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Relying on AIS? There may be more out there than what you see on the screen.

Aship’s captain recounted to me a story of coming onto the bridge at sea:

“I have a vessel two miles ahead, sir,” said the second mate on watch.

“Oh yes, that’s a yacht,” the captain said.

“How do you know it’s a yacht, sir?” the second mate asked. “Because I am looking out the window,” the captain said.

The Automatic Identification System (AIS) seems to solve so many problems when you are navigating in poor visibility or at night. Your onboard unit will tell you where all the other vessels are and show their heading and speed. With the information displayed on your radar and/or chart plotter, it looks like you know everything that is going on around you. I find that AIS gives you such a positive presentation that you feel reassured, especially since AIS transmits information about what you are doing to those other vessels.

AIS sounds like the perfect navigation tool for collision avoidance, but stop and think: AIS is only showing those vessels that have a working AIS.

What about smaller vessels that might not have an AIS on board? On vessels with AIS, is it actually working? Are the AIS returns shown on the displays giving valid information? There are a lot of variables.

All ships over 300 tons are required to have a working Class A AIS system, so you should pick those vessels up on your screen, but I have come across a large dredger at night

that was not transmitting an AIS signal. I also have seen a ship heading west across the North Sea while showing Amsterdam, to the east, as its destination.

With yachts, AIS is an optional (as opposed to mandatory) piece of equipment at the helm, and yachts of cruising size will generally have a Class B AIS, which transmits less frequently and reliably than Class A units do, especially in crowded waters, and especially with older units.

Thus, you always have to ask, What isn’t my AIS showing? On a passage recently, I found myself focusing on the ferries going in and out, and I was ignoring the possibility of picking up the navigation lights of a passing yacht or small fishing boat. Yachts with AIS showed up bright and clear on my display, but the situation made me realize that without a working AIS these days, my own boat could simply disappear from the view of big ships where the navigator tends to look at his display rather than out the windows. (He obviously hadn’t heard my ship captain story.)

Ships and yachts fitted with AIS are like a big club, with all members talking to one another. Anyone without AIS is excluded from membership. If you are not transmitting an AIS signal, then you become a second-class citizen, greatly reducing your chances of being seen.

And don’t think that having a receive-only AIS will do the job. You might be able to see other vessels with it, but they will not be able to see you.

With the rising popularity of AIS on board, some skippers may be tempted to rely more on their screens than their own eyes. And that could be a costly mistake.