The short answer: Class A is built for vessels required to carry AIS, such as larger commercial craft, and transmits more often at higher power. Class B is designed for leisure boats and is what almost every yacht and motorboat owner fits. Unless your vessel is required to carry Class A, Class B is normally the right choice.
The decision matters less than the installation. A well installed Class B unit with a good antenna and clean power outperforms a badly fitted anything.
Both classes transmit your position, course and speed and receive the same from other vessels. The differences are in transmit power, reporting frequency and priority. Class A reports more frequently and has priority on the shared channels; Class B reports less often and gives way to Class A traffic in busy areas.
On the busy waters of the Solent and around Portsmouth Harbour, that means your Class B signal is still seen, but at a slower update rate than a ferry or ship.
A receiver shows you other vessels but does not make you visible. A transponder does both. Many owners start with receive-only built into a VHF and later want to be seen, especially when crossing shipping lanes or sailing at night. If being visible matters, fit a transponder from the start.
AIS works on VHF frequencies, so the antenna matters as much as the unit. You can fit a dedicated AIS antenna or share the main VHF antenna through an approved splitter. Height gives range; a masthead antenna sees further than one on a pushpit rail.
Coaxial cable quality and connector workmanship make a real difference. A water-soaked or badly made connector at the masthead can halve your effective range with no obvious symptom.
A transponder needs a clean, fused 12V or 24V supply and its own GPS antenna, or a GPS source it is configured to use. It then connects to your plotter over NMEA 2000 or NMEA 0183 so targets appear on the chart. Getting that data path right is where many DIY installs stall.
A transponder must be programmed with your vessel's MMSI and details before it transmits. In the UK, the equipment should be added to your ship radio licence. Your installer can programme the unit where the manufacturer allows, but the licence itself is the owner's responsibility.
Mount the unit somewhere dry and accessible, route coax without sharp bends or crushing, and seal deck glands properly. Test transmission after installation by checking your vessel appears on another receiver or an online AIS service.
Safety note: AIS draws little current, but it connects into the same DC system as everything else. Supplies should be fused at the source and wired to marine standards.
Within Class B there are two technologies. Older and simpler units use a method that waits for gaps in the channel before transmitting. Newer SOTDMA Class B units reserve their own transmission slots, report more frequently and at higher power, and behave much more like Class A. They cost more but give a noticeably better presence on busy channels.
For a boat that regularly crosses shipping lanes, sails at night or cruises in congested water, the SOTDMA variant is worth considering. For occasional coastal use, a standard Class B unit is usually enough.
Set sensible alarm thresholds on the plotter so close approaches are flagged without constant nuisance alarms in a crowded harbour. Learn what the closest point of approach and time to closest approach figures mean and use them alongside a proper lookout, never instead of one.
Remember that not every vessel carries AIS, and that small craft, some fishing boats and many leisure boats will not appear on the screen at all. AIS is an aid that improves your picture of the water, not a complete one. Radar and eyes remain essential in poor visibility.
Most leisure boats in the UK are not required to carry AIS. Many owners fit Class B voluntarily for the safety benefit of being seen.
Yes, with a proper AIS splitter designed for the purpose. A dedicated antenna avoids the splitter but needs a second cable and mounting point.
Common causes include an unprogrammed MMSI, no GPS fix, a poor antenna connection or a unit set to silent mode. Online apps also depend on nearby shore receivers.
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