The British captain of the Iranian supertanker Shoush out of wartorn Khorranshahr sounded unperturbed as he chatted lazily over ship-to-ship radio with reporters on this tugboat off the coast of Oman Wednesday.

His ship, like three dozen other massive crude-oil carriers, swung gently with the tide, at anchor in a 144-square-mile anchorage 20 miles north of the Omani captial of Muscat, waiting out the war before venturing through the Strait of Hormuz. Nearby was another British-crewed Iranian tanker, the Minab, whose captain sounded equally untroubled about lying-to in Arab waters.

"We're waiting for sailing orders from our owners, the government of Iran," the captain of the Shoush said.

The other supertankers anchored at comfortably safe intervals of at least a mile apart throughout the sparkling calm waters of the Oman Gulf, flying flags of most of the major seafaring nations, reported that they were no better nor worse off than the Iranian vessels.

All were victims of what already amounts to a partial blockade of the Persian Gulf -- a voluntary blockade imposed by economic realities.

According to shipping authorities in Muscat, most of the ships dropped anchor here to await decisions by thier owners or charterers whether to steam away from the troubled region for pay the tripled costs of war-risk insurance imposed by marine underwriters following the outbreak of the Iran-Iraq war 10 days ago.

Since the war started dozens have paid the price and gone on to oil-loading ports as far north as Kuwait in the Persian Gulf. Others have turned back to the Indian Ocean to steam elsewhere for cargoes of crude oil. Some continue to stand fast here, joined each day by new arrivals so that the average number waiting in the vast anchorage remains at about 40, some days a few more, some a few less.

The result has been a decrease of about a third in the number of tankers passing through Hormuz, the vital 20-mile strait between Oman and Iran through which 60 percent of world oil supplies are shipped, sources in Oman said.

"The world has a vital interest in keeping the strait open and I suppose that is why President Carter is so concerned, but there are degrees of closing and they don't necessarily involve hostile military action," a petroleum source here said.

"Insurance rates have already discouraged some tankers that would have gone but didn't. Armed convoys would shoot the rates even higher and discourage even more traffic. The international seamen's unions probably would balk at manning the ships through the strait under those conditions, too, so you might have amounted to a shutdown of the striat with no hostile military action at all," he added.

But shipping officials in Oman, through whose territorial waters most of the Hormuz traffic must sail, said they were confident that, although the numbers have gradually shrunk since the war began, the strait will remain peaceful and oil from all of the gulf countries except warring Iraq and Iran will continue to flow.

"The Iranians have acted very responsibly in the strait so far," an Omani shipping source in Muscat said. He said that on only one occasion, three days ago, was there a mildly threatening gesture by an Iranian warship, a gunboat, against an Iraqi merchant vessel in international waters in the Gulf of Oman, about 100 miles southeast of the strait. "One of our tugboat captains observed the Iranian gunboat approach the Iraqi ship, but after coming close enough for identification, the Iranian Navy vessel turned and left the Iraqi alone," he said.

Omani officials said that their patrol craft have been especially active in Oman's territorial waters, particularly in the broad anchorage where the dozens of idle empty supertankers are awaiting sailing orders from their owners.

Paradoxically, the Omani concern has less to do with the war against pollution, said the harbor master of the port of Qaboos, just north of Muscat.

"With so many ships at an anchorage that usually has only 10 or 15, we worry about captains cleaning out their tanks and leaving the oil residue to wash in on us, so we are patrolling the anchorage very carefully," he said. "We won't even let them put a lifeboat in the water."

The harbor master said that Oman also has refused to permit the anchored ships to shut down power and leave skeleton caretaker crews to sit out what may be a long wait. "If one of them drags his anchor and drifts without power [and] without enough crew aboard to handle the emergency, we would have the baby in our laps, wrecked," he said.

As a further discouragement to ships that might want to risk a long wait in what until now has been a free anchorage, the harbor master hailed the ships in the anchorage Wednesday and informed their captains that beginning Oct. 10 if they want to stay they will have to pay regular harbor dues and maintain full time shipping agents in Qaboos, by order of the Omani government.