Every season the same conversation happens at the pontoon. Someone has just been overtaken by a container ship in poor visibility, or nearly cut across a fishing boat in the dark, and now they want AIS. The next question is always the same : Class A or Class B, and is Class B+ worth the extra money. The answer depends less on the size of your boat than on where you actually sail, who you sail among, and how you plan to wire it in.

What Class A and Class B actually do

Both classes transmit the same core information on the same two VHF channels (AIS1 and AIS2, around 161.975 and 162.025 MHz). Both broadcast your MMSI, position, course over ground, speed over ground and heading. The differences are in how loudly you shout, how often, and how politely you queue for airtime.

  • Class A transmits at 12.5 W, uses SOTDMA (Self-Organised Time Division Multiple Access) to reserve time slots, and reports position every 2 to 10 seconds when under way, faster when manoeuvring. It also transmits voyage data : destination, ETA, cargo type, draft, IMO number.
  • Class B "classic" (CSTDMA) transmits at 2 W, listens for a free slot before speaking, and reports every 30 seconds at cruising speed. No voyage data, no ETA.
  • Class B+ (SOTDMA) transmits at 5 W, uses the same slot-reservation scheme as Class A, and reports every 5 to 15 seconds depending on speed. It gets prioritised over classic Class B in congested waters.

In real terms : in an empty stretch of the Bay of Biscay, a classic Class B is fine and a big ship's watchkeeper will see you at 15 or 20 miles. In the Solent on a Cowes weekend, or entering Rotterdam or Marseille, classic Class B messages get dropped from crowded time slots and you can effectively vanish from the ECDIS of the ship that matters most. Class B+ largely fixes that.

Who is legally required to carry what

Class A is mandatory under SOLAS for commercial vessels above 300 GT on international voyages, all passenger ships regardless of size, and most fishing vessels above 15 metres in EU waters. If you are on a recreational yacht or a small charter boat under 24 metres, no regulation forces you into Class A. You are free to fit Class B, Class B+, or nothing at all.

Some charter operators in busy areas (Croatia, the Balearics, the Greek islands) now require an AIS transponder in their fleet spec, but Class B+ satisfies that. If you plan to charter your boat in Croatia or add it to a professional fleet, check the operator's technical brief before ordering hardware.

When Class A actually makes sense on a yacht

There are three cases where a recreational owner might genuinely want Class A instead of B+ :

  1. You cross serious shipping lanes regularly. Channel, Gibraltar, Bosphorus, Malacca. Being seen at 12.5 W with a 2-second update rate when a bulker is doing 22 knots at you is not a luxury.
  2. You operate commercially and want ECDIS-grade voyage data. Pilotage services, harbour authorities and VTS operators expect Class A behaviour from workboats and commercial charter above a certain size.
  3. You want the pilot plug and the full input set. Class A units accept rate-of-turn sensors, heading from a proper gyro, and integrate more deeply with commercial bridge systems.

For everyone else, Class A is overkill. It costs roughly three to five times the price of a Class B+ unit, draws more current, requires a Minimum Keyboard and Display (MKD) or an approved equivalent, and demands more of your VHF antenna installation. Unless you have a reason, Class B+ is the sensible choice for a cruising yacht above roughly 10 metres.

Antenna, GPS and power : the install that actually matters

An AIS transponder is only as good as its RF chain. This is where most disappointing installs go wrong, not in the choice of Class.

VHF antenna. You have two options : a dedicated AIS antenna (tuned around 162 MHz) or sharing your existing VHF antenna through a splitter. A dedicated masthead antenna gives the best range, but on a sailboat with only one masthead lead, a zero-loss active splitter that prioritises voice VHF on transmit is the standard solution. Never use a passive splitter with a Class A or Class B+ transmitter : you will lose several dB in both directions and cook the splitter. If your VHF is aging, it may be worth revisiting the whole radio chain before adding AIS. Our guide on choosing a VHF for your boat covers the antenna and cable choices that also determine AIS range.

GPS. Every AIS transponder has its own internal GPS receiver, and you must mount its dedicated antenna with a clear sky view. Do not rely on feeding position from your plotter. The AIS unit's internal fix is what gets transmitted, and if it is degraded, you appear to jitter on other people's screens, which is exactly what annoys ship's officers about small-boat AIS. Details on antenna placement principles are in our piece on choosing a marine GPS.

Power and data. Class B+ typically draws 200 to 400 mA on standby with brief transmit peaks. Class A draws more, and needs a stable supply. Both should be wired to a dedicated breaker, not piggybacked off the plotter. For data output, most modern units offer NMEA 2000, NMEA 0183 at 38400 baud (AIS speed, not the 4800 used by GPS sentences) and often USB or Wi-Fi. If your boat is being progressively converted to a single backbone, the approach in our NMEA 2000 wiring guide applies directly : one drop, one T, correct LEN accounting, terminators at both ends.

Transponder vs receiver, and the MMSI question

An AIS receiver is cheap, sees other traffic, and transmits nothing. Fine as a first step, useless for being seen. A transponder both receives and transmits. If you buy an AIS to solve the "will the ship see me" problem, you need a transponder, full stop.

To transmit, you need an MMSI assigned by your national authority (ANFR in France, Ofcom in the UK, and so on). It is the same MMSI as your DSC VHF, and it must be programmed into the AIS unit. On most Class B/B+ units this is a one-shot operation from the manufacturer's software : get it wrong and the unit is locked to the incorrect MMSI, and you will need a factory reset or a return. Enter the number twice, then check it on an external AIS receiver or a public tracking site before you go to sea. While you are in the paperwork mindset, this is also a good moment to review your emergency procedures : the flow in how to send a Mayday and use DSC assumes your MMSI is correct across VHF and AIS.

What to check on the spec sheet

Once you have decided Class B+ (or the rarer case of Class A), the shortlist comes down to a few practical points :

  • SOTDMA confirmed, not just "5 W". Some older 5 W units are still CSTDMA-based.
  • Integrated splitter or not. An all-in-one box saves a shelf and a coax run, but ties the failure modes of VHF and AIS together.
  • NMEA 2000 certified if you have a modern backbone, with a clear PGN list including 129038, 129039 and 129794.
  • Silent mode switch : useful for delivery crews and for anyone who does not want their weekend anchorage broadcast.
  • SART and MOB reception handling : the unit should raise a proper alarm, not just log a target.
  • Firmware update path. AIS regulations evolve, and a unit that cannot be updated is a five-year purchase, not a fifteen-year one.

The final question is not just "which box" but "what do I do with the data". AIS traffic is a stream of position reports, and the same data your plotter uses to warn you about a CPA is worth logging alongside your own track, your engine hours and your maintenance events. That is the kind of picture the Oria Box is designed to build from your NMEA network, so a decision you made about a transponder in February keeps paying off when you are reading back a season of voyages in November. What would you actually want to see on that replay : just your own track, or the ships that passed within a mile of you.