What Happened
- Since Iran's effective blockade of the Strait of Hormuz began on February 28, 2026, fewer than 100 commercial ships have crossed the strait in the entire three-week period, compared to roughly 100 ships per day under normal conditions.
- Iran has declared the strait "open to all except the US and its allies" — but in practice, it has been selectively allowing passage based on the flag state, cargo, and geopolitical alignment of each vessel.
- China-linked vessels have fared best: 11 China-linked ships transited the strait between March 1 and March 15, benefiting from Beijing's close ties with Tehran.
- Two Indian vessels carrying LPG under the Shipping Corporation of India were permitted to pass, reflecting Iran's interest in maintaining relations with New Delhi.
- A Pakistan-flagged Aframax tanker loaded with crude from Abu Dhabi became the first confirmed non-Iranian cargo vessel to transit the strait while broadcasting its AIS (Automatic Identification System) transponder location.
- Approximately one in five ships that did transit switched off their AIS transponders — a practice known as "dark shipping" — suggesting covert passages outside Iran's sanctioned list.
- Around 21 tankers in total have transited the strait since the conflict began — a fraction of the pre-blockade rate of several hundred tankers per week.
Static Topic Bridges
Maritime Traffic Monitoring: AIS and Dark Shipping
The Automatic Identification System (AIS) is a maritime tracking system that uses VHF radio to broadcast a ship's identity, position, course, and speed to other vessels and shore stations. Under SOLAS (Safety of Life at Sea Convention), AIS is mandatory for all vessels over 300 gross tonnes on international voyages. AIS data is the primary tool used by port authorities, coast guards, shipping companies, and intelligence agencies to monitor maritime traffic.
- AIS operates on two VHF channels (161.975 MHz and 162.025 MHz) and transmits data every 2–10 seconds for moving vessels.
- Class A AIS transponders are mandatory for large vessels; Class B is used by smaller commercial craft.
- "Dark shipping" — deliberately switching off AIS transponders — is used to conceal vessel movements, evade sanctions, avoid hostile surveillance, or conduct illicit trade. It is a violation of SOLAS and flag state regulations.
- The US, EU, and UK use satellite-based AIS monitoring (commercial providers like Windward, Kpler, and SpaceKnow) to track sanctions violations and dark shipping patterns.
- During the 2022–present Russia sanctions regime, dark shipping became widespread to conceal Russian oil exports; the same tactics are now being used in the Hormuz crisis.
Connection to this news: The one-in-five transit rate of ships going "dark" in the Hormuz strait indicates that some vessels are passing through without Iranian authorisation, exploiting the difficulty of physically blocking every transit in a busy international waterway.
Selective Passage and Geopolitical Leverage
Iran's selective approach to permitting passage through the Hormuz strait is not unprecedented — it reflects a deliberate strategy of using maritime access as geopolitical leverage. By allowing ships from friendly or neutral states while blocking those of adversaries, Iran attempts to drive wedges in international coalitions and reward non-aligned countries.
- China, as Iran's largest oil customer and a strategic partner, benefits from Tehran's willingness to allow Chinese-linked vessels passage — reinforcing their bilateral "Comprehensive Strategic Partnership" (signed 2021, part of a 25-year cooperation agreement).
- India's non-aligned stance — condemning the blockade while abstaining on some UNSC votes and maintaining economic ties with Iran — may be reflected in Iran allowing limited Indian vessel transits.
- Pakistan, despite being a US security partner historically, may benefit from its trade ties with Gulf states and Iran's interest in maintaining relations with neighbouring countries.
- This selective enforcement creates a two-tier shipping regime: Western (US/EU/UK/Japan-flagged or destined) vessels are blocked; Global South-flagged or neutral vessels may pass.
- For insurance purposes, Lloyd's of London and other underwriters have designated the Persian Gulf as a "war risk zone," dramatically raising premiums for any vessel attempting transit.
Connection to this news: Iran's selective passage policy effectively transforms the Hormuz blockade from a total embargo into a tool of diplomatic coercion — penalising Western-aligned nations while incentivising others to distance themselves from the US-led coalition.
The Tanker Trade: VLCC, Aframax, and the Physics of Oil Shipping
Crude oil and LNG are transported by sea in specialised tankers classified by size. The major categories include VLCC (Very Large Crude Carriers, 200,000–320,000 DWT), Suezmax (120,000–200,000 DWT), Aframax (80,000–120,000 DWT), and smaller Panamax vessels. LNG is carried in cryogenic tankers at -162°C.
- VLCCs typically load at major Gulf export terminals (Ras Tanura, Jebel Ali) and carry 2 million barrels of crude oil per voyage to refineries in Asia, Europe, and the US.
- Aframax tankers are mid-sized, capable of transiting ports too shallow for VLCCs, making them common for intra-regional crude movements (e.g., Abu Dhabi to South Asia).
- LNG tankers require dedicated regasification terminals at the destination — Europe has rapidly expanded its floating storage and regasification units (FSRUs) since 2022 to reduce Russian gas dependency.
- Under the blockade, VLCCs loaded at Gulf terminals are stranded, unable to discharge — a growing "floating storage" situation that raises operational costs and risks of accidental spills.
- Iran's own tanker fleet (often operating under flags of convenience) has reportedly been transiting freely, enabling it to continue exporting to China and other buyers.
Connection to this news: The Pakistan-flagged Aframax transit — the first confirmed non-Iranian cargo ship to broadcast its position while crossing — is a bellwether for whether Iran's selective policy creates an opening for non-Western shipping to resume, potentially easing the worst supply disruptions.
Key Facts & Data
- Pre-blockade: ~100 ships/day through Hormuz; post-blockade: fewer than 100 total in three weeks (March 1–20)
- Only 21 tankers transited since February 28, 2026
- China-linked vessels: 11 transits in March 1–15 period
- Indian LPG vessels under Shipping Corporation of India: permitted to transit
- ~1 in 5 transiting vessels switched off AIS transponders (dark shipping)
- Lloyd's of London classified Persian Gulf as a "war risk zone"
- Iran's declared position: strait open to all except US and its allies
- 3,200 vessels stranded west of the strait, representing ~20,000 seafarers