STOPPING IN SHALLOW BASINS
A vessel in very shallow water drags a volume of water astern which can be as much as 40% of the displacement. When the vessel stops this entrained water continues moving and when it reaches the vessel’s stern it can produce a strong and unexpected turning moment, causing the vessel to begin to sheer unexpectedly. In such circumstances accompanying tugs towing on a short line may sometimes prove to be ineffective.
The reason for this is that the tug’s thrust is reduced or even cancelled by the proximity of the vessel’s hull and small underkeel clearance. This causes the tug’s wash to be laterally deflected reducing or even nullifying the thrust. The resultant force on the hull caused by the hydrodynamic action of the deflected flow may also act opposite to the desired direction.
