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Chuck Husick: Techno-Talk, July 2003 BoatUS Magazine  - Updated June 2009

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. A personal locator beacon (PLB) can be used as a man overboard beacon, however these devices are not designed to activate automatically and are therefore useful in a MOB situation only if the person carrying the device can activate it. When activated the PLB will emit two radio signals, one on 406 MHz for detection by the COSPASS-SARSAT system, the other a low power signal on 121.5 MHz that may be detected by a receiver carried on board for that specific purpose. (Marine VHF radios cannot receive this signal.) Reception range for the PLB's VHF signal may extend to only a mile or two, depending on the height of the receiving antenna on the boat and the sea conditions. Receivers in aircraft may be able to home on the 121.5 MHz signal from distances of 5 miles or more, depending on sea conditions. Unlike EPIRBs, PLBs do not have integral strobe lights. The proven value of a strobe light as a location aid makes equipping each crew member with a personal strobe a very worthwhile precaution.

Work now underway will likely result in the incorporation of an AIS signal transmitter in some future EPIRBs and perhaps in the PLBs. The AIS "distress" signal would be noted on the AIS receivers/ chartplotters of all vessels in radio range, improving the likelihood of detection of the signal. The elevation of the vessel's VHF antenna would provide an advantage when compared with the low elevation of a hand held direction finding receiver.

Some of the MOB alarm systems operate by continually monitoring for the presence of a low-power radio signal from each of the MOB devices carried by crew members. Such systems will create an alarm if the signal from any of the devices is absent for more than a few seconds.

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.

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. Experience has shown that a strobe light can be an invaluable aid in locating a person in the water. A crew overboard pole that is deployed when the MOB incident is first noted can be extremely useful since it will tend to drift in a manner similar to the drift of the person in the water.





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