I bought my first AIS transponder the winter after a near miss off Cap Gris-Nez. A ferry came out of the haze doing 22 knots, I was close-hauled and slow, and for about ninety seconds I had no idea whether he had seen me. He had, as it turned out. But I spent that winter reading and decided I would never cross a French shipping lane again without putting my own boat on everyone else's screen.
That is the part new owners miss. A receive-only AIS shows you the ships. A transponder shows the ships to you, and shows you to the ships. For coastal France, where ferries, fishing fleets and the busiest traffic separation scheme in the world all share the same water, the second one is worth the extra money.
What you are actually looking at
AIS data arrives in two flavours. Class A is fitted to commercial vessels over 300 gross tonnes and transmits often, with high priority. Class B is the leisure version, and it comes in two sub-types that matter when you are shopping. The older CSTDMA units transmit at 2 watts and get out to roughly 5 to 7 nautical miles. The newer SOTDMA units push 5 watts and reach 10 to 15 miles, and they grab a guaranteed time slot rather than waiting for a gap. On a quiet Atlantic coast the difference is academic. In the Dover Strait, where the airwaves are saturated, I would pay for SOTDMA every time.
Prices in 2025 have settled into a sensible band. A capable Class B transponder runs from around 700 to 1,100 pounds depending on whether you want a built-in GPS and a VHF splitter so it can share your masthead aerial. The AMEC Camino-108W sits near 708 pounds, the Si-Tex MDA-5 with splitter near 1,116 pounds. You do not need the dearest one. You need one that transmits.
If you are still deciding how AIS fits with the rest of your kit, it pairs naturally with the navigation apps for French waters you already run on a tablet, since most overlay AIS targets straight onto the chart.
Where AIS earns its keep in the strait of Dover
Crossing from England, you will meet the Channel Navigation Information Service, jointly run by Dover MRCC and CROSS Gris-Nez. It is a mandatory reporting area for the big ships, monitored 24 hours by radar and AIS. Two facts are worth memorising before you go.
First, CNIS broadcasts traffic information on VHF channel 11 every 60 minutes, and every 30 minutes if visibility drops below two miles. Set channel 11 on your second radio and leave it running. You will hear what is coming long before you see it.
Second, the lanes have direction. The south-west lane is the French side, the north-east lane the English side, and you cross them at right angles to the traffic flow, not diagonally. Your AIS plotter will show this beautifully: a river of magenta triangles all pointing the same way, and your own little boat crossing the current. Watch the closest point of approach figure your unit calculates. If it says you will pass within half a mile of a 300-metre container ship, alter early and alter obviously.
The detail of timing and tidal set across that lane deserves its own read, and I would go through the Dover Strait TSS for small craft before your first attempt rather than learning it underway.
Ferries, fishing fleets and the bits AIS hides
French coastal traffic is not only ships. Brittany ferries between Roscoff, Saint-Malo and the English ports run fast and to a timetable. The cross-Channel boats out of Calais, Dunkerque and Cherbourg are relentless. AIS shows all of these with name, speed and heading, which lets you do something genuinely useful: call them by name on channel 16, or better channel 13 for bridge-to-bridge, and ask their intentions. A skipper who hails "Armorique, Armorique, this is the yacht two miles on your port bow" gets a far better response than a vague securite call.
Here is what the screen will not show you. Many French fishing boats under 15 metres carry no transponder at all, or switch it off to hide their grounds. Small open boats, kayaks and the August armada of jet skis are invisible. So is anything that has lost power. AIS is a layer of awareness, not the whole picture. I keep a proper visual watch and treat a clear screen as good news, never as permission to stop looking.
One more local quirk. In the approaches to busy ports you will sometimes see AIS aids to navigation, virtual marks transmitted by the authorities that have no physical buoy. They are real and you should respect them, but do not go hunting for a buoy that is not there.
A note on the fishing fleets, because they catch out more visitors than ferries do. The Atlantic ports of Brittany and the Vendee send out large fleets that work in loose clusters, often at night, often without lights you would expect, and frequently towing gear that extends well astern. The ones that do transmit will show a speed of three or four knots and a heading that wanders, which is the AIS signature of a boat actively trawling. Give them a wide berth on the side away from their gear. The ones that do not transmit are the reason a radar overlay still earns its place on the same screen as your AIS. Where I cruise the two layers together catch almost everything the eye cannot, and the eye catches the rest.
Setting it up so it actually helps
A transponder is only as good as its data feed. Three things to get right:
- Programme your MMSI correctly when you commission the unit. It can usually only be set once without returning the device, so check every digit. The MMSI is tied to your ship radio licence, the same one behind your VHF DSC and MMSI registration in France, so the numbers must match.
- Enter your real boat dimensions and the GPS antenna offset. If the ferry plotter thinks your 10-metre yacht is a 30-metre fishing vessel, his collision logic gets it wrong.
- Set a sensible CPA and TCPA alarm. I use 0.5 nautical miles and 10 minutes inshore, opening it to a mile offshore. Too tight and the alarm cries wolf all day, too loose and it warns you when it is already too late.
Feed the output into your chartplotter and your tablet both. On passage I run the fixed plotter for the radar overlay and a phone app for the easy pinch-to-zoom when I want to read a target name fast. They draw the same targets from the same transponder, so there is no conflict, just redundancy. If a chartplotter is on your shopping list this winter, the chart cards for French waters you choose will decide how much detail sits under those AIS triangles.
Worth it?
For coastal hopping along Brittany or the Med in settled summer weather, a receive-only set will keep you informed and costs a fraction of a transponder. But the moment you commit to a Channel crossing, a Biscay passage or a night in the ferry lanes, transmit becomes the safety item, not a luxury. Being seen by a bridge officer who is watching twelve screens at once is the cheapest insurance on the boat. Mine has paid for itself in calm nerves alone, and I have never again spent ninety seconds wondering whether the ferry knows I exist.
