Chuck Husick: Techno-Talk, July 2003 BoatUS Magazine -

MOB Beacons
Anyone who has participated in a man overboard (MOB) drill knows how difficult it can be to keep the “victim” in sight in daylight and calm seas. Waves more than a couple of feet high can make the task much more difficult. In darkness or in rough water, a person going overboard might be lost from view in less than a minute even if his personal strobe light is operating. A minute’s delay can be critical even for a sailboat moving at 6 knots since the boat will have moved 600 feet in 60 seconds and will be a mile distant in only 10 minutes.

Although electronic technology can’t prevent a person from falling overboard, it can tell everyone on a boat that someone is in the water, automatically record the location of the person at the moment he or she went into the water and guide the boat to the person’s position. A personal MOB beacon consists of a waterproof personal emergency radio transmitter — small and light enough to be worn whenever one is on deck — and an alarm monitoring and direction finding receiver on the boat.

The personal locator beacon (PLB) typically transmits a low power signal on the civilian VHF emergency frequency, 121.5 MHz. While use of this frequency for EPIRBs is now discouraged (since the COSPASS-SARSAT satellite system is not designed to respond to signals on this frequency) it is ideal as a short-range homing signal and is transmitted along with the 406 MHz signal from EPIRBS and the PLBs discussed in the preceding issue.

Boats carrying personal MOB beacons equipped with monitor and direction finding receivers are permanently tuned to the 121.5 frequency. The low power drain receiver is always on and will sound an alarm whenever it receives the unique signal from the MOB beacon.

The MOB beacons are waterproof and can be turned on manually or set to automatically activate if submerged to a depth of a few inches. Spray or rain will not activate them. Typical power output is limited to between 3 and 100 milliwatts by the need to limit the power drain imposed on the necessarily small batteries. However this relatively low power level is sufficient to trigger the man overboard alarm.

Since the VHF radio signals travel in essentially a straight line from the antenna on the beacon to a receiver on the boat, it will be able to track and home in on a MOB up to about a mile distant. Receivers in aircraft or helicopters will typically be able to receive the signals at distances up to about five miles.

Typical MOB beacons are rectangular in shape and barely larger than a pack of cigarettes. They can be set to activate immediately upon submersion or may be manually switched on by the wearer. Since a number of MOB incidents involve an injured crewmember who may not be able to immediately activate the beacon, it is important to set the PLB for automatic operation. Rain or spray will not activate the beacon.

An interesting alternative MOB beacon design builds the transmitter into a full-function waterproof wristwatch. Anyone who dives in the open ocean and has surfaced at a distance from the dive boat will recognize the value of a dive watch that can also inform the dive boat that they have surfaced and direct the boat to their position in the water. The cost of the MOB beacons range from about $130 for the conventional variety to just under $400 for the wristwatch model.

The receivers used to monitor for MOB signals draw very little power and can be operated continually even on boats where conserving electrical power is important.

In addition to sounding an audio alarm when a MOB beacon signal is detected, the receiver can trigger the MOB function in the GPS, automatically storing the boat’s position as the MOB waypoint. The monitor receiver may have a built-in directional antenna and signal strength indicator or it may use an accessory handheld direction-finding antenna that is brought on deck only when needed.

Monitor receivers are available at prices ranging from less than $500 to more than $4,000 for units designed for use on official SAR vessels.