Page 42 - CHIRP annual digest 2016.pdf

CHIRP Maritime

keeping out of the way “until finally past and clear” – Rule forestall its continued progress. The dimensions of the
13(d) – may meet further situations ahead that require vessel’s turning circles to port and starboard determine the
action, represented in Figure 1 by crossing vessels C, D and domain’s width. The domain’s length ahead is determined
E. The normal avoiding action for B of turning to starboard by the distance a vessel will progress after initiating a crash
has been blocked by the presence of A and a dangerous stop (Full Astern from normal sea speed). The zone either
situation can very rapidly develop. side ahead is determined by extreme rudder movements in
combination with controls on engine movements that
Scenario 2: could also bring the vessel to a standstill.
Overtaking as above but A and B may be any size and the
Whilst some of these measurements are available from trials
location need not be in traffic routing of any description.
Vessel B follows close astern of A and passes close. data it also follows that, particularly for very large vessels, the
differences between their loaded and ballast conditions could
A number of traffic surveys by radar and AIS have shown give them very different manoeuvring characteristics for each
that this situation can often occur. B is following A and of the different conditions. It should also be noted that for
closing. The two vessels however are following course lines single (fixed pitch, right handed) screw vessels (e.g. the most
that are virtually identical but B continues to follow A right commonly found and represented by A in Figure 1) the
astern. The speed differentials between A and B are such forward lobe of the domain will be biased to starboard whilst
that B is overtaking A. for a twin screw vessel it will be symmetrical (e.g. B, C, D and
E in Figure 1). In right handed, single screw controllable pitch
If the distance between A and B continues to close, the equipped vessels the zone will be biased to port as it would
risk of collision escalates in inverse proportion to that
be for a vessel equipped with a fixed pitch left handed
distance. propeller. Mariners should always seek to maintain distances

If A is keeping a lookout around the full 360°, as would be from other vessels that prevent these domains from crossing.
If such crossing of domains does occur, avoidance options
correct, the closing of B may cause concern. If A stopped
for any reason – such stoppages do occur – the closing available to mariners rapidly diminish.
speed between A and B would significantly increase and B
would be left with less time to take appropriate avoiding Issues raised
action. Evidence suggests that vessels are increasingly navigating
between commonly used waypoints. Rigid adherence to
Reports suggest that B often leaves any action until very
the course line exacerbates the increased potential for
late, causing extreme discomfort and even alarm to A,
which may be much less able to take action due to collision that this “waypoint convergence” creates.
manoeuvring encumbrance (see Scenario 1). In congested and/or restricted waters vessels should be
switched to hand steering. Auto-pilots should be dis -
The reasons for the identical course line could be various
engaged from any GPS input for steering and should never
but there is suspicion that increased accuracy of GPS
positioning coupled with following tracks based on be permitted to automatically alter course without the
direct intervention of the responsible officer of the watch
waypoints that tend to be universally used (e.g. WP A in
Figure 1), often places vessels in coinciding positions. This (OOW). Such devices are there to control the vessel in
uncongested waters but even there OOWs should
occurs more often than it would have done when the
COLREGS were last revised before the introduction of high remember that the auto-pilot does not possess any
lookout function.
accuracy position fixing systems such as GPS. The ability
of some auto pilots to steer on an input from GPS as an
In congested waters Masters should be mindful of increased
alternative to a compass is another possible factor. Further workloads on OOWs as well as possible lack of experience
danger may occur when the autopilot is enabled to alter
and should ensure that their attendance on the bridge is
course independent of human involvement. available whenever such circumstances arise. For their part,
OOWs should remember that their authority is delegated
Manoeuvring Domains to them by the Master. He (she) remains ultimately res -
In Figure 1 the vessels depicted are within a shaded zone. ponsible for their actions and should be called in good time
This zone is the manoeuvring domain. It represents the to offer advice or take full control in the event that the
boundary of the minimum area within which a vessel can situation is becoming too complex for the OOW alone.

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