Collision Avoidance
Collision
Avoidance is the main job of the watch officer during his sea watch. At
any time he needs to know what is happening around his ship and who
else is there. The traditional way of doing this is to watch the sea
surface and the horizon closely at all time. Even if one or two
lookouts are posted on the bridge wings the watch officer remains
responsible to detecting all ships and possible hazards around
him. If he has spotted a target he must find out if there is a risk of
collision and if so he needs to react to avoid collision. He can do
this through a change of course or a change of speed or a combination
of both. Whatever he does must be in accordance with the Rules of the
Road.
COLREG
The
Rules of the Road (COLREG) are an international convention to avoid
collisions at sea. They consist of 38 rules grouped into 5 parts. Part
B is about the steering and sailing rules and there you find the rules
about the lookout, safe speed, how to detect a risk of collision and
how to react if such a risk exists. It also includes rules about the
conduct of vessels in bad visibility and the responsibilities between
different kinds of vessels. It tells you how to behave in head on
situations, crossing situations, when overtaking another vessel or if
being overtaken.
RADAR
Radar
is very important on the bridge. In bad visibility it is the only means
to detect another vessel or other target in time for efficient
collision avoidance. It can also be used for navigation purposes and to
receive weather information by seeing wave or rain echoes.
The
radar antenna is placed in the rigging of MIR and sends electromagnetic
waves
to the front, back and the sides of the ship. If these waves meet metal
they
get reflected and return to MIR where the radar antenna collects them
again. In
accordance to the time and intensity of the refection the computer
calculates the size and distance of the object. In case of no other
ship inside the chosen radius the radar switches on standby modus, but
returns automatically if any reflection appears. In the case of a
possible collision an acoustical alarm is given. Unfortunately there is
one problem with this useful instrument if it is used on square-rigged
ships. Their own sails produce blind patches in accordance to the
position of sails. This is a reason why very often no sails are set in
misty weather.
ARPA - Automatic Radar Plotting
Aid
Having
worked a lot with ARPA during my last voyage I want to give those who
are no professional navigators a short inside into this technique.
ARPA stands for Automatic Radar
Plotting Aid. It is a radar system of the high end of technology.
On
MIR we have an ARPA which allows us to change between different
projections so that every watch officer can choose the one he likes
best. If working with ARPA in connection with ECDIS (Electronic chart
display and information system) the "Relative Motion / North Up /
off-centre / true vectors" projection has proved to be most handy and
so this was the projection which was normally installed when we were on
the watch.
The
system we use on MIR also allows us to choose between different ranges.
This starts from 0,5 miles and goes as far as 96 miles. However, in the
waters where we were sailing 96 miles is absolutely useless as radar
cannot "look around corners". I really doubt that it is of any use
anywhere because anything 96 miles away is not really relevant for
navigation except for a landfall towards a very distinctive coastline
after crossing an ocean. I have no experience on it being used in
connection with weather forecasts, but maybe it is possible to receive
some information about bad weather ahead from the 96 miles picture.
For
our voyage we normally used a range of 12 miles. However, when in
narrow waters often 6 Miles or for an approach even 3 miles seemed to
be more handy. In the open sea speeding with 15+ Kt. we sometimes use
the 24 miles screen. On a radar screen the own ship normally appears in
the centre. However with modern tools this is not the rule any more. We
used our radar screen decentralised to give us more information
of what is going on ahead.
The
ARPA now projects us the targets not only like a green spot or azimuth
as
traditional radars do. It already shows the vector of motion so
that we can decide whether a target is of interest for us or not. Those
which might come near to us or might cross our track can acquired with
a track ball and we can receive further information about them: speed,
course, how close they will come, when this will be. Additionally
every target we acquire will appear on the ECDIS so that we can watch
it's moving not only while standing directly in front of the radar
screen but also from the chart desk. Having an ECDIS-display in his
cabin the master can watch it while he is working on his desk. Another
display is standing on the training bridge so that everybody on board
can at any time see where we are and what is going on in our
surrounding.
However,
if you are not near the display and not near the radar screen the ARPA
also gives acoustic warnings for each vessels that comes into the
chosen closest passing distance. While in relative motion projections
any standing bearings can easily be seen in true motion you need to
take bearings to find out if there is a risk of collision. You can do
this directly on the ARPA screen and it is actually not much different
from taking bearings to vessels in sight. However, being a school ship
MIR also has a traditional radar set on the bridge to give the students
the
possibility to train working with simpler technology. This is a north
up projection which is only connected with the gyro compass and works
just in the same way as the radar on any yacht. It is also used as
backup in case the ARPA does not work or if people are working aloft on
the fore mast near the antenna of the ARPA radar and it therefore has
to be switched off. The antenna of the traditional radar set is
situated on the mizzen mast.
After
2 weeks in good weather with good visibility one morning we were
anchoring in deep mist. We could not see the main mast from the bridge.
As I knew from previous radio contact the KRUZENSHTERN was anchoring
just 3 cables
from us. However, on the radar she looked not different than a buoy and
if I had not known she was there I would have never thought it from
what information the ARPA gave me. It takes a lot of experience to
interpret a radar picture correctly and it is the same with ARPA. Only
if you permanently practice this during times with good visibility you
will be able to see something on the screen in bad weather.
Professional navigators are only allowed to navigate with ARPA in bad
visibility if they have had some special ARPA training and prove to
have a certain amount of practical hours of work with ARPA every year.
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was updated 10/07