The Strait of Hormuz isn’t open, but thanks to intrepid shipowners—some working in collaboration with the U.S. military—it isn’t quite closed either.
In recent weeks, clumps of ships, some of them the world’s largest tankers laden with oil and liquefied natural gas, have traversed the dangerous crossing, creating a tiny relief valve for the global economy.
Some ships are sailing “dark,” as it is known in the industry, turning off lights and traveling without navigational beacons known as AIS, or Automatic Identification Systems, that help prevent collisions. Turning off the service makes ships harder to spot electronically and less prone to Iranian attacks.
To navigate the strait, some ships stay in contact with U.S. military officials, who use radar, drones and other tools to monitor traffic and help them transit safely. The U.S. advises them when to go dark and how to respond to Iranian threats, according to shipowners and U.S. officials.
The flow of ships through the strait unscathed is a test of Iran’s stranglehold on global energy markets—and a test of the leverage Tehran has at the negotiating table with the U.S., where freedom of navigation remains a key sticking point.
Iran’s state media has said that the Islamic Revolutionary Guard Corps Navy, part of the regime’s powerful military arm, will maintain control and management of the waterway. In the past week, the IRGC has attempted to lay sea mines, and it fired five one-way attack drones, according to U.S. Central Command.
The U.S. has responded by sinking IRGC mine-laying boats and bombing missile and drone sites, according to Centcom. The U.S. has described the strikes as defensive and maintains that the ceasefire is still in place.
In ongoing talks, Iran has insisted it will play a role going forward in approving ship traffic, including possibly charging tolls.
This week, the U.S. imposed sanctions on the Persian Gulf Strait Authority, an entity that Iran established to screen and charge commercial ships wanting to cross the waterway.
A small number of the crossings are taking place through a part of the Gulf that the U.S. cleared earlier this month as part of its short-lived “Project Freedom.” That operation, which involved U.S. naval and air escorts, was halted after Iran began attacking vessels, and Saudi Arabia restricted U.S. access to its bases and airspace.
While Project Freedom didn’t last, it left a relatively safe path through the Gulf. U.S. forces used underwater robots to clear it of mines in the weeks leading up to the operation.
A Greek supertanker laden with two million barrels of crude communicated with the U.S. while it crossed the waterway off the Omani coast earlier this week. The ship was stuck in the Middle East Gulf since early March and is now headed to India to deliver its cargo.
Two Greek shipowners who had vessels pass through in recent days said they are being contacted by clients to ask if they can get their cargoes through in the same way.
Most of the transits this month followed the Iran-specified route by hugging its coast in the north of the strait or passed dark, so were difficult to track. Only a small number used the U.S. designated path, hugging the coast of Oman, Kpler data shows.
“Everyone’s waiting for a small window to open to get their ships out,” said Michelle Bockmann, a maritime intelligence analyst at Windward.
A Chinese-owned vessel, the Vicstar, pulled off a dark transit on May 17 carrying fertilizer from the United Arab Emirates to Brazil after being stuck in the Persian Gulf for nearly three months, according to a crew member and Kpler records. The crew member said the vessel turned off AIS at night and sailed out along the Omani coast.
The handful of vessels getting through on any given day still pales in comparison with before the war, when more than 100 ships would traverse the channel a day. And traffic has all but ground to a halt when skirmishes have broken out, including twice this week when the U.S. hit Iranian drone and missile sites near the Strait.
Capt. Tim Hawkins, a spokesman for U.S. Central Command, said the U.S. continually communicates and coordinates with ships transiting the Strait of Hormuz.
There is a strong financial motivation for the hundreds of ships stuck in the Gulf to get out, according to Alessandro Gelli, a tanker ship broker at BRS Shipbrokers.
Operators spent $10,000 to $15,000 a day in fuel and crew costs to keep large crude carriers sitting still, maritime analysts say. Insurance costs have jumped, and crews are getting paid double because of war premiums.
Sailing without AIS creates dangers. Ships can’t see one another or locate themselves on electronic charts, and instead rely on radar to navigate, which is reliable but requires an experienced person at the helm, seafarers said.
The person at the helm also needs to anticipate the intentions of other vessels. Further, radar also doesn’t show the names of other ships, making communication, coordination and maneuvering to avoid collisions challenging.
“There are obviously safety and security risks” associated with sailing dark, said Claire Jungman, director of maritime risk and intelligence at Vortexa.
Some insurers are comfortable insuring ships even as they make the two-to-three hour journey in dark mode.
“Clearly, insurers like to know that the AIS is on,” said Marcus Baker, global head of marine and cargo at broker Marsh. But he added that “it’s not always a condition of the insurance.”
Insurers take divergent views of what all agree are steep risks, brokers say. Some quote higher rates if ships have a naval escort, for example, on the assumption it makes the vessel more of a target, said David Smith, head of marine at broker McGill & Partners. Others quote lower premiums if there is an escort, because of the perceived greater protection against attacks, he added.
The U.S. government’s view of the risks is reflected in the terms of its $40 billion reinsurance program. This federal safety-net, backed by U.S. insurers led by Chubb, was announced in early March by President Trump as a way of opening the crucial waterway to shipping but has yet to be used.
It will be available only for ships with a naval escort, Chubb CEO Evan Greenberg told investors last month.
The economics of war-zone shipping-insurance incentivizes success. Ships that make it through safely get around 50% of their money back, Smith said.
Rates overall are currently around 2.5% to 4% of the ship’s value, compared with a typical 0.25% in peacetime, according to McGill. So a $100 million vessel might get back, say, $1.5 million of a $3 million fee when it makes it through the strait.
Write to Rebecca Feng at rebecca.feng@wsj.com, Jean Eaglesham at Jean.Eaglesham@wsj.com and Costas Paris at costas.paris@wsj.com