Man Overboard Device (MOB)

What sets a MOB device apart

An EPIRB tells the world a ship is in distress and gives its last position; a SART helps a rescue unit close the final distance to survivors already in a raft or boat. Neither is fast enough or precise enough for the first minutes after someone falls overboard, when the ship is still moving away and every second changes the search area. A MOB device closes that gap: it is carried or worn by the individual, activates automatically on immersion or manually by a pull cord, and transmits directly to receivers on the parent vessel so the bridge gets an alarm and a bearing within seconds, before any shore-based rescue chain is even involved.

Main components

Personal transmitter unit

Worn on a lifejacket or harness, contains a water-activated switch, battery, and either an AIS transmitter broadcasting a dedicated MOB message on AIS channels or a DSC unit calling the ship's own receiver, depending on the system fitted.

Shipboard base station

Fixed on the bridge, listens continuously for the personal unit's signal, triggers an audible and visual alarm, and on AIS-type systems plots the position directly on the ECDIS or radar display so the officer of the watch has a bearing and range immediately.

Activation mechanism

Automatic immersion switch as the primary trigger, with a manual pull-tab as backup, since a person can go over the side without the automatic switch making full contact with water in a heavy sea state.

Selection points

Compatibility with the ship's existing AIS or GMDSS equipment is the deciding factor: an AIS-MOB unit is only useful if the bridge has an AIS receiver actively displaying MOB targets, not just transmitting the ship's own data. Battery replacement interval, floatation, and whether the unit is integrated into standard-issue lifejackets or carried separately also drive the choice.

Regulations and class

MOB devices are not yet a blanket SOLAS carriage requirement across all ship types, though IMO's AIS-SART and MOB signal standards define the transmission format, and many flag states and offshore or passenger vessel operators mandate them through company or charterer policy. Where fitted, class typically requires the units in the safety equipment inventory and included in emergency drills.

Typical faults

• Battery expired and not replaced on schedule, so the unit transmits nothing when triggered
• Bridge AIS receiver not configured to alarm on MOB targets, so the signal is received but not flagged to the watchkeeper
• Unit stowed separately from lifejackets rather than integrated, so it is not actually worn when needed
• Automatic immersion switch fouled by salt residue, so it fails to trigger despite full water entry

What to look for in a supplier

• Confirmed compatibility with the ship's specific AIS or GMDSS receiver model
• Battery life and a clear replacement or return-for-service schedule
• Integration options with existing lifejacket stock rather than a separate item crew forget to wear
• Documented alarm testing procedure the crew can run during drills

Confirm the bridge receiver is actually set to alarm on MOB signals, not just log them: a system that silently records a position on a screen nobody is watching gives the ship nothing in the first critical minutes.