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Concepts

Hydrodynamic Interaction

Forces generated by the passage of a large vessel in close proximity that affect a smaller vessel (typically a tug) — causing loss of control, unexpected movement, capsize, or collision.

Definition

Forces generated by the passage of a large vessel in close proximity that affect a smaller vessel (typically a tug) — causing loss of control, unexpected movement, capsize, or collision.

Mechanism

When a large vessel moves through water, it generates:

  • Bow pressure wave — pushes vessels away at the bow
  • Suction zone amidships — draws nearby vessels toward the hull
  • Stern wave — turbulence and suction at the stern

A small tug caught in these zones can be:

  • Drawn toward the large vessel's hull uncontrollably
  • Pushed against a berth or structure
  • Capsized by sudden heel induced by suction forces
  • Dragged along or underneath

Bank cushion, bank suction, and squat are related channel effects that can alter a vessel's manoeuvring response near a bank or confined channel. Treat them as hydrodynamic mechanisms, not as interchangeable findings: the accident page should retain the wording and evidence status of the underlying report.

Key risk factors

  • Narrow channels limiting separation distance
  • High speed of the passing vessel
  • Small tug size relative to the large vessel
  • Tug operating beam-on to the passing vessel

Distinction from girding

  • Girding: force comes from the tug's own towline
  • Hydrodynamic Interaction: force comes from the passing vessel's water displacement

These can co-occur — a tug in a towing operation can be both at risk of girding and affected by hydrodynamic interaction.

Accidents linked to this concept are grouped by vessel type.

Tug (11)