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The Official Magazine of the Bluewater Cruising Association

AIS Follies

Rob Murray

Beneteau First 435 Sloop
November 3rd, 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 5 others, 4 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 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.

Antenna connections

Antenna connections

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 at 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 2/5 of the expected instances, and they picked up 2 or 3 targets. With the replacement antenna, I saw them 4/5 of the expected instances. Surprising to all of us was that with my rig, I saw 5 or 6 targets, against the 2 or three 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 their’s, 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!

All images in this article are licensed with a  Creative Commons Attribution-Share Alike 3.0 Unported


  1. Brian Short says:


    thanks for that report and for doing the work. . I am about to install an AIS on board Carpe Ventus (Benny 445 oceanis). I have been told to install a SS whip antennae on the cockpit railing but this seems rather low to me. Can you enlighten me as to where your’s is installed? I am thinking it could go on the spreader. I believe that putting it at the mast head will allow for too much interference between the two antennas (AIS and voice),. I am getting my mast pulled this winter so it is a prime time to install the coax if need be.

    As well, it sounds like the Shakespeare emergency antenna is a good bet for a permanent installation? Which model are you referring to as their web site listed about 30 different types.

    Enjoy Mexico, the weather up here is frightful!


    1. Rob on Avant says:

      Ours is atop the Stern radar mount frame, about 12-13′ above the water. As with all antenna, higher is better. I would look for a spot higher than the stern rail, that’s both a bit low and subject to damage from lines, etc.

      The emergency antenna is a white plastic rubber duck style with a suction cup base mount and attached coax. It’s quite distinct from their regular products. It would not be suitable as a permanent antenna, but it’s a great backup. They come in a plastic stowage case ready to be put away and never used. Here’s the link:

      1. Brian Short says:

        Ok. thanks again

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