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Heads-Up Cruising: How AIS Really Works

Sometimes being a one trick pony ain't all bad. AIS may only serve one purpose—collision avoidance—but it does it in a very smart way. After all, staying in one piece isn't the worst thing when spending time afloat.

"It was quite late and I was going home from my yacht club to my house in my powerboat, and it turned out to be very foggy that evening,” says Anders Bergström of a recent encounter in the archipelago off Stockholm, Sweden. “I also have a radar on my boat. I was going pretty fast. I’m an old navy officer, and I’m quite used to radar navigation, so I was doing 35 knots in darkness and fog, because I trust my systems. I saw on the AIS, a boat was approaching from behind an island, so I couldn’t see it with radar of course.”

Bergström identified the boat and its course and speed—he could see it was a commuter ferry that goes from island to island—and he also knew where she was headed. “I could turn clearly and make more way for him to come into the channel where I was coming,” he says. “In these conditions, I could maintain my speed and pass him at a safe distance, and also give a better way for him to come into this channel.”

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Without AIS, the scenario could have unfolded much differently as Bergström’s radar would have simply revealed the 118-foot ferry as it came out from behind the headland of the island, and one or both vessels would have needed to adjust course and speed, or worse, required evasive action. “And the captain called me afterwards on the VHF and thanked me for giving him so much room on the turn, Great job,he said,” Bergström laughed. “But you’re going very fast.

Bergstrom loves such stories, and not just because he’s a former Swedish naval officer and experienced boater with both power- and sailboats. He’s also a senior partner with True Heading, a company that makes Seapilot navigation systems and apps that offer AIS functionality to recreational boaters and has now expanded to the U.S. Bergström has an understanding of the system’s capabilities from his navy days, but also because members of True Heading’s management team served on the committee that wrote the standard for the development of AIS for the International Maritime Organization in the 1990s.

The Automatic Identification System, or AIS, is a tracking system that uses transponders operating on two dedicated VHF radio frequencies to share data—including vessel name and type, position, speed, course, and rate of turn—between boats, and is updated frequently (transmission rates vary with vessel type and speed), as most of us know by now. Many large commercial vessels are required to carry it (see “AIS Regulations Under Review,”). Targets are identified on a standalone AIS unit with its own display or on the chartplotter of an integrated multifunction display, or via apps running on Wi-Fi-enabled mobile devices. Identifying target vessels by name allows a boater to communicate directly with their crews—some systems facilitate one-touch DSC calls—to make smart decisions when in close contact. Likewise, carrying an AIS transponder on your boat helps the crews of other vessels see and identify you, and communicate directly too.

There are three types of AIS devices. Class A transceivers are intended for large vessels including those covered under the Safety of Life at Sea regulations (SOLAS) of the International Maritime Organization (IMO) and cost upwards of $3,000. Class B transceivers are designed for smaller vessels and can range from around $800 to $1,800, and send and receive AIS data. Both types receive all Class A and B signals while also transmitting their boat’s ID, but there are also plain AIS receivers starting at around $500 for those boaters willing to remain unseen by other AIS users.

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Of course, AIS only works if all involved parties have a device onboard that’s set up properly. That includes making sure the AIS device has correct information input upon installation, and is connected to a VHF antenna, as well as hooking up all relevant data inputs as needed.

Since truly effective AIS requires participation of both parties, you may consider the Mini Automatic Radar Plotting Aid (MARPA) that’s included in most radars as a reasonable substitute. MARPA tracks radar targets and calculates their movements to determine any threat of collision. That’s a handy tool, since many smaller boats and many recreational vessels still don’t have AIS.

As we know, radar also has its limitations. “On the snake-like portions of the Tenn-Tombigbee Waterway (and a few similar portions of the Mississippi, Cumberland, and Tennessee Rivers) where ‘visibility’ is restricted by land mass rather than fog, darkness or rain, AIS complements the other tools very nicely, and becomes the primary source of data used to formulate a safe navigation solution,” says Bob McLeran, an experienced boater who has a Vesper Marine AIS in his quiver of navigation tools. “It is not uncommon that a commercial vessel (tug and tow, frequently) will not respond if not contacted by vessel name, so AIS helps with this situation by identifying the other vessel by name, which in turn allows one to initiate VHF contact by name, rather than trying to contact a vessel at such and such a location.”


But all that data on an MFD screen in a crowded waterway or harbor can be overwhelming. Some boaters fear screens with AIS targets completely covering the chart. However, increased processing horsepower and smart AIS filtering software is helping with that issue. “The ability to deal with the clutter is really important to us,” says Jeff Robbins, CEO of Vesper Marine. “Rather than have a screen full of icons, you can set the filters and say, ‘I only care about a target if it’s going to come within a half-mile of me.’ Immediately upon visibility of that target [to the system], you know how close it’s going to come to you. It’s not really important to see 100 icons on the chart. It’s important to see the two or three that you care about.” Just like with a radar or a depthsounder, smart boaters create AIS presets and alarms for different aspects of cruising: Crowded harbors get one setup, while offshore legs and low-visibility situations may require other settings.

AIS helps simplify the lives of boaters in other ways as well. Vessel tracking allows loved ones to monitor your progress on the water from any computer or app-equipped mobile device. “My wife is very happy that she knows where I am,” says Bergström. “I’m boating a lot by myself. She’s actually following me using Seapilot when I’m out, because she has my boat available in her data from the system.” Cruising off Sweden, Bergström also uses AIS to find friends anchored out in the islands. “It’s great for spontaneous meetings in the evenings,” he says.

Also when cruising, Bergström makes note of similar boats using remote anchorages. “My sailboat has a draft of 2.3 meters [7½ feet], which is the limit here in the Stockholm archipelago for a lot of places,” he says. “So if I see a boat here of a similar type and the same draft is moored in an anchorage, I mark it on my chart because I know I can get in there, and I can use that place the next time I come to this area.”

Of course, no electronic navigation tool or device is a substitute for common sense and good situational awareness around the boat. But AIS offers an aspect of communication that can identify problems early and help you make good decisions, and communicate to solve problems before they get serious. And it’s hard to put a price on that peace of mind at the helm.

This post originally appeared here.