Barry and Margaret Wilmshurst
Tue 16 Jun 2009 12:58
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:
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
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.