Page 62 - Collision Avoidance Rules Guide

devices required by law. It undermines the law that requires the equipment if
shipowners fail to train their masters in its operation and use. Ironically the
ship’s management had a policy that required masters to teach ARPA skills to
officers when it did not properly train the masters in the first place.

Plotting or equivalent systematic observation
Even continuous observation by a competent person is unlikely to be
accepted as proper use of radar to obtain early warning of risk of
collision if the bearings and distances of approaching vessels are not
taken at regular intervals and carefully evaluated by plotting or by
some equivalent method.
There are differences of opinion as to which form of manual
plotting, either true or relative, should be used, but it is generally
agreed that both methods have their advantages. The principles of rela-
tive plotting should be understood by all observers, as this is the
method which enables the closest position of approach to be deter-
mined. The true plot is simpler to understand, and is considered by
many to be superior when there are several targets on the screen.
Alterations of course, or speed, by the observed vessel, carried out
simultaneously with, or shortly after, an alteration by own vessel, are
likely to be more readily detected by a true plot than by a relative plot.
The term ‘equivalent systematic observation’ would apply to the
use of one or more of the various types of radar aids which are avail-
able, ranging from simple plotting devices to automatic radar plotting
aids - ARPA (see page 30). It may even be accepted as applying to the
recording of ranges and bearings at regular and frequent intervals by
an observer who has no such aids at his disposal and who may find it
impracticable to plot because of the particular circumstances.
In regions of high traffic density it may be impracticable to make,
and to evaluate, a comprehensive manual plot, but, provided the radar
is being carefully and continuously observed, it should be possible to
discard some targets which are obviously going to pass well clear
and concentrate on those with which a close quarters situation seems
likely to develop.

Assumptions on scanty information

The determination of risk of collision, in both clear and restricted
visibility, must be based on several successive observations taken as
accurately as possible. The table on page 228 gives the change of

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