Rule 17 Stand-On Vessel Class: Tankers Are Named in 31% of Extremis Events and Represent 22% of the Tracked Fleet

The Setup

Rule 17 of COLREGs requires the stand-on vessel to maintain course and speed while the give-way vessel maneuvers. In practice, 75% of the 72,113 Rule 17 handoffs recorded over the past 45 days reach extremis — the point where the stand-on vessel takes independent action because the give-way vessel failed to yield.

That 75% figure is a fleet-level average. It obscures a vessel-class concentration that changes how analysts should triage.

The Chain

Breaking the 54,085 extremis events by stand-on vessel class reveals an uneven distribution. Tankers appear as the stand-on vessel in 31% of cases. Cargo and container vessels account for another 28%. Bulk carriers add 19%. The remaining 22% is split across tugs, passenger vessels, and unclassified tonnage.

Tankers make up approximately 22% of the actively tracked fleet in the same 45-day window. The 9-percentage-point overrepresentation means a tanker is named as stand-on roughly 40% more often than a random draw from fleet population would predict.

The asymmetry is not uniform across context. In approach channels — where give-way inaction rates already run high — tankers account for 38% of extremis stand-on events. In open water, the share drops to 26%. The channel concentration is the more actionable number: it reflects encounters where sea room is constrained and maneuvering options narrow fast.

The give-way side has its own pattern. Fishing vessels and high-speed craft are the give-way vessel in 41% of tanker-as-stand-on extremis events. Both vessel classes have lower COLREGS classification certainty in the encounter model, which means the geometry is more ambiguous at classification time and the extremis trigger fires later.

The Implication

Tanker over-representation as stand-on vessel has two readings. The first is operational: tankers respond more slowly to course changes, so give-way vessels may underestimate how long a maneuver takes to materialize — and delay their own action past the extremis threshold. The second is screening-relevant: tankers carry the highest cargo value per incident and face the steepest liability exposure in COLREGs disputes. A tanker that reaches extremis in an approach channel is already inside a high-documentation scenario regardless of outcome.

Neither reading is resolved by knowing that 75% of Rule 17 handoffs reach extremis. The class distribution is the layer below that number.

What to Watch

The fishing-vessel and high-speed craft share of give-way partners is worth tracking separately. Both classes show the highest rate of late classification — the encounter model assigns context (approach channel, crossing, head-on) with lower certainty when one vessel has irregular speed profiles or short AIS transmission intervals. A give-way partner that gets classified late produces an extremis trigger that looks cleaner in the record than it actually was.

If the tanker-as-stand-on rate in approach channels climbs above 40% over the next reporting cycle, the channel geometry in the top-5 congested approach corridors warrants its own review.

Limitations

Stand-on vessel classification is derived from encounter geometry and relative bearing at first detection. In approach channels with strong current or pilotage requirements, the classification can be ambiguous at detection time. Approximately 7% of extremis events in channel contexts carry a low-certainty classification flag and are excluded from the class distribution above.

Overwatch tracks pairwise encounter and Rule 17 compliance state across the global AIS feed. Coverage is limited to vessels broadcasting AIS.