The Strait Dope

Supertankers carry about 90 percent of Persian Gulf oil exports through the Strait of Hormuz each day, satisfying some 20 percent of worldwide demand. For maximum safety, the International Maritime Organization suggests that the huge, difficult-to-maneuver ships travel within a designated channel while in the strait, but that channel is only a few miles wide. With such a narrow passage, many experts fear that an attacker (read: the Iranian military) could "close the strait."

The Iranians appreciate the concern: Explicit threats to the strait are a key component of their foreign policy. Alternate routes could only carry a fraction of the oil, so a disruption could cause a major price spike that would severely threaten the global economy.

But the conventional wisdom may be wrong. Regardless of how we assess the credibility of Iran’s threats, we should also assess Iran’s capabilities. Iranian military exercises apparently emphasize three weapons in the strait: small suicide boats, mobile antiship cruise missiles, and sophisticated sea mines. Using these tools, how hard would it be for Iran to disrupt the flow of oil?

The answer turns out to be: very hard. Iran would have to disable many of the 20 tankers that traverse the strait each day — and then sustain the effort. Iran cannot rely on the psychological effects of a few hits. Historically, after a short panic, commercial shippers adapt rather than give up lucrative trips, even against much more effective blockades than Iran could muster today. Shippers didn’t stop trying during World War I. Nor did the oil trade in the Gulf seize up during the 1980s Tanker War, when both Iraq and Iran targeted oil exports.

Instead, tankers tend to move around dangers. The strait is deep enough that even laden supertankers can pass safely through a 20-mile width of good water, not just the 4-mile-wide official channel. Tankers already take other routes when it is convenient; during a conflict, they would surely scatter, as they did in the 1980s. Although the strait is narrow compared with the open ocean, it is still broad enough to complicate Iran’s effort to identify targets for suicide and missile attacks. The area is too large to cover with a field of modern mines dense enough to disable a substantial number of tankers, especially given Iran’s limited stockpile.

What’s more, tankers are hard to damage with mines or the small warheads on modern missiles. And a big ship pushes a tremendous amount of water out of its way when it is moving; tankers’ bow waves would fend off most small boats attempting suicide attacks. Terrorists hit the USS Cole and the Limburg because their targets were stopped.

Surprisingly, oil tankers also do not burn well. They generally have too much fuel and not enough oxygen to sustain a blaze. Only a tiny fraction of their bulk contains sensitive equipment that, if damaged, would disable the ship. The suicide attack on the Limburg was a lucky shot that hit a boundary between a full cargo cell and an empty one full of air, so the fuel-air mixture caught fire. Even so, three days later, the ship was able to move under its own power, and after repairs, it returned to the global tanker fleet. Over five years of the Iran-Iraq War, 150 large oil tankers were hit with antiship cruise missiles, but only about a quarter were disabled.

So what? By presuming that Iran can easily close the strait, Western diplomats concede leverage, and the current U.S. habit of reacting immediately and aggressively to Iranian provocations risks unnecessary escalation. Iran would find it so difficult, if not impossible, to close the strait that the world can afford to relax from its current hair-trigger alert.