Every cargo vessel underway carries a declared estimated time of arrival (ETA) for its next port. The ETA appears in AIS broadcasts as a free-text field, in carrier vessel-schedule tools as a structured timestamp, and in port community systems as the value the harbour master’s office uses for berth planning. The same vessel’s ETA can differ between sources, and any individual ETA can move by hours or days during the voyage. This guide explains why.
Where the initial ETA comes from
The initial ETA for a voyage is computed by the carrier from a planned voyage routing, an assumed sea-passage speed, and a planned arrival window negotiated with the destination port. A voyage from Shanghai to Rotterdam at an assumed 16-knot mainline speed takes about 28 days; the initial ETA is the departure timestamp plus that figure plus any planned waypoint stops for bunkering.
What changes the ETA
ETAs are revised in transit by every factor that affects voyage length or arrival window. Weather routeing decisions taken to avoid storms add hours or days. Speed adjustments to optimise fuel consumption (slow steaming) extend voyages by a day or two. Berth congestion at the destination, a missed pilot window, or a delayed cargo readiness can move the planned arrival window. AIS-reported ETAs are particularly noisy because they are operator-entered — a careless deck officer who has not updated the ETA field for two days will be broadcasting an obsolete number.
How PortWatch handles ETA
PortWatch records the ETA as last reported and timestamps the report. An ETA that is stale relative to the timestamp is flagged on the vessel profile. ETAs are not interpolated forward from the reported speed and heading; the directory shows what the vessel is reporting, not what it would be reasonable to predict from physics alone.