The tiny, 17-ship Omani Navy is now guarding what is arguably the most important and vulnerable waterway in the world, the Strait of Hormuz, through which 60 percent of the Western world's oil moves each day.

An average of 77 ships a day -- one every 19 minutes -- passes through the strait and the majority of them are oil carriers.

Both the east and westbound shipping channels through the strait lie off Oman's Musandam Peninsula, within Oman's territorial waters.

The tankers that ply the oil lanes through the 24-mile-wide entrance to the Persian Gulf provide Japan with 96 percent of its oil. About 38 percent of America's petroleum and 56 percent of Europe's oil comes from the Gulf.

Before the collapse of the shah, Iran had replaced the British in policing the strategic choke point.

"When the Iranians were on the northern shore, we were very much the junior partner," said a senior British naval officer attached to the Royal Oman Navy. "We thought the major responsibility for surveillance was in the hands of our friends. Now all that has changed, and we don't see much of the Iranians now.

"In June the U.S. State Department got upset about the possibility of a terrorist attack on one of their tankers," said the officer, who asked not to be identified. "They must have had some intelligence. As a result, we dusted off our plans but we weren't too concerned because we thought we could handle it. We were joined by a U.S. destroyer for a while."

Another alert came July 23, which is both the Egyptian national day and the anniversary of the Omani sultan's ascension to the throne. Oman is virtually the only Arab state supporting President Anwar Sadat of Egypt and the Camp David accords, a position which has generated Palestinian antagonism.

In 1971 the Palestinians tried, unsuccessfully, to sink an Israeli ship in the Red Sea with a missile carried in a speedboat. It was feared that an attack might be made on shipping in the Strait of Hormuz.

The 17 Omani vessels guarding the strait include two Britist-built 37-foot, missile boats flying the sultan's flag, each armed with a French-made surface-to-surface missile. In addition, the Omani Navy has four smaller gun-boats, two support ships, an assortment of landing craft, a wooden dhow -- "very good for sneaking up on people" -- the royal yacht and a topsail schooner for training purposes.

All the skippers and more than half the officers are expatriates, mostly British, either on loan or on contract to Oman. But there is an intense program of "Omanization" going forward, and it is hoped that by next year Omani officers may begin coming into command positions.

It is thought unlikely that any power would actually try to blockade the channel, "not even Iran in its present unpredictable mood," said the officer. The primary danger, in Oman's view, is that a terrorist group might acquire mines.

Given the depth of the channel and the swiftness of the current, it would take a very sophisticated mining effort to close the strait to all ships, and only the most modern of the world's navies could accomplish it. But it would not take such an effort to frighten the ship operators or the big insurance companies, such as Lloyd's of London, which could effectively close the strait to shipping by refusing to take the risk. A nervous Lloyd's has put the Persian Gulf on and off its high-premium war-risk zone list in recent months.

"You can buy World War II mines off the shelf from one merchant of death or another," the naval officer said, "and with the silicon chip revolution there will be no limit to what you can do in the future."

In order to improve its naval forces, and especially its mine-seeking capabilities, Oman has suggested a plan whereby oil producers and oil importers would contribute to a fund to equip the Omani navy. "We are seeking $100 million," Oman's secretary for foreign affairs, Yusef Alawi, said in a recent interview. "And don't forget it is not Oman that needs the strait kept open. Our oil is found south of the strait, and is piped straight to the Arabian Sea."

Iraq has roundly denounced the Omani plan on the ground that Oman sought to involve the Europeans and Americans in the effort. Saudi Arabia also poured cold water on the project in public. But according to Alawi, the Iraqis have offered to contribute money and equipment and the Saudis have not forally turned down the plan. The Omanis care less about where the money comes from than they do about buying modern equipment.

"It is for the good of the world," Alawi said.

Oman's requests are expected to be comparatively modest because the Omani Navy does not want to make the same mistake the shah did by buying equipment too sophisticated for its forces to use. And Oman does not want to delay the process of Omanization by keeping expatriate officers on any longer than necessary.

The united States, which sent a military mission here in September to review all of Oman's defense needs, is worried about the perception of vulnerability in the strait. If it is perceived by the states in the region that the industrial world could be held to ransom, it would affect U.S. relations with the Gulf states and Saudi Arabia, sources say.

Virtually all states in the region, however, would like to avoid direct superpower involvement in the Gulf, although Soviet and American naval ships frequently pass through the strait. It is worrisome enough to the Persian Gulf states to have a 23-ship American fleet prowling around the Arabian Sea, outside the entrance to the Gulf, while Iran threatens to put the American hostages on trial.

Meanwhile, the Omani Navy puts to sea in its small ships to shepherd tankers through the narrow waters. In November, Oman moved the shipping chanels farther north away from their shore both to protect fishing, the principal livelihood of the Musandam Peninsula and to give big tankers more room to maneuver. There has not been a major collision or grounding in the strait since the Omani Navy took over the responsibility after the fall of the shah.