Strait of Hormuz Closure: How Iran’s Conflict Fuels a ‘Shadow Fleet’

The Strait of Hormuz is effectively closed to most commercial traffic. Since February 28, 2026, following joint military strikes by the United States and Israel on Iran, oil tanker traffic through the world’s most critical oil shipping choke point has dramatically decreased, dropping by more than 90%.

Iran has threatened to destroy any ships, including oil tankers, attempting to transit the strait. Insurers are assessing coverage on a ship-by-ship basis, and the international maritime community has acknowledged a ship’s crew’s right to refuse sailing into the area. As of March 6, more than 400 tankers were stranded in the Persian Gulf.

However, some vessels are still navigating the strait. These are largely what maritime professionals call the “shadow fleet” – vessels that operate outside established international regulations. They exist since the oceans aren’t governed like land, where borders are closely monitored and rules are enforced. At sea, the system is largely voluntary.

A System Built on Trust, and Easily Circumvented

The tracking of ships relies on a voluntary system. The International Convention for the Safety of Life at Sea (SOLAS) requires most commercial vessels to carry a radio transponder broadcasting their identity, position, speed, and heading. However, there’s no mechanism preventing a crew from switching it off or broadcasting false information. When a vessel goes “dark,” it simply disappears from tracking maps.

A ship’s flag state – the country under which it’s registered – is theoretically responsible for its regulation and inspection. But in practice, registration is often a commercial transaction. Some companies choose flags of convenience, whereas others may even operate under flags that don’t actually exist, effectively becoming stateless.

Insurance provides a degree of enforcement, as mainstream insurers require adherence to safety standards and sanctions. However, vessels can avoid these requirements by obtaining insurance from “unknown” providers or engaging in ship-to-ship transfers to obscure their activities.

The Rise of the Shadow Fleet

The shadow fleet didn’t emerge because the maritime system failed, but because it’s built on voluntary participation. For decades, it worked because opting out was too costly. However, international sanctions have changed that calculation. Iran began building a parallel system in 2018, and Russia expanded it significantly in 2022.

Now, with the Strait of Hormuz largely closed to legitimate trade, the only vessels still moving are those that disregard the rules. According to maritime intelligence firm Windward, approximately 1,100 dark fleet vessels represent 17-18% of all tankers carrying liquid cargo.

How Vessels Operate in the Shadows

A typical shadow fleet operation involves purchasing an aging tanker, registering it through a shell company under a flag of convenience, obtaining opaque insurance, and switching off its transponder when approaching sensitive waters. Cargo is often transferred ship-to-ship at sea to avoid detection. If scrutinized, the vessel can change its name and re-register under a different flag.

In December 2025, the United States seized the tanker Skipper, which was flying the flag of Guyana despite never being registered there – rendering it stateless. Another vessel, the Arcusat, even altered its International Maritime Organization (IMO) identification number, the maritime equivalent of removing a vehicle’s VIN.

Implications for Global Oil Supply

The current situation highlights the fundamental nature of maritime regulations. They are not commands, but agreements. The system worked when compliance was the most economical option. Now, as sanctions and geopolitical tensions increase, opting out becomes increasingly viable. This poses a significant risk to the stability of global oil supplies and the enforcement of international trade regulations.

The existence of the shadow fleet doesn’t necessarily indicate a failure of the rules, but rather reveals their inherent limitations. It sends a message that circumventing the system is possible, potentially encouraging more vessels to operate outside the bounds of international law.

The opinions and views expressed are those of the author alone and do not necessarily represent those of the Department of the Navy or the U.S. Naval War College.