Three acronyms, three prices, three very different search-and-rescue outcomes. When a locker fills with water or a crewmate goes over the rail at night, the beacon you chose six months ago in a chandlery is suddenly the most important piece of kit you own. EPIRB, PLB and AIS MOB beacons all promise to raise the alarm, but they talk to different networks, they alert different people, and they are useful in different scenarios. Getting this choice right is less about brand loyalty than about matching the device to how, where and with whom you actually sail.

What each beacon actually does

All three live in the "distress beacon" drawer at your local chandler, but only two of them talk to satellites, and only one is designed to stay with the boat.

  • EPIRB (Emergency Position Indicating Radio Beacon). Registered to the vessel. Transmits on 406 MHz to the COSPAS-SARSAT satellite constellation, with a 121.5 MHz homing signal for the final approach by SAR assets. Category 1 units float free from a hydrostatic release when the boat sinks past roughly 1.5 to 4 metres. Category 2 units are manually deployed. Battery autonomy is typically 48 hours of transmission at low temperatures.
  • PLB (Personal Locator Beacon). Registered to a person. Also transmits on 406 MHz, also picked up by COSPAS-SARSAT. Smaller than an EPIRB, no float-free housing, shorter minimum transmission time (usually 24 hours). Designed to be clipped onto a harness, a lifejacket or a drysuit and to go where you go, whether that is a sailboat, a sea kayak, or a mountain ridge.
  • AIS MOB beacon. Not a satellite device at all. It transmits an AIS distress message on VHF marine channels 87B and 88B, sometimes with a DSC alert on channel 70. Range is line-of-sight, so typically 4 to 8 nautical miles depending on antenna heights. It shows up as a MOB icon on the chartplotter of your own boat and any AIS-equipped vessel within earshot.

The confusion is understandable, because all three flash, all three are red or orange, and all three are marketed under the umbrella of "personal safety". But the AIS beacon is a very different animal from the two 406 MHz devices. It is a tool for local rescue by whoever is nearby, most likely your own crew. The EPIRB and PLB reach a coordination centre ashore, which then tasks the nearest resources.

How the signal gets to a rescuer

Understanding the chain matters, because the weakest link decides your rescue time.

A 406 MHz beacon (EPIRB or PLB) transmits a burst that includes a unique hex ID and, on modern units, a GNSS position. Low Earth orbit and geostationary satellites relay this to a Mission Control Centre, which passes it to the relevant Rescue Coordination Centre (in France, the CROSS). If your beacon is registered properly, the CROSS operator already has your boat name, hull colour, emergency contacts and normal cruising area on screen. Time from activation to first alert is usually a few minutes with a GNSS-equipped beacon, longer without.

An AIS MOB beacon works completely differently. It self-activates on water contact, acquires a GPS fix, and starts broadcasting position reports as an AIS message roughly every minute. There is no satellite, no coordination centre. It relies on a receiver being within VHF range and someone paying attention to the plotter. This is why the AIS beacon shines for man-overboard scenarios : your boat is 50 metres away, not 500 kilometres, and the person still on board can start a recovery run immediately. It is worth reading up on how AIS traffic is decoded on a modern plotter so you know exactly what a MOB target looks like on screen at 03:00.

Matching the beacon to your sailing

There is no single right answer. There is a right answer for your kind of programme.

Coastal day-sailing, mostly within 6 miles of shore, small crew. The dominant risk is a person going over the side, not the boat sinking. A per-person AIS MOB beacon integrated into each lifejacket gives the remaining crew an instant target on the plotter and, via DSC, alerts nearby vessels. An EPIRB is not required by regulation in this zone in France, but a manual Category 2 unit in the ditch bag is cheap insurance.

Offshore coastal, 60 miles offshore category. A Category 2 EPIRB registered to the vessel becomes the anchor of the safety plan. Add AIS beacons on lifejackets for MOB response. A PLB in the skipper's pocket is a sensible redundancy if the EPIRB is trapped in a capsized hull.

Ocean passages, high-latitude cruising, delivery skippers. Category 1 EPIRB with float-free bracket, one PLB per crew clipped to their harness, AIS beacons on every lifejacket. The layering is deliberate : the EPIRB handles vessel loss, the PLB covers separation from the boat and from other crew, the AIS handles the recoverable MOB. Combine this with a proper offshore comms plan, which we cover in more depth in our guide to satellite communicators for offshore sailing.

Charter operators. One EPIRB per boat is the minimum. If your clients are inexperienced, AIS MOB beacons on the lifejackets pay for themselves the first time a guest leans too far over the pushpit. PLBs are harder to justify in a rotating-guest context because they need to be registered to a named individual.

Registration, batteries and hydrostatic release

A beacon that is not registered is still useful, but it will cost the SAR service extra minutes while they figure out who they are looking for. In France, 406 MHz beacons must be declared to the ANFR. The registration includes MMSI (for the vessel), owner contact, secondary contact ashore and vessel description. Update it when you sell the boat, change phone numbers, or change your typical cruising area. A beacon registered to a previous owner in Brittany, activating off Corsica, generates exactly the wrong initial response.

Battery replacement is not a DIY job on most units. Expect a factory or authorised-dealer service every 5 to 10 years depending on model. Diarise it. A dead battery on a 15-year-old EPIRB has been the punchline of more than one post-incident report.

Hydrostatic release units (HRUs) on Category 1 EPIRBs have their own service life, typically 2 years. This is the same discipline as the HRU on your liferaft cradle : cheap part, catastrophic consequence if neglected. Note the expiry on the same maintenance calendar as your lifejacket inflator cartridges and firing heads, because the failure modes rhyme.

Layering with VHF DSC and satcoms

A distress beacon is one voice in a wider signal plan. It is not a substitute for a properly configured DSC-capable VHF, which remains the fastest way to raise a coordinated response near the coast. If your MMSI is not programmed, or your GPS is not wired into the VHF head, your red button does much less than you think. This is worth a five-minute audit at the nav station, and we walk through it in our note on how to properly configure your VHF for safety.

Similarly, a beacon does not replace the practised drill. Knowing which button raises what alert, in the dark, with wet gloves, cannot be improvised. Run the drill at least once a season, including the full MOB recovery sequence with the AIS beacon actually keyed on (in test mode, obviously, not live).

Two-way satellite communicators sit alongside beacons but do not replace them. They let you describe the situation, negotiate a response, and confirm you are OK once help is on the way. A one-way 406 MHz alert cannot do any of that. Serious offshore boats now carry both, and increasingly a third layer via Starlink for routine comms.

The honest answer

If someone forces you to pick one device with one budget, the answer depends on the single most likely bad day you will have. Coastal skippers whose worst-case scenario is a crewmate in the water should start with AIS MOB beacons on every jacket. Offshore skippers whose worst-case is losing the boat should start with a registered EPIRB. Solo sailors and delivery crews who might be separated from both the boat and the crew should carry a PLB on their person, full stop. Most experienced skippers eventually end up with all three, layered, because they cover genuinely different failure modes.

Beacons only work if their maintenance windows, registration details and battery expiries are tracked as rigorously as engine hours. That is exactly the kind of quiet, boring recordkeeping the Oria Box and the Oria platform were built to automate alongside your voyage log, so the day you actually need to press the red button is not the day you discover the HRU expired last winter.