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Understanding The Many MOB Devices

Man-overboard (MOB) devices, also called crew-overboard devices, represent a booming and fast-changing area of marine radio communications. Recreational boaters want these devices for peace of mind, and they are particularly relevant for those who go boating with children and pets, for those who boat single-handed, and for sailboats where crew need to move around on deck in sea conditions where falling overboard is more likely. Selecting a MOB device involves asking three big questions:

First, What do I Want to Happen?

Look for these MOB device acronyms which each do a different thing when they are triggered:

  • PLBs (Personal Locator Beacons; also called a “406” device) transmit a signal received by satellite which is relayed to the Coast Guard. Nearby boats are not notified. Because PLBs are monitored by satellite they will work even when you are far offshore.
  • AIS devices (Automatic Identification System; also called AIS-SART for Search-and-Rescue Transponder) transmit alerts to nearby AIS-receiver-equipped vessels – often commercial vessels. They are not monitored by the Coast Guard.
  • DSC (Digital Selective Calling) devices alert all VHF-equipped vessels within range that are equipped with DSC. This usually also alerts the Coast Guard, but might not if you are far offshore. Almost all boats have a VHF radio with DSC, so this is the best option for alerting nearby boats. This is usually the most useful audience for a MOB alert!
  • Bluetooth devices sound an alarm on a tethered cell phone app or base station in your boat. Bluetooth devices are the most inexpensive and compact option but (so far) they can not be tracked after a device has left the boat.
  • Radar-SARTs (Search-and-Rescue Transmitters) send a signal when hit by X-band radar, drawing a line on the radar screen to the location of the MOB. Rare for recreational boaters.
  • GPS (Global Positioning System) enabled devices will transmit their lat/lon location
  • RLS (Return Link Service, also just called “receive”) devices will provide a confirmation that your signal has been received. These are new in 2022.

Some of the above can be combined, so a "PLB / AIS / GPS" device will send a signal to the coast guard and nearby commercial vessels and this will include the device’s lat/lon location.

MOB devices are either manually triggered, triggered when an inflatable life jacket deploys, triggered when in contact with water, or triggered by distance away from a base station. Some devices also include a flashing light and/or an engine kill switch.

Second, What Kind of Boat Do I Have?

Your boat should inform your choices above. Bluetooth devices are not as reliable on large boats (above 40’ LOA) or on metal boats. Radar-SARTs will be much less useful if you have no radar, AIS devices if you have no AIS, and so on.

If your boat is equipped with the radios triggered by the technology of the MOB device, keep in mind that an alarm will sound on your own boat if a person falls off. So while a bluetooth device will sound an alarm on your own boat, so will a DSC device–if your boat has a VHF radio equipped with DSC. This means that the DSC device is probably more capable: it will alert both your boat and other nearby boats with radios as long as you have a DSC radio.

Third, What Kind of Boating Am I Doing?

A single-handed sailor would not want an alarm to sound on their own boat if they fall overboard, as they are single-handed! No one would be aboard to turn the boat around and pick them up. Alternately, if no commercial vessels are present where you normally go boating, an AIS-equipped MOB might not be a good choice as commercial vessels are the ones that use AIS.

Boaters sometimes tell you that the PLB (a.k.a. “406”) device is the obvious best choice as it alerts the Coast Guard, however this may not be true. In many situations your best chance of rescue will be the crew on other boats nearby. Coast Guard response may be the slowest option. If you boat in winter, cold water makes a fast response critical. When your own crew or nearby boats are alerted to a MOB, they will also call the Coast Guard just as a PLB would have.

If you boat overnight at the helm while your family is sleeping below or you are concerned about your pets/children, an alarm on your own boat may be the most important feature of an MOB device. (Note that submitting a public MOB distress call for a pet constitutes a false report, and is illegal.) Of course, a manually-deployed MOB will not save a pet or young child, and pets do not wear the kind of life jackets that can automatically deploy an MOB device.

Resources:

Activity idea:

  • Talk through some scenarios: e.g., Crew on a sailboat very far offshore falls overboard with a PLB. The Coast Guard will know the location of the MOB but the boat will not. The boat must establish communication with the Coast Guard to learn the location of their MOB. They did not hear an alert from the crew falling overboard, and may not have noticed.

Linking to videos that discuss particular radios as examples is not intended as a recommendation or endorsement. 

Updated 2023-Feb-10 by Christian Sandvig, SO-CM