The Offshore Voyaging Reference Site

AIS—See And Be Seen

One thing that most sailors dread is poor visibility. Throw in high levels of shipping traffic and you have a perfect recipe for sleepless nights.

The waters of the English Channel are some of the busiest in the world, and fog is a common challenge especially around Ushant on the French coast and the Channel Islands further east. And wouldn’t you know it, both are areas where there are very busy Traffic Separation Schemes (TSS) to zone ships leaving and entering the narrows of the Channel. Being caught out in the shipping lanes as the fog descends is a memorable experience, to say the least.

So as we departed our winter berth in the lovely French port of Camaret in early April to return to our home port of Falmouth for a brief haul out, I had one eye on the weather, as we would be passing just to the East of Ushant (and the TSS). We have a good radar and know how to use it, so I was confident that if the visibility did close in overnight I could work our way through the shipping as I’ve done many times before, but it would mean that I couldn’t leave Lou on watch on her own as she has (luckily) little experience of such pleasures.

Like most boats we have a radar reflector, although this seems somewhat pointless as we are sailing around in 12 tonnes of oddly shaped aluminium. And I have to say that from bitter experience I have very little faith in them anyway, having had more than one near miss in fog with yachts carrying radar reflectors that didn’t show up at all until we made visual contact, at far too close a range for comfort.

When we fitted out Pelerin with her electronics package, we decided we’d try one of the Simrad AIS 50 Class B units, which would allow us to track the movements of most suitably equipped ships, with the added ability to transmit our details. We’d used it during our earlier crossing last year, and been very impressed with it, but we felt a night passage with the likelihood of lots of shipping would be a real test for it. Clearing Ushant as darkness fell, a brief conversation with the French Coastguard ended with a laconic warning for us to “watch out tonight as Ushant TSS is full of shipping”.

And they weren’t kidding—at one stage we had 23 ships in sight (the night was crystal clear), as well as two trawlers—and all of them were being picked up by our AIS unit. By scrolling the cursor onto each ship on our plotter, in a matter of seconds we had all of the necessary information regarding that vessel, including its course and speed, but most essentially its Closest Point of Approach (CPA) and the Time to Closest Point of Approach (TCPA), which would tell us whether any risk of collision existed. As a result we were able to plan a safe passage through the shipping well in advance, making smaller course alterations than might have been necessary if made at the much closer range that would have allowed us to make a visual or radar derived decision. And our guard zone alarms at ten minutes to CPA and 1 nautical mile worked perfectly.

We powered up our radar, and avoided using the AIS for a while, to make a comparison between the two systems. The radar worked fine (and I’d still rather have it over AIS in fog), but we both concluded that it was far simpler to use the AIS for ship tracking and collision avoidance.

What was particularly useful was the ability to plot ships in an overtaking situation, always difficult due to merging of radar targets, or with one or other vessel being hidden from sight behind the other. Several ships we were tracking were travelling at twenty knots or more, so the ability to choose which ones might present the greatest threat was a real help. And it was hardly surprising that the two trawlers we had in sight were both AIS-equipped as there have been several tragic losses of fishing vessels attributed to collisions in these waters in recent years.

We shared the watch for a couple of hours during the busiest stretch, before going back to our solo watchkeeping routine, and both of us used the AIS on several further occasions through the night to good effect. And we both felt that it made for a far safer and more relaxing passage than would have been the case without it. We agreed that it was very reassuring to know that our own details might be seen by any watchkeeper monitoring AIS aboard the ships that passed us in the night, thus helping us in our efforts to keep out from under them.

Of course there may be drawbacks. Class B AIS has its limitations, such as limited transmission distance and the possible filtering out of information from Class B sets in busy areas compared with commercial Class A transmissions. And like every other watchkeeping aid from visual to radar, small boats are entirely dependent on the guy on the bridge doing his job and maintaining a proper watch, and AIS is no different in that respect. But I have had less to complain about with ships over the years than many other yachtsmen, and think that most watchkeepers will (in time) make more use of Class B AIS transmissions.

It may well be the case that in the middle of an ocean or in remote areas there may be less call for AIS. But most voyages have a beginning and an end near harbours, and that usually means more traffic to cope with, and we’re all for anything that improves our ability to navigate safely through busy waters. And given the choice of being able to actively read shipping movement via AIS, backed up with even limited active reception of our own details, compared with solely relying on a radar reflector, we know which we’d choose—every time.

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Victor Raymond

Colin, I just installed the new Em-Trak B100 AIS transceiver on Rajah Laut. So far I am able to see about 60 miles out but that is on the hard. We will see what happens next week on the high seas.


Hi Victor

Sounds good, and it’ll be interesting to hear how it performs on the water.

We have a maximum range of 128M with our Simrad unit, which is perhaps a little over the top for our needs (!), but we’ve often picked up ships within that range when simply playing with the unit.

Some of these ships are huge, and their aerials so high I’m sure you’ll at least match the range achieved on the hard.

Best wishes


Dick Stevenson

Hi all,
I really fun, creative, and interesting use of albatrosses (
has collected data suggesting that far fewer vessels are using their AIS than I would have predicted. Data is still coming in and only certain international waters have been surveyed, but about 1/3rd of vessels in these areas appear to have their AIS turned off.
I doubt this is the case near big western commercial areas such as the UK and the Atlantic coastal waters of the US, but open water might be a different story. A bit sobering for those who feel AIS was a big boost to safety at sea.
My best, Dick Stevenson, s/v Alchemy

Marc Dacey

A couple of thoughts on AIS: We have the Vesper XB8000 on at the moment in our winter berth as a not-particularly-secure anti-theft device, I suppose, but I can see wanting to turn it off in certain areas of the world, the Sunda Straits and the Red Sea being two. But I can’t see how turning it off in any of the customary sea lanes is necessary or desirable, given that its range, mast head to mast head, exceeds that of RADAR and provides critical TCPA data even in daunting conditions. I consider it the best thing that ever happened to RADAR in that it is so complementary.