At 6:48 p.m., the British frigate Battleaxe crosses a hypothetical line off the coast of Oman in the southern Gulf. On the navigator's chart it is labeled: ``Silkworm envelope.'' There are no bells, no warning sirens, no flash-ing lights. The officer on the bridge simply informs the appropriate crew members to proceed with ``close up.''
For the next nine hours certain hatches and doors are required to stay closed. The ship's fire brigade is on alert. Everyone sleeps fully clothed.
``The state means we are better prepared to deal with damage should we pick up a Silkworm,'' the officer on the bridge explains.
He is referring to the Chinese-built Silkworm antiship missiles Iran has positioned at key points along the 24-mile-wide Strait of Hormuz.
The bulky missile with a 50-mile range and a speed of six miles per minute, is only one of an array of threats facing merchant seamen who regularly transit the strait and naval warships like the Battleaxe sent to the Gulf to protect them. There are Iranian warships, open motor launches, mines, and aircraft as well.
That means this narrow patch of sea is not only the choke point for the Gulf, it is also the equivalent of home turf for Iran's Navy, Air Force, and Revolutionary Guards Corps.
It is the one place in the Gulf where Iran could, if it so chose, quickly unleash the full fury of its unconventional forces in a ``jihad'' (holy war).
British naval officers and other Gulf analysts discount the threat of an all-out ``holy war'' in the strategic waterway.
They say that Iran needs access to open sea lanes at least as much as any of the other oil-exporting Gulf states. And they note that Iran's ``rules of engagement'' in the Gulf seem to preclude direct attacks on warships. Instead Iran has concentrated its hit-and-run raids on unescorted tankers.
But the British, like the Americans, French, Soviets, and others with warships in the region, are taking no chances.
``I have a full operations team ready. All weapons are ready to go, and the chaps are on the upper deck,'' says Comdr. T.J. Norman-Walker, captain of the Battleaxe, as his ship proceeds through the darkened strait under a nearly full moon.
The Battleaxe's defensive systems include air radar out to 114 miles, detailed sea surface radar out to 24 miles, and an array of electronic sensors. The computerized equipment in the ship's operations center can locate and track in flight objects as small as an artillery shell.
In addition, a contingent of Britain's Royal Marines are positioned at the bow of the ship with Javelin shoulder-launched missiles, and along each side of the bridge with high-powered binoculars, night-vision scopes, and small arms. They are looking for anything suspicious. Each man is wearing a headset wired directly into the ship's operations center - in effect, the ship's war fighting room.
The difficulty for both radar operators and the marines on the upper deck is trying to differentiate among unlit fishing dhows, darkened speedboats used by Iranian smugglers, and the 42-foot motor launches - ``Boghammers'' - that the Iranian Revolutionary Guards use for most of their hit-and-run raids in the busy waterway. Boghammer is the Swedish manufacturer who sold 50 of the fiberglass speed boats to Iran.
Fishing dhows, Boghammers, and pleasure craft all look the same on radar, if they show up at all. And in choppy and hazy seas they can be very difficult to spot visually.
Crew members on watch duty are regularly rotated to avoid problems of stress and fatigue. But the routine in the Gulf sometimes seems never ending. ``During exercises you are on the go all the time,'' says Robbie Robson, chief gunner. ``Here, it is completely different. You never know what is going to happen.''
A new threat of attack from Iran was added to the list when an Iranian F-4 jet fighter fired two missiles at a Liberian tanker in the strait on Feb. 2. Both missiles missed but naval officers see the incident as an expansion of Iran's methods of attack. Nor is Iran the only worry. Last weekend, an Iraqi jet was flying uncomfortably near a US Navy ship when the jet fired a missile at Iranian installations. It was an Iraqi jet that, in a case of mistaken identity, fired an Exocet missile into the USS Stark, killing 37 crew members.
``It might not happen, but we are expecting anything,'' says Radar Operator Chris Sharp.
He says technicians in the operations center watch for aircraft coming in low and zig-zagging or boats approaching very quickly. Operations center personnel attempt to identify the approaching object with the help of computers.
``If we still don't know what it is and it is closing, we contact it and ask, `Who are you and what are your intentions?''' Mr. Sharp asks.
``If the answer comes back, `I'm an Iranian F-4 [jet fighter] on my mission,' then we say, `Please stay out beyond five miles.'''
British officers explain that what they are primarily concerned about are mistakes or crossed signals that might lead to an unintended attack.
``You find everyone in the Gulf talking to keep the picture clear and prevent accidents,'' says Lt. Cmdr. Guy James, the Battleaxe's principal warfare officer.
He notes that in the event of an apparent Silkworm attack, naval officers would take special precautions to ensure that what they were seeing on their radar screens wasn't a low-flying private jet or other innocent aircraft.
Lt. Commander James says that officers on the Battleaxe would expect a series of tip-offs before an incoming Silkworm cleared the horizon. At night, marine sentries would probably see the flash of light from the Iranian mainland.
Next, radar technicians would pick up a fast-moving object and determine it wasn't a commercial aircraft. Meanwhile, the operation center's computer would have automatically started plotting height and speed, and electronic sensors would be employed to identify the type of radar or guidance system. That would identify it as a Silkworm, according to James. ``The whole thing is providing a picture to the commander so he can decide what to do,'' he adds.
Ultimately, a Seawolf missile would be fired to destroy the Silkworm and decoys would be deployed to draw it off target. Hand-held missiles and other close in weapons would be at the ready.
The Silkworm warhead is large enough to sink a warship. But naval officers say that under a convoy situation, the missile would probably veer toward the largest tanker in the group, because of the design of its guidance system.
The only Iranian Silkworms launched in the Gulf knocked out Kuwait's oil export terminal and hit two tankers anchored off Kuwait last October.