Safety | Techniques & Best Practices


Collision Avoidance 2.0

By Evans Starzinger

The single biggest advance since radar, AIS has come of age for recreational boats, allowing us to automatically communicate our course and speed with other vessels, and avoid accidents.

Photo of a large container ship with a small boat near it out at sea
Photo: John Whitman

In 1998, while my wife, Beth Leonard, and I were on our maiden voyage aboard our brand new 47-foot sailboat from Florida to the Chesapeake Bay, we came as close as we ever have to getting run down by a freighter. Approaching the traffic separation scheme at the mouth of the Chesapeake in the dark, the radar showed a steady stream of commercial vessels in both the incoming and outgoing lanes. Those lanes converge off Cape Henry, just south and east of the Chesapeake Bay Bridge-Tunnel that connects Virginia Beach with Virginia's eastern shore. A traffic separation scheme is one of the few places where a sailboat — even under sail — must give way to commercial traffic, so I was looking for an opening that would allow us to cross the nearly 4-mile-wide zone without interfering with the containerships, bulk carriers, tug and tows, and fishing vessels that were traveling into and out of the Bay. Unfortunately, even at our best motoring speed of 8 knots, and crossing the channels at right angles to minimize the distance, we needed half an hour to get from one side to the other.

After hanging around the edges of the outbound channel for 20 minutes, I thought we had an opening. We made it across the outbound channel, but as we entered the inbound channel 15 minutes later, I heard a call on the VHF. "Fishing vessel near the Bay Bridge, this is the freighter Wallensia." I was not a fishing vessel, and I was five miles from the Bay Bridge, so it didn't occur to me that Wallensia was hailing us. Only after I picked up an oversize dot on the radar closing at an alarming speed on a collision course did I connect the repeatedly heated attempts on the part of the freighter to contact the mysterious fishing vessel with us. I scrambled for the VHF and was roundly cursed for my efforts. I explained we were a sailboat, crossing as fast as we could, and said we would do whatever they told us to do. More expletives followed, and then I was told to maintain course but to go faster and get the $&@* out of the way. I pushed the throttle to just below redline and squeaked across in front of the freighter, close enough that its bow wave rolled our boat from gunwale to gunwale.

If we'd known what AIS was, and if AIS had been available for recreational vessels at that time, we would have given almost anything to have had one aboard that night. AIS (Automatic Identification System) allows vessels to automatically communicate their course and speed with each other and identify when there's risk of collision. It's the single biggest advance in collision-avoidance technology since the development of radar. If you navigate in waters with a great deal of commercial traffic, make trips of several hundred miles along the coast, or take your boat offshore, an AIS unit may well be worth the investment.

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Installing An AIS

Each vessel in the AIS system is identified by a unique MMSI (maritime mobile service identity) number. When you purchase an AIS in the U.S., the first thing you'll need to do is provide the manufacturer with an MMSI, which they'll program into the unit. If you have a DSC radio onboard, then you probably already have an MMSI, and that's the one you should use in the AIS as well. If you don't know what it is, you'll have to look it up on your radio, or find the slip of paper you wrote it down on when you programmed the radio. If you don't already have an MMSI, you can get one for your vessel either from the FCC or, if you do not intend to leave U.S. waters, from BoatUS ( for free.

AIS transceivers will need both a VHF antenna and a GPS antenna, while receivers need only the VHF antenna; most take a GPS feed from another GPS unit onboard.

You can either "split" your current VHF antenna between your VHF and AIS, or install a separate independent antenna for the AIS. If you use your current VHF antenna, you'll need to install an antenna splitter (available from West Marine, The VHF antenna cable plugs in one side of the splitter box, and the VHF and AIS plug in the other side. The AIS will transmit its signals at regular intervals, except when you key the VHF microphone to talk. Using a splitter is usually a quicker and simpler installation job, but the splitter will slightly degrade the AIS and VHF transmissions. Whether you use a splitter or not, it is best to install the AIS antenna as high as possible to maximize the AIS range.


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For a comparison of Class A and B transceivers, check out the U.S. Coast Guard's page at: