Iran and the Strait of Hormuz, Part 2: Swarming Boats and Shore-Based Missiles

11 MINS READOct 6, 2009 | 18:55 GMT

Editor's Note: This is part two in a three-part series examining Iran's ability to close the Strait of Hormuz. Iran knows its navy is no match for the ubiquitous and powerful U.S. Navy, so any credibility Iran may have in its threat to close the Strait of Hormuz rests on its asymmetric assets like small speedboats and more conventional weapons like anti-ship missiles and naval mines. In this segment, Stratfor considers the first two options, which present a clear but limited danger to traffic in the strait. Click to read Part 1 and Part 3.

In addition to its fast attack missile boats, which are part of the conventional navy, Iran also has much smaller speedboats employed by the naval arm of the Islamic Revolutionary Guard Corps (IRGC). These vessels gained some notoriety in January 2008 when they were used to harass U.S. warships in the strait.

There are many ways these boats can be employed against tanker traffic in the strait, but most involve massing them in swarms to overwhelm any shipboard defenses. Scenarios include using these small, highly maneuverable vessels to launch rocket-propelled grenades (RPGs) and other ordnance at larger vessels or packing them with explosives for use in suicide attacks. Although an RPG peppering is unlikely to do more than irritate a conventional warship that displaces nearly 10,000 tons, U.S. war-gaming has suggested that suicide tactics could present a danger to warships as well as tankers trying to maneuver in the cramped waters of the strait.

The example that quickly comes to mind is the American guided missile destroyer USS Cole (DDG 67), which was struck by a small boat in a suicide attack in the Yemeni port of Aden in October 2000. At the time, however, the Cole was moored to a pier in the cramped waters of a port and its defenses were further hindered by restrictive rules of engagement. Underway in the Strait of Hormuz and engaged in a shooting war, U.S. warships would be subject to far less restrictive rules of engagement and would be keenly on guard against approaching vessels of any sort.

Moreover, modern warships — though hardly as agile or maneuverable as small boats — are heavily armed. U.S. surface combatants not only employ five-inch naval guns but also generally have multiple .50-caliber heavy machine guns arranged to cover all quadrants and often 25 mm Bushmaster cannons. Indeed, a potential attacker could well find a Bushmaster mounted amidships not far from where the USS Cole was struck on any Arleigh Burke-class destroyer it encounters in the strait. In addition, the U.S. Phalanx Close-In Weapon System, designed as a final line of defense against anti-ship missiles, is being upgraded to include optical and infrared sensors for use against surface targets.

In addition, the size of the small IRGC boats significantly limits the amount of explosives they can effectively deliver. A single strike could be managed by effective damage control on the targeted ship, as was the case with the Cole, where a small boat packed with explosives detonated against the warship's hull on the water line. Such a strike could well achieve a "mission kill" (scoring enough damage to prevent the ship from continuing to carry out its mission), but it would not likely sink the ship.

Also, the distance between the shoreline where such boats would lurk and the shipping lanes where ships transit the strait is considerable (on the order of 10 nautical miles), and even with suboptimal visibility, the armaments on a modern U.S. warship give it a substantial range advantage. Once hostilities commenced, swarms of small boats approaching alert warships would likely suffer considerable losses while closing the distance to the point where they could inflict damage themselves.

While a large tanker would lack the defensive and damage-control capabilities of a U.S. warship, its size would provide it with its own sort of protection. The bow wave alone would make it difficult for small craft to make contact with the hull. The flow of surface water along the hull of such a large, moving ship creates strong currents toward the ship's stern. This would not necessarily prevent a small boat from making contact with the hull, but it would certainly complicate the effort. Indeed, though these small boats are maneuverable, they are not designed to operate a dozen miles from shore; the sea state itself in the middle of the strait could present its own challenges.

In addition, crude oil does not easily ignite, so a supertanker's load can actually serve to absorb explosions if such contact does take place. Indeed, tankers' compartments for crude have long been segmented, limiting the damage from any one point of impact. Double hulls have been standard in new construction for nearly a decade now and will be required for all tankers by next year. This combination of design features and sheer size further limits the effectiveness of not only small boats but also anti-ship missiles and naval mines.

Though crude oil could certainly be spilled if both hulls were breached, even a series of impacts by small boats would have trouble doing more than bringing a large tanker to a slow halt. It is worth noting that when the French oil tanker Limburg was attacked by a small boat filled with explosives in 2002 in the more open waters of the Gulf of Aden, it burned for several days before being towed to port for expensive repairs.

Shore-Based Anti-Ship Missiles

Iran is also known to have a considerable arsenal of shore-based anti-ship missiles, although the exact composition of that arsenal is unclear (and has likely been distorted by the Iranians, in any case). Indeed, the same intelligence problems that surround Iran's nuclear program extend to its arsenal of anti-ship missiles and naval mines.

Some of these missiles are U.S.-made, predating the Iranian revolution and fall of the Shah, and many were used in the Iran-Iraq War. Even in those days, Iran had begun to field Chinese missiles like Beijing's copy of the Soviet SS-N-2 "Styx," known as the "Silkworm." A number of improved variants have been spun off from this basic design, including one reportedly built in Iran. Although slower and "dumber" than more modern anti-ship missiles, this class of weapons carries a bigger punch: a warhead weighing about 1,000 pounds. Warheads on Iran's newer and smarter anti-ship missiles are one-half to one-third of that weight.

These newer weapons include a considerable quantity of Chinese C-801 and C-802 anti-ship missiles (including indigenously built copies). The C-801 is a derivative of the widely proliferated French Exocet and U.S. Harpoon, while the C-802 is an improved version of the C-801. It was one of these missiles — almost certainly provided by Tehran — that struck the Israeli warship INS Hanit off the Lebanese coast during the conflict in southern Lebanon in the summer of 2006. Iran is also thought to be building an indigenous copy of the C-801 and to be engaged in other domestic manufacturing efforts based on the various missiles in its arsenal. Iran's own production efforts not only cloud the size and composition of its arsenal but also allow it to work around limits to its industrial base and to tailor weapons for its own specific needs.

The C-801/802 missiles carry with them not only a warhead weighing some 300 pounds (similar to the amount of explosives a small boat might carry) but the kinetic energy of high-speed impact, which can lead to more extensive damage deeper inside the hull of the ship. It is worth recalling here that the recent history of anti-ship missiles vs. ship-board defenses — not only the Hanit but also the HMS Sheffield in the Falkland Islands campaign in 1982 and the USS Stark during the so-called "Tanker Wars" in 1987 — has come down consistently in favor of the anti-ship missile. (Of these three ships mentioned above, only the Sheffield sank — and then only after high seas took her down while under tow days after being struck.)

Missiles like the C-801/802 also have improved range and guidance systems. Even the shortest-range models (about 25 miles for the oldest Silkworms) have the reach to cover the strait's designated shipping lanes from the islands of Qeshm and Larak. Longer-range variants put much of the Persian Gulf and the Gulf of Oman at risk from Iranian shores.

This is not to say that a warship equipped with modern defenses does not have the ability to decoy or destroy a modern anti-ship missile; it does, and Iran's arsenal is hardly immune to modern countermeasures and defensive systems (they do not currently appear to field the most threatening classes of modern anti-ship missiles). But if Iran had the element of surprise, it could score some initial hits. And the situation could be further complicated once hostilities commenced, depending on whether Iran chose to expend its missile arsenal in single shots, hoping to survive and get lucky over time, or tried to score hits with larger salvos. The understanding of the performance of shipboard defenses at relatively close range against a large salvo is largely theoretical, since there is little operational experience in this area.

Iran has elements of its anti-ship missile arsenal deployed in batteries not only along its coast but also on key islands within the Persian Gulf near the Strait of Hormuz — with the islands of Qeshm, Sirri and Abu Musa most likely harboring significant quantities of anti-ship missiles. As a general rule, Iranian anti-ship missiles are launched from trucks and the batteries are mobile. Hence, they can be quickly repositioned as needed in a time of crisis. Fired from the coast, these missiles would emerge from the clutter of the shoreline and have very short flight times before impacting ships in the strait, leaving little time for defensive systems to react.

But the anti-ship missile option also presents fundamental challenges for Iran. Iran has only so many launch vehicles for its arsenal, so only a fraction of its anti-ship missile stockpile can be brought to bear at any given time. These batteries are not useful hidden in hills dozens of miles from shore. Most anti-ship missiles — including Iran's — do not have a terrain-following capability, so they must have a relatively straight, clear shot at the ocean, with no major obstructions. This limits the depth within Iran from which launchers can threaten the strait, and it increases their vulnerability to American naval and air power.

In addition, an anti-ship missile's maximum range generally exceeds — often greatly exceeds — the range at which it can acquire and guide itself to a target. This means that in addition to the actual launch vehicles, anti-ship missile batteries must be linked to search and fire-control radars. However, when these radars are activated and radiate, they are vulnerable to being pinpointed and jammed or hit with anti-radiation missiles. And without a battery's link to a search and fire-control radar, the effectiveness of its missiles is severely degraded. While some missiles can certainly be fired "blind" in the hope they can find targets on their own when their seekers activate, or against targets closer to shore, the effectiveness of Iran's anti-ship arsenal depends largely on its vulnerable search and fire-control radars.

Iran can also use air-launched anti-ship missiles of similar capability (and with similar payload limitations) in targeting vessels in the strait and the Persian Gulf. But fighter aircraft are much larger than anti-ship missiles and would provide additional warning when spotted by powerful American ship-borne radars. Moreover, Iran's air force would be subject to rapid attrition at the beginning of any air campaign, and the United States would be able to quickly establish air superiority. Iran's air force is in such a poor state of readiness that even in the early hours of a conflict it would not likely be able to sustain a high sortie rate for any significant length of time.

Thus, Iran must anticipate significant attrition of its anti-ship missiles once hostilities commenced, and it would certainly see an erosion of its ability to fully exploit the remaining missiles over time. So while Iran's anti-ship missile arsenal could play a role in interdicting commercial traffic in the strait — and it would probably be an effective tool for a limited or controlled escalation — it would not be able to sustain anything more than a short-term campaign to close the choke point.

To make it impassable for any length of time requires a different kind of weapon, one that is often far more primitive and difficult to counter — the naval mine.

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