said. That wasn't dangerous? "No, just like any other ship," he said.

Vernaci had plenty to do later as the tugs maneuvered the tanker through a 100-degree turn so it could berth facing seaward, as Coast Guard regulations require.

Once the Descartes was tied up, workman connected three stainless steel pipes into a "closed loop" system - two to take the LNG to storage tanks and one to bring back vapor displaced from the storage tanks to the ship. The lines first were precooled with liquid nitrogen from a tank on the ship, and frost from air moisture condensation formed quickly on the outside of the uninsulated sections of pipe.

LNG is so cold that the 1,900 feet of pipe carrying it from the terminal to the storage tanks actually contract five full feet in length. The pipeline is punctuated with accordion-pleated joints and stretch space on curves to allow for that.

Like the ships, the pipelines and storage tanks are heavily insulated but include no cooling equipment. Any LNG that warms up sufficiently to become gas again is returned to the ship to be used as supplemental fuel. The storage tanks are equipped with elaborate emergency venting devices, although the destiny of most of the LNG is to be reheated back to a gas anyway and fed into ordinary natural gas mains. Some is loaded in liquid form onto refrigerated trucks and shipped out.

At Cove Point, the ship-to-storage pipelines are encased underwater in a 6,400-foot tunnel, a concession to esthetic considerations raised by the Sierra Club. Industry men complain privately about such things. The tunnel cost $30 million more than projected spending, while more than 150 permits of various kinds had to be obtained for Cove Point construction, according to assistant resident engineer Edward S. Schwartz. "You'd be amazed at some of the hassles we got into," he told a group of tourists recently.

The permits are granted, however, and Cove Point soon will be watching the pipes frost and groan as they do in Everett when the LNG comes through. At 17,500 gallons per minute, the Descartes is emptied in 12 to 14 hours.

THe Distrigas men discount the possibility of sabotage on the pipes, noting heavy plant security while LNG is moving an emergency shutdown equipment they say could minimize any spillage.

"The industry is ready to go into big importing," said Arthur Knight, president of Boston Tow Boat and one of the persons instrumental in getting LNG facilities here. "We're aware of the eyes of the world upon us, but we can't behave like we're handling eggs all the time. It has to be done on a routine basis, and that's the way it is," he said.