Page 33 - Collision Avoidance Rules Guide

to define this type of vessel it became apparent that it would not be
satisfactory to specify a minimum size or draught and it was decided
not to restrict this category to very large ships. The more appropriate
term ‘vessel constrained by her draught’ was therefore used.
The main factor which must be taken into account is the space
available for manaeuvre rather than the depth of water beneath the
keel, but the restriction of space must be due to relatively shallow
water which would not necessarily be a danger to other vessels in the
vicinity. The signals are mainly intended for use by vessels unable to
make an appreciable alteration of course, especially to starboard, due
to passing between shoals which are, however, deep enough not to
restrict other vessels. The signals should only be shown when the abil-
ity to alter course is severely restricted. On passing clear of the area
of relatively shallow water the signals must no longer be displayed.
A very large fully loaded vessel will not be justified in displaying
the signals even in crowded waters, or a traffic separation area, if there
is sufficient deep water on either side to permit course alterations.
In order to clarify this matter further the following item of guid-
ance for the uniform application of certain rules has been approved
by the IMO Maritime Safety Committee:

Clarijication of the definition ‘Vessel constrained by her draught’,
Rule 3(h). ‘Not only the depth of water but also the available
navigable water width should be used as a factor to determine
whether a vessel may be regarded as constrained by her draught.
When determining this, due account should also be taken of the
effect of a small underkeel clearance on the manaeuvrability of the
vessel and thus her ability to deviate from the course she is following.
A vessel navigating in an area with a small underkeel clearance but
with adequate space to take avoiding action should not be regarded as
a vessel constrained by her draught.’
In 1987 the fifteenth Assembly of IMO adopted an amendment to
Rule 301) changing the words ‘available depth of navigable water’ to
‘available depth and width of navigable water’. The purpose of this
amendment was to confirm and strengthen the above clarification.
Special rules may apply in some harbours, rivers or inland water
areas requiring signals to be shown by vessels over a certain size or
exceeding a certain draught. In such cases the condition that the abil-
ity to deviate from the course must be severely restricted to justify
showing the signals is unlikely to apply.

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