Collisions at Sea
15 March 2017 | Puerto Los Cabos Marina, San Jose del Cabo, Mexico
Along with an easy tool to tell us exactly where we are (GPS), modern marine electronics gives us an excellent tool (AIS) to avoid collisions at sea.
Equipped with an inexpensive AIS receiver, available as an added built-in feature in marine VHF radios, you can see where all of the big ships are, and many smaller vessels also. You can set an alarm to warn if another vessel is projected to come within a specific distance of you. If you equip your boat with an AIS transceiver (also called a transponder), then you can broadcast your position so that other vessels can see you on AIS.
The image above shows a screenshot, from Ubiquity's recent passage down Baja, showing the electronic information we get from AIS. We see the location of other vessels, and we can analyze crossing situations resulting in potential collision. This screenshot shows a projected closest point of approach (CPA) to another vessel of under .2 nm, pretty close.
My crew on S/V Ubiquity tends to look at these diagrams and get tense. I say relax. The threatening vessel in the screenshot above is over 50 nm away* and will not be close to S/V Ubiquity for hours. It does not hurt to have advanced warning of a vessel that could later pose a threat, but any worry or course change now is premature. Most likely, the two vessels will end up passing many miles from each other.
If a close vessel appears a possible threat, then that may warrant a course change, or perhaps a VHF call to that vessel, especially at night. Between San Diego and Ensenda I made a VHF DSC call to a cruise ship that passed 1.5 nm from me at night and confirmed that they did see me on AIS and also on radar.
The truth is that the ocean is huge and mainly empty. When sailing 40-100 nm off the Baja coast there was almost never another vessel we could see. The vessels my crew worried about because of the AIS targets passed so far from us to appear quite distant.
Personally, I feel that spending too much time on visual watch-keeping is a waste of time in clear conditions way off-shore. A visual sweep of the horizon every twenty minutes I consider adequate, plus setting an AIS alarm and monitoring AIS targets.
Of course, when closer to the coast, and especially if there is small fishing vessel traffic in restricted visibility, it's different, and can be quite tense, even with radar. But otherwise, relax and note but don't fret about distant AIS targets.
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*Coming down the coast of Baja we saw AIS targets far beyond normal VHF AIS range, sometimes beyond 100 nm, which could only result from AIS repeaters installed in the Baja area.