AIS
JJMoon Diary
Barry and Margaret Wilmshurst
Tue 16 Jun 2009 12:58
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A word about AIS: Every commercial
ship of over 300 tonnes must now carry an Automatic
Identification System for transmitting
its details over the marine VHF frequencies. Yachts are not obliged to
do so but there is equipment available at moderate cost that can read the
data only or read the data and transmit limited information about the
yacht at low power. We came to the conclusion that this was a coming
technology that would be a useful aid to safety. The small boat equipment
can either be self-contained, showing the information on a small screen, or can
be linked to existing computers and chart plotters.
After considerable research we opted for a transponder that would send as
well as receive and would link to our electronic charts on the laptop and
the radar/chartplotter which we use for navigation back-up.
The results are very interesting.
Whenever a commercial ship (warships are excluded) comes within about 30 miles
of us a ship-shaped symbol appears on the chart with a vector line
indicating course. Put the cursor on the ship and a panel appears
with:
Name
Identification numbers
Range
Course
Speed
Closest
Point of Approach (in nautical
miles)
Time of
Closest Point of Approach (in
minutes, or negative minutes if the ship has passed)
Type of vessel
Type of cargo
Destination
Changes of course
Rate of change of course.
It is the CPA and TCPA that are most
useful in giving clear guidence about which ships to keep an eye on
and when to take avoiding action. On the first night out of
Noumea Mags was entering the log when she noticed a ship right on the line
of our projected course, heading towards us with a CPA of 1.6 miles.
However, the target was 22 miles away and travelling at 15 knots so no
panic for an hour or so. In the event the ship made a slight alteration of
course and passed a safe distance away. Some of this information is
available on radar but it is much more clearly presented with AIS and in a much
more stable form. Also, the radar uses a good deal of power and we only
turn it on when necessary. The AIS transponder uses very little and it can
be run all the time. AIS does not obviate the need for
radar, for example fishing boats, the bane of yachties' lives, do not show
up, but together they make a very good safety system.
When a big ship approaches, especially in
a narrow channel it is sometimes necessary to call up and find out what their
intentions are or explain our own plan for avoidance. Knowing the name of
the vessel gives much greater confidence than: "large red freighter in
approximate position XX XX YY YY; large red freighter
in.............". Alternatively, one can enter the nine digit
identification number and initiate a VHF call directly to the
bridge.
The system also has entertainment
value. It is really very interesting to see what is passing, what they are
carrying and to where. Of course coastal states are very interested in
this. While in New Zealand the coastal radio station was identifying large
ships using AIS and calling them up to require them to move out
of zones restricted for environmental reasons.
We have sailed half way round the world
and seen very few ships. We now realise that, in some areas at least,
there is quite a lot going on out there that never
meets the eye.
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