Page 113 - Collision Avoidance Rules Guide

required to take action at an early stage. If action is not taken in good
time there is a danger that the vessel being overtaken may take action
which could confuse the situation.
A power-driven vessel which approaches another power-driven
vessel from a direction approximately 22.5" abaft her beam may be
in doubt as to whether she is an overtaking vessel or a crossing
vessel. There should not be any doubt at night because a crossing
situation is indicated if a side-light is seen, but the aspect cannot be
determined accurately by day. Rule 13(c) requires such a vessel to
assume that she is overtaking and keep out of the way. As the other
vessel may ascertain that a crossing situation exists, and take action
to avoid a vessel crossing from her own starboard side, the vessel
which is to starboard should preferably turn on to a parallel course
and subsequently pass ahead.

Interaction
It is now generally accepted, as a result of model tests and practical
experience gained by ships replenishing at sea, that when two ships pass
close to one another, on roughly parallel courses, forces of attraction
and repulsion are set up between them. This effect is known as interac-
tion, It will be greatest in shallow water and when the two vessels are
moving at high speed in the same direction with little difference of
speed between them. In the case of two vessels passing on opposite
courses interaction will have little effect, but in overtaking situations
the course of one or both of the vessels may be affected to an appreci-
able extent, especially when a large vessel is overtaking a smaller one.
The maximum distance between two vessels at which interaction
may be noticed will vary with the size and speed of the ships and the
depth of water. It may be over 300 metres in some cases. Even in
deep water interaction may be experienced by fast vessels overtaking
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