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Lessons Learned: AIS Follies

Rob Murray

Beneteau First 435 Sloop
April 1st, 2016

We did a crossing from near La Paz, Mexico (Muertos) to Mazatlan, a distance of some 195 miles. As often happens, other boats chose to take the same weather window, and we sailed in company with five others, four of whom had AIS transceivers aboard. We formed an impromptu radio net via VHF to keep in touch and amuse ourselves on the crossing, and chatted idly as we made the 36 hour crossing.

Some of those on passage never did show up on our transceiver’s target list, and others only sporadically or when very close by, so we decided to do some investigation into the causes when we got to Marina Mazatlan.

Since I thought I was particularly clever in my installation, and I did seem to have the best target list (most targets and targets at the greatest range) on my transceiver, we used Avant‘s station as a benchmark and changed out the installation of the antennas and cables on the boats that were not showing up well.


Class B AIS transponder (CC BY 2.5)

Class B AIS transponder (CC BY 2.5)

Class B AIS transceivers are pretty simple devices, but pretty power poor on their transmit side. They need a good antenna and clean cabling to get their digital signal clearly transmitted with their tiny 2 watts of power. When you are moving less than 2 knots, they transmit your position every 3 minutes, and when you are under way at more than 2 knots, they transmit it every 30 seconds. One of the main issues with AIS is the ‘black box’ nature of the technology. You buy it, you hook it up, and if you see targets at least some of the time, you think it’s working just fine.

Since I had a Shakespeare emergency VHF antenna, I loaned that to the two other boats, one at a time, and they used it to replace their antenna and cable setups for the test. They powered up, left the units on for 30 minutes or so, and recorded what they saw (number of targets and range), while I recorded how often I saw their output signal (expecting to see them every 3 minutes like clockwork, as they were moving less than 2 knots). Then they disconnected their antennas and cables, plugged in the replacement and then looked at the results, while I again recorded how often I saw them. As we were within a few hundred feet of each other, we expected good results.

With the existing antennas, both of which were professional installations less than 2 years old, I saw them some 2 out of 5 of the expected instances, and they picked up 2 or 3 targets. With the replacement antenna, I saw them 4 out of 5 of the expected instances. Surprising to all of us was that with my rig, I saw 5 or 6 targets, against the 2 or 3 they were picking up with their existing rigs. With the replacement antenna, they each saw 11 or 12 targets, meaning my install, while better than theirs, was still far from perfect.

If you have an AIS aboard, it’s worth taking the time to compare with other boats and even getting out your spare antenna once in a while to check that the system is working as it should. If you see another boat on AIS while on passage, ask them if they see you. The results could surprise you!

Attribution: All images used in this article are licensed under CC BY 2.5


  1. Heather Cudmore says:

    A most interesting article with good information and advice. Thank you.

  2. David Ward says:

    thanks for a eeally informative post. The real life test is worth a hundred desk reviews. Could I suggest you submit it to the UK Yachting Monthly who have this issue published a test review of a widd range of AIS tranceivers.

  3. Carole Downes says:

    Nautimoments had a Digital Yacht AIS send and receive unit and did comparisons with buddy boats. We found that without fail we were always visible to the other boats. Another one to add to the list.

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