The approach to Kolkata is one of the more demanding river passages a deep-sea vessel will make. It is a long stretch of pilotage through a shifting, silt-laden channel where the navigable water is defined less by the shoreline than by the sandbars beneath it. The banks move. The channel moves with them. Tidal streams run hard, and in the bends they do not run parallel to the intended track — they set across it.
That cross-set is the part crews underestimate. In open water, a current is a correction: a few degrees of drift angle, adjusted and forgotten. In a bend in a confined channel, the same current becomes a force acting on the vessel’s pivot point at precisely the moment the hull is already swinging. The margin between the intended track and the bank is measured in ship-widths, and the time available to notice a developing problem is measured in seconds, not minutes.
Add a large container vessel — high windage, significant inertia, a pivot point that shifts as she gathers or loses way — and the geometry becomes unforgiving. A turn started slightly late, or a rudder that responds slightly slower than expected, does not produce a small error. It produces a track that runs wide onto the outer bank.