The Offshore Voyaging Reference Site

AIS Revisited

I’ve written before on the potential benefits of AIS for small craft, and having used it far more since then, it’s time for an update. After being initially impressed with it, and the capabilities it offers beyond radar, has it lived up to that first impression? A good test was when we recently crossed to Morocco west of the Straits of Gibraltar, a remarkably busy stretch of water, which is where AIS should (and indeed, did) come into its own.

We have a stand-alone Simrad AI50 unit, interfaced to our plotter. When we first installed it we tended to rely on the smaller unit to examine shipping information, partly because we were still getting the hang of the plotter. But since then we have learned to rely almost entirely on the AIS overlay on the plotter, because it’s all so much easier to take in on the plotter’s large screen, especially when there are many AIS-enabled ships around.

As the chart information on the smaller unit is (necessarily) very basic, the plotter also allows far more detail, relative to topography and other charted features such as shipping lanes. But it also allows you to make better use of the additional range advantage of AIS. We can pick up ships at 60nm quite commonly, and can foresee groups of ships bunching together that may affect us as they converge on the lanes or a natural turning point. Being able to gauge the relative speed of each individual ship in a group to see who’s overtaking who can be helpful when you’re planning to thread your way through a big fleet.

So Far, So Good

But there are a few glitches. One small drawback with AIS is that whilst it gives you Closest Point of Approach (CPA) data, it doesn’t specify whether the approaching vessel will pass ahead or astern of your vessel. You still have to work that out, and tracking via the radar overlay, especially at medium to close range can help in that regard.

And you can’t always believe what it says on the screen—not all ships display the correct (or full) information—we spent ages trying to work out what a tug was doing crossing ahead of us, where the AIS said it was towing, the lights said otherwise, and the radar confirmed that there was no tow. And as AIS is still being phased in for smaller fishing vessels we’ve yet to see any of the fleet down here so equipped, so it’s back to radar and eyeball Mark 1 when they are around.

Radar Isn’t Perfect Either

Which highlights one of the weaknesses of radar, as nearly all of the fishing vessels along this coast are wooden, and they just don’t show up well on radar. Successfully tracking small wooden vessels (or GRP yachts, for that matter) in poor visibility is part science and part art, in my view. You really need to know how to set up your radar to get the best return, and then watch it carefully for any sign of faint or irregular echoes, although there is a price to be paid in terms of power consumption for such constant usage. AIS uses very little power, so it can be left on when on passage with a pre-set guard zone and alarm, but that is definitely not a reason to be complacent when there is every likelihood of non-AIS equipped boats being around.

We intend heading down the African coast in a few weeks, an area that supports a big fishing fleet. And as we get further south, we’ll start to encounter more of the artisanal fishermen who work off the beach in pirogues. Reading the pilots and talking to friends who’ve sailed there already, many of these pirogues work well offshore, and are not even lit at night, let alone equipped with AIS. Being wooden and very low in the water, there’s no doubt that they will be almost impossible to pick up with radar, except perhaps at very close range in the calmest of conditions. The risk of running one down on a dark night is a real one, so we’ll do all we can to avoid that, staying well out to sea, watching the radar and maybe carrying extra crew for that leg to help with the watchkeeping, but it’s still a real cause for concern.

And AIS? Well, there’s no doubt that if you do much of your cruising where there’s lots of shipping or visibility is often poor, it has to be worth considering. But otherwise perhaps it should be viewed as a luxury rather than a necessity, unlike radar which has many uses far beyond ship avoidance. But AIS does take a lot of the guesswork out of dodging shipping in busy areas, and if used alone, or better still in conjunction with a well tuned radar and a watchful set of eyes—it’s well worth having.

Further Reading

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Will Taylor

Do naval vessels use AIS?

John Harries

Hi Will,

In our experience British, Danish and Canadian naval vessels do. American ones do not transmit although I believe they do receive.



Great information here.

I recently learned of the following web site that tracks commercial traffic. It is an “extension” of AIS. Of course, to access it, you will need WIFI or a cellular data connection.

The site will display the track of the commercial vessel (or yacht if they have AIS) and display photos of the vessel, displacement, speed, etc. Really interesting data.

Keep in mind the data is slightly delayed, so the boat may have traveled a bit since the past/last update.

We find it useful while navigating on the Chesapeake Bay for ID of traffic and validation that they are moving or at anchor.

They even have an app for the iPhone, although I have not downloaded it yet to try it out.

Best Regards

Trond Hjerto

I like this article.
As for the radar let us hope that someone that has fitted so called Broadband radar will write, I hear that you can pick up Lobsterpot bouys and seagulls swimming on the water. Yeah, I know… It sounds like a Chuck Norris “truth”. But they are supposed to be a giant leap from the older technologies.
As far as running the AIS on the chartplotter I like what you say, but you seem to have left out one benefit. When you have a big Tanker coming towards you, it is good to check the depths around yourself and the tanker. If I head in towards shallower water where there is plenty of water for a little sailboat and highly unlikely or even impossible for the tanker to go. Just keep an vigil eye on that depthmeter. I don’t want to end up grounded and I certainly would not like to end up like the sailboat in last weeks Cowes Regatta. It got hit by…you guessed it, a tanker. The Hanne Knutsen, with a Norwegian owner.
You say have problems reading if you are on collision course, going in front of or behind the ship. First off, any small boat, especially slow ones, will bounce along.
1. The heading arrow will flop back and forth, as the boat twists in the waves.
2. The boat will slow down up a wave and speed up downhill.
Try inshore in calm waters and you will have a easier time finding the results.
I tend to make a slight change of course 2-4 degrees in good time before you are forced to make your decision. I would usually aim towards the stern, rather than the bow. If it improves the situation all is good and you take the necessary steps to make a safe passing. If it does not maybe you need to go around the bow, which I consider less safe, depending on your distance. Most cruiseships I have talked to require a maximum one half a nm distance.


William daley

We have been using AIS for several years now (simrad AI50). Interfacing with the chartplotter to overlay on chart/ radar is much better than watching (and mentally integrating ) 2 separate displays. Another benefit of transmitting AIS that is not often mentioned is that we should appear on that approaching tanker’s display as an ICON associated with our radar return, which is much more likely to get noticed, when a faint return alone may not.


interesting stats on the number of sailboat and pleasure powerboats using AIS by country

Bill Attwood

Hi John and Colin,
I want to report a good experience with a marine electronics supplier, and hope that this is the appropriate thread. I have a Vesper AIS installed, a stand-alone unit with its own tiny screen. It works well and I have no issues with it. However, as I am now using TZ iBoat with an iPad as my electronic navigation system, I noted that they have a $10 add-on which allows AIS data to be displayed on the iBoat chart. At more or less the same time I discovered Em-Trak and wished I had known about them earlier. They are a subsidiary of SRT Marine Systems, the largest manufacturer of AIS mother boards worldwide, and have a range of products serving the market from commercial to leisure. I’ve bought a “black box” AIS engine from Em-Trak which will communicate with my iPad over WiFi. It also supports Bluetooth (but not for Apple products!). At last to my point. I want to praise them for their excellent customer support. They react quickly and constructively to problems. If anyone is considering a new AIS transponder, I can only recommend the firm. I have NO commercial interest in them, but feel that the rarity of good customer support in the area of marine electronics deserves a cheer.
Yours aye,

Marc Dacey

My only call to Vesper for our otherwise solid XB-8000 unit was to get the pin assignment order. The tech took a picture of a connector end and manually labelled it in (probably) MS Paint and sent me a JPG, allowing me to fix the problem from then until now. I was impressed.

Bill Attwood

Hi Marc
I am also a fan of Vesper – and indeed of any NZ sailing product. I have Kiwi-slides and Kiwi-grip on my boat. ?

John Harries

Hi Bill,

Absolutely the right place, and a great idea to give credit where credit is due. Thanks.

Rolf Niebel

Hi Colin, hi John,
I want to install an AIS transponder (I have a receiver but want to switch to a transponder).
My question is about positioning the antenna.
Em-Trac and other manufacturer say the antenna should be installed minimum of 3,5 metres above deck to be safe from HF radiation.
The Radar-pole on the aft-deck which I could also use as an antenna-base is only 2,4m.
Should I use a spiltter and share the VHF-antenna with my VHF-radio?
Best regards.

Marc Dacey

I will mention our experience here. We have a SH GX-2200 VHF as a helm radio and it comes with AIS reception and we get that data via the mast top antenna. Our AIS transponder is the Vesper XB-8000 and it has its own whip on our first port spreader, well above the deck but below our Furuno radome. We have found that height adequate to both receive and transmit AIS data and, on a practical level, easier to reach from deck than the mast top.

We use the Vesper data to feed into the B&G helm plotter and therefore have redundancy in terms of seeing AIS targets. This was most helpful coming down the St. Lawrence last summer and certainly was helpful in fog in the Maritimes as the number of “working vessels” increased (save for the lobster boats that do not like to tell other fishers where they are doing well, evidently.

We also presented an AIS target to the Canadian Coast Guard, which was equally helpful. We could be “seen” at least 12 NM ahead by sailboats our size and farther by larger ships. AIS also complemented our RADAR and we were never really surprised by overtaking ships making twice or more our night time passage speed. While we own a powered splitter, I would likely use it strictly for the VHF and a rail-mounted whip rather than for both AIS and VHF together on one antenna, which might compromise two means of communication at once if the antenna was damaged or otherwise out of action. Hope this helps.

John Harries

Hi Rolf,

I don’t think the hight recommendation is anything to do with RF safety, but rather to get a high enough height of eye to have decent range.

Anyway, we started off with a splitter but found that we got an annoying click on the VHF every time the AIS transmitted, so we went over to an antenna on the radar stand. Ours is about 4 meters above the water. That works fine, although the range we see ships on AIS is much reduced, but still a quite acceptable 10-15 miles. I’m guessing the click might have been a problem with the splitter, rather than intrinsic to the technology, but even so we like the radar stand option because it’s simpler, and also gives us a spare VHF antenna in case of dismasting.

Paul Kanev

Do you consider the increased AIS range from a 12 watt class A transponder worth the added costs over a 5 watt class B system? Many thanks
Paul Kanev

John Harries

Hi Paul,

I haven’t looked at that, but I don’t think I would pay a bunch of money to get extra transmit range. Ships seem to see our class B at a minimum of five miles or so, and that’s more than adequate.