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Some thoughts on AIS

Jun 22, 2011

To the editor: Several years ago, when the manufacturer of our chartplotter offered a software upgrade that processed AIS signals, we decided to buy an AIS receiver and give it a try. Installation was straightforward even though we used the cheapest generic AIS receiver we could find, importing it from overseas via the Internet. As useful as we have found AIS, we did discover a couple ways that this wonderful invention can get us in trouble.

AIS answered that one nagging question that bothered my wife every time we crossed the shipping channel, or otherwise encountered a commercial vessel. Namely, are we staying far enough away from that big fellow who is not going to share the space? With AIS on board, knowing the ship’s name, and where it was bound was nice, but knowing immediately the closest point of approach (CPA), and how soon that would happen (TCPA) quickly became critical need-to-know information. Once she discovered that alarms could be set for her personal comfort levels, there was no going back to simple watchkeeping.

Of course, life was sometimes complicated by the sheer volume of information available. When we encountered high-traffic areas like just offshore the St. John’s River Inlet in Florida, or off Savannah, Ga., the chartplotter would alarm with multiple targets, looking more like a pinball machine than a navigation instrument. Eventually, even my wife conceded that we couldn’t stay a mile away from everyone. We reached a compromise and safety was maintained. We tolerated closer — but still safe — encounters with the Big Dogs and the 800-pound Gorillas of the sea.

It might just be my impression, but I think that some of the big guys treat you better when you know their name from the AIS, and use it on the VHF. Nevertheless, there are some really considerate captains out there who respond well when you call their ships by name. We are truly grateful to the captain of a Norwegian Cruise Line ship coming out of Port Canaveral, in Florida, who stopped his ship to let a small fleet of sailboats pass by him safely.

The recent popularity of “Class B” AIS transceivers has, in my opinion, somewhat diminished the utility of AIS. We have long ago learned to tolerate close encounters with recreational vessels not unlike our own. It seems a little pretentious that every average-sized powerboat now announces its proximity by setting off the AIS “close encounter” alarm. At night this might be useful, but I look forward to some future software update that will allow us to switch off these little guys when they get tedious.

Even more annoying is the growing propensity of ”Class B” AIS users to leave the AIS running at the dock, day and night. There is a large powerboat in the creek around the corner from where we keep our boat Clairebuoyant with one of these “always on” AIS transceivers. Apparently she is pointed directly at us since the chartplotter cannot be convinced that we are not in imminent danger. The alarm sounds every time we drift a little in the slip.

AIS, however, became so convenient that we became overconfident, and we confused the precision of the information AIS provided with having sufficient information to navigate safely. There is an axiom that one should never rely on only one source of information when navigating. As comfortable as one gets with AIS, we found at least two opportunities in which AIS could lead to real trouble.

Early on in our AIS experience, we used it to plot our crossing of Chesapeake Bay’s shipping channel. Commercial ships crossed our course at two or three times our speed. Before AIS, it was sometimes difficult to properly time these encounters. After AIS, it was much easier to plot any required change in course. That is, until one day we altered course away from a ship to open up a more comfortable CPA, and suddenly found that our CPA dropped from several thousand feet to several feet. We quickly realized that while the AIS had calculated the CPA with great precision, it did not tell us that the encounter was behind, not in front of the ship. We had altered our path to go directly into the ship’s path. While this was immediately clear to us in daylight, a nighttime encounter could have resulted in confusion, and with only minutes to recover from the error, possible disaster.

Our second AIS-related close call was more serious. Returning from the Abaco Islands, we were motoring north up the Florida coast at night when we encountered, via AIS, a southeastern-bound tug on an intersecting course. On his starboard bow, and with AIS showing enough distance (CPA) to pass in front of him, we held our course. As we closed with the tug, the CPA diminished enough that we became uncomfortable with crossing in front of the tug. We decided that it would be safer to alter course to pass behind the tug. Of course, AIS allowed us to alter course enough to clear the tug without going too far off our northbound course. As we closed with the tug, my wife found him with the stabilized binoculars and her Ocean Navigator School of Seamanship (ONSOS) captain license training kicked in. “Quent, he’s showing running lights that he has a 200-meter tow!”

Our rudder was hard over to port by the time she finished the sentence and we headed away from the tug and his towline. Had she not known what to look for, we would likely have snared the towline, rode it back to the barge, and drowned when the barge ran over our sailboat. It would have been our fault. We were so confident in the AIS, that we were not watching the radar, which would have clearly shown the tow.

We still use AIS, but we learned an important lesson that night. AIS is now one tool among the many tools necessary for safe navigation.

Quent Kinderman lives in Maryland and sails his boat Clairebuoyant, a Pearson 424 sloop, on Chesapeake Bay.

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