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.

The Three Basic Types Of AIS Units

  • 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.
  • 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.
  • 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.End of story marker

Evans Starzinger has completed two circumnavigations under sail, and has written technical articles on a broad range of subjects. He's just returned to the Chesapeake Bay after solo sailing in Maine for the summer.

Published: October/November 2013

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