Hydrodynamic interaction continues to be a major contributory factor in marine casualties and hazardous incidents. Typical situations involve larger vessels overtaking smaller ones in narrow channels where interaction has caused the vessels to collide and, in one case the capsize of the smaller vessel with loss of life.
Situations in which hydrodynamic interaction is involved fall into the following categories:-
- Vessels which are attempting to pass one another at very close range. This is usually due to their being confined to a narrow channel.
- Vessels which are manoeuvring in very close company for operational reasons, particularly when the larger vessel has a small under-keel clearance.
- Vessels with a small under-keel clearance which stop rapidly, when approaching an enclosed basin, resulting in unexpected sheering. Included in this category is the reduced effect of accompanying tugs which may sometimes be experienced in these circumstances.
When vessels are passing there are two situations:
- overtaking and
- the head-on encounter.
Interaction is most likely toprove dangerous when two vessels are involved in an overtaking manoeuvre. One possible outcome is that the vessel being overtaken may take a sheer into the path of the other.
Another possibility is that when the vessels are abeam of one another the bow of each vessel may turn away from the bow of the other causing the respective sterns to swing towards each other.
This may also be accompanied by an overall strong attractive force between the two vessels due to the reduced pressure between the underwater portion of the hulls.
There are other possibilities, but the effect of interaction on each vessel during the overtaking manoeuvre will depend on a number of factors including the size of one vessel relative to the other, the smaller of the two vessels feeling the greater effect.