Page 51 - Collision Avoidance Rules Guide

the locality should be selected but the scale should be changed at
regular intervals. The scale should not be changed when there is a
dangerous target at close range.
When two radar displays are available and in use it may be advan-
tageous to select a different range scale on each display to avoid the
necessity of switching scales.

Nassau-Brott
If the master of the Nassau was relying upon radar to justify his speed in
reduced visibility it was not good seamanship to have kept his radar perma-
nently on the short range. It is a matter which I have thought about and
discussed with the Elder Brethren, and we are agreed upon what I am about
to say. They should have extended the range periodically at intervals appro-
priate to the circumstances to inform themselves of the general situation
and, in particular, of the probable effect of the approach of otherwise invis-
ible vessels upon the action of the vessel known to be, and seen to be, ahead
of them, the Brott. (Mr Justice Hewson, 1963)

Intelference

Proper use of controls should normally enable ship echoes to be
distinguished from clutter due to waves and precipitation but such
interference may sometimes be so severe that even large targets may
be obscured. Echoes from small craft are especially likely to remain
undetected when such interference is present.
The effect of rain clutter is much less when using lOcm wave-
length than when using 3 cm. This is often also the case with clutter
caused by sea return. Vessels fitted with two radars, one of each
wavelength, would be expected to make use of the 10 cm wavelength
for detecting other vessels in conditions likely to cause severe clutter,
particularly in heavy tropical rain squalls.
In 1979 the Atlantic Empress and the Aegean Captain, two very
large laden tankers, collided off Tobago near the edge of a tropical
rain squall. The Aegean Captain had just passed through the heavy
rain. Both vessels were proceeding at full speed and in each case
detection was not achieved until the range was less than 2 miles. At
the subsequent inquiry in Greece it was considered that both vessels
did not make effective use of their radar equipment and were
proceeding at excessive speed for the conditions of visibility.

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