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Collision Avoidance 2.0


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.

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.

How Does AIS Work?

An AIS transceiver has an integral GPS and a VHF transmitter. It broadcasts the vessel’s name, GPS course, speed, and, if enabled, data on its route and cargo on a VHF frequency (most units are dual frequency using VHF 87B and 88B). AIS units on other vessels receive that data and, assuming both vessels continue on at their current course and speed, calculate when and how close they’ll be at their closest point of approach (CPA). The units sound an alarm if they will be closer than some minimum preset distance.

In a potential collision situation, the AIS units can do two things that radar can’t. First, AIS can “see” around corners, over islands, and through sea clutter that would block a radar return from a vessel on a collision course. Second, it gives you a specific vessel name to hail on the VHF radio. You’ll be answered much more quickly and be sure the vessel answering is the correct vessel if you hail it by name than if you call, “Vessel near the Bay Bridge.” Some AIS units will interface with your VHF and allow you to make a DSC (digital selective calling) call to a specific target, but our experience has been that a VHF 16 or 13 (bridge-to-bridge channel for ships) voice call is answered more frequently and faster than a DSC call.

On this screen zoom, the highlighted boat (with the brackets around it) will pass our boat — at center — 4.3 nautical miles away in just over 14 minutes. (Photo: Vesper Marine)

If we had had an AIS when we were entering the Chesapeake, the alarm would have gone off long before we realized the danger, probably before we entered the first traffic separation zone. We might even have been able to plot a course behind the freighter by angling across the outbound channel. If we’d gotten into a collision situation, we could have done one of two things. As the give-way or burdened vessel, the vessel that had to change course to avoid the collision, we could have stopped at the edge of the outbound channel and allowed the freighter to pass before proceeding across. Or we could have hailed the freighter using its name, visible on our AIS screen, and asked what its captain preferred that we do to stay out of the way.

In situations where there’s more room to maneuver, the give-way vessel should take clear and decisive action to avoid the collision. Where it’s unclear which is the give-way vessel, you should hail the other vessel on VHF 16 or 13 and discuss each other’s intentions and on which side you’ll pass one another.

The Three Basic Types Of AIS Units

1. Class A transceivers are required on vessels over 300 tons. These are the most powerful (12 watts) and full-featured units, but are also quite expensive (around $2,500-$3,500 depending on features). The broadcast frequency of Class A transceivers increases with boat speed, from every 10 seconds at 3 knots up to every 2 seconds above 23 knots. The greater speed and more limited maneuverability of Class A vessels make this frequent updating necessary to avoid collisions.

2. Class B transceivers were specifically designed to be used on recreational vessels and are significantly less powerful (2 watts) but also significantly less expensive (about $500-$1,000). Class B transceivers broadcast their course and speed once every 30 seconds at any speed over 1 knot.

3. Class B “receive-only” units will receive signals from other vessels, but don’t transmit your own vessel’s information. Thus you can see other AIS-equipped vessels, but they won’t see you on their AIS. These are the least expensive units ($300-$500).

The range of AIS signals will depend on both the antenna height and the transmit power. Generally you can see Class A units on ships at a range of about 50 nautical miles (although you occasionally see them at more than 100 nautical miles) and Class B on pleasure vessels at a range of about 12 nautical miles. Some AIS units have their own built-in screen for displaying the position of other vessels, but most are meant to display AIS information as overlaid data on chartplotter or radar screens. This usually entails a bit of not-too-difficult wiring from the AIS box to your plotter or radar screen. Some AIS units, including some Class B units, also broadcast their data on Wi-Fi; this data can be picked up by tablets like the iPad and displayed on the iPad plotter program, iNavX, with no extra wiring.

Several websites, such as, track vessels worldwide and show their current location and course on Google maps. This information can be displayed on an iPad or computer aboard if you have an Internet connection, and this has become the “poor man’s” AIS for those who do not want to install a Class A or B unit. Be aware that if you do have a Class A or B unit and are broadcasting your information, anyone can go on those sites and search by vessel name to find your last reported position and, in some cases, your track, including course and speed, to that position. If you prefer privacy, purchase a Class B “receive-only” unit.

If you navigate in areas with a lot of AIS-equipped vessels, you can sometimes get an annoying number of collision alarms. To minimize this problem, it is very useful to buy an AIS with three types of collision filters:

  • CPA distance – The distance to the closest point of approach
  • TCPA – The time to the closest point of approach
  • Target speed – How quickly the other vessel is moving

If you’re sailing in open, uncrowded waters, set the minimum CPA distance to around 1 nautical mile and the TCPA to around 30 minutes. The collision alarm won’t go off until another vessel is within 30 minutes of passing within 1 nautical mile of your position, giving you plenty of warning that a vessel is crossing your track, and time to maneuver, while keeping the number of false alarms to a minimum. If you’re navigating in much more crowded waters, you might set the minimum CPA to 0.2 nm and then TCPA to 10 minutes.

In most situations, you’ll want to leave the target-speed filter set at zero, so the alarm will sound if there’s a stopped or very slow-moving vessel in front of you on the water (like a fisherman picking up a pot, or a ship waiting for a pilot). However, if you’re navigating around marinas or docks where a number of stationary vessels have their AIS turned on, you can set a minimum target speed filter to something like 0.2 knots to filter out alarms for the docked/stopped vessels. Note: It’s polite to other AIS users to turn off your AIS transceiver when docked or moored.

This summer, thanks to our AIS, I passed through New York Harbor singlehanded with no drama whatsoever, and with complete awareness of all the commercial vessels around me. We’ve found the AIS to be particularly useful in high-traffic areas like that one and in heavy fog. Most ships these days transmit AIS, though we’ve seen the occasional one that didn’t (typically in those cases, it’s broken).

Useful as AIS is, don’t let yourself be lulled into thinking that you’re seeing all the vessels in your vicinity. A large percentage of recreational and fishing vessels are not equipped with AIS transmitters.

AIS is another, albeit very helpful, aid to navigation. Never rely on one single aid to navigation. Keep using your eyes and the radar to be sure you don’t find yourself in a close encounter you didn’t see coming.

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.

For a comparison of Class A and B transceivers, check out the U.S. Coast Guard’s site.

Evans Starzinger

Contributor, BoatUS Magazine