The shadow fleet, quietly expanding

The headline figure that drives the Hormuz crisis — how much oil is moving — is partly a story about vessels that aren’t supposed to be there. Sanctioned tankers keep sailing. The fleet that carries the cargoes those sanctions were supposed to stop has roughly doubled in the past three years. Hormuz is one of its primary corridors.

The dark fleet vs the shadow fleet.

The trade press uses the two terms interchangeably, but they describe slightly different problems. The dark fleet refers to vessels that disable or spoof their AIS broadcasts — the operational signal that a ship is operating outside the regulated maritime system. The shadow fleet refers to the ownership-and-flag layer: tankers held through shell companies in flag-of-convenience registries, often with no insurance from a member of the International Group of P&I Clubs. The two overlap, but a vessel can be one without being the other. A shadow tanker with a clean AIS record is harder to spot from the dashboard. A dark vessel with conventional ownership is easier.

Estimates of fleet size vary by methodology, but the trade consensus places the combined dark + shadow tonnage at somewhere in the 600–900 vessel range, with most analyses biased low because identification depends on observing irregularity. What the analyses agree on is the trajectory: the fleet has grown roughly 2–3× since 2022, driven first by Russian oil under the G7 price cap, then by sustained Iranian export volumes despite the OFAC SDN designations.

Why Hormuz is the corridor.

Iranian crude leaves Kharg Island and Mahshahr and crosses the Strait of Hormuz to reach buyers in China, Syria, and increasingly intermediated through ship-to-ship transfers in the Gulf of Oman. The strait isn’t optional for that flow; the only Iranian export route that bypasses Hormuz is the Goreh-Jask pipeline to Iran’s Indian Ocean coast, which has run at a small fraction of its 0.35 mbpd nameplate capacity since opening. The rest moves by tanker through the strait, and a meaningful share of that is OFAC-listed.

The current closure phase has not stopped these flows. Some of the Iranian fleet has continued to load and sail; some has loitered at anchor in the Gulf waiting for the operational picture to resolve. The Straits homepage Sanctioned Tankers block surfaces every vessel currently visible on AIS in the strait window whose name matches an OFAC SDN entry. At zero hits the block reads as “clean,” but zero hits doesn’t mean no sanctioned tanker is moving; it means none is broadcasting under a matched name from inside the bbox right now.

The rename problem.

OFAC publishes vessel designations by name and IMO number. The IMO number is permanent — it follows the hull through the registry — so a designation by IMO is durable. A designation by name alone, by contrast, decays the moment the operator paints over the funnel. Trade-press analysis suggests that within months of a high-profile sanctions designation, a meaningful share of the listed vessels appear under new names through fresh shell companies in registries with limited transparency.

Match strategy on the dashboard reflects this. We join the OFAC SDN list against AISStream positions by IMO when the IMO is present in both records, and by normalised name otherwise. The IMO match is high-confidence; the name match is biased toward false-negatives (we’d rather miss a match than surface a falsely-flagged vessel). Vessels carrying sanctioned cargo under a fresh-rename + clean-IMO combination escape the join entirely.

What the dashboard actually tells you.

The Straits sanctioned-vessel block is best read as a floor, not a ceiling. A non-zero count means at least one OFAC-designated vessel is, right now, broadcasting under its designated name from inside the strait. A zero count means the join didn’t find one. Neither is a definitive statement about the underlying flow.

For the underlying flow, look at the AIS gap detector instead. The number of tankers that were broadcasting actively from inside the strait core area and have stopped broadcasting for between 3 and 24 hours is one of the cleanest available signals on shadow-fleet operational intensity. A vessel doesn’t need to be on the SDN list to want its position obscured during a politically-hot loading or transfer.

What changes when the strait reopens.

The shadow fleet’s economics depend on the Hormuz corridor staying open. A fully-closed strait would force sanctioned operators onto the same Cape-route detour that the major lines have taken — doubling voyage time, consuming extra bunker, and exposing the vessel to JWC Cape-route premiums. Some sanctioned cargoes will absorb that cost; many will not.

Conversely, a normalisation of the strait — declisting by JWC, carrier resumption — would re-open the cheapest possible corridor for sanctioned flows just as it re-opens it for legitimate trade. The same Cape-route detour whose cost the major lines have been absorbing is also the cost that the shadow fleet has been avoiding by transiting Hormuz under the radar of formal enforcement.

The cleanest forward indicator on the dashboard is the sanctioned-tankers count over time, paired with the AIS-gap baseline. A persistent uptick in either, at any verdict (open, restricted, closed), is the market saying the shadow fleet’s operational tempo has shifted — independent of what the headline crude price is doing.

Live OFAC SDN cross-join at /#sanctioned. AIS gap detector at /#shadow-fleet. Per-indicator sourcing at /methodology.