VHF radio and distress signals

There are four calls, and the first three run from the most to the least urgent. Mayday is for grave and imminent danger: people or a vessel needing help at once. Pan-Pan is for urgency: assistance is needed, but the danger is not grave and imminent, for example a broken rudder or a request for medical advice. Sécurité is a message for everyone, such as a navigational hazard or an important weather forecast. The Mayday relay is different: it is a Mayday that you pass on yourself because you hear it and nobody answers, or because the vessel in distress cannot transmit.

Look at how they begin. The Mayday goes straight to “This is” with the boat's name; the other three first say whom you are talking to: “All stations” or a specific station. It is best to relay a Mayday directly to the Coastguard; if you cannot reach them, to all stations. In a relay two different boats must stay clear: you, who are transmitting, and the one in distress. The Mayday line by line and the handling of the emergency are in the topic “Distress and rescue”.

The phonetic alphabet is used to spell out letter by letter the names and abbreviations that the radio, with its noise, makes hard to understand: the boat's name, the call sign, the name of a place. Each letter always has the same word, as in the table: from ALPHA to ZULU. The listener must not have to guess. When the name matters, for example your boat's name in a distress call, you can spell it letter by letter.

The parachute rocket climbs high and then descends slowly: it is there to be seen from far away. In clear skies it is fired vertically; if it is windy it is tilted about 15° downwind; under low cloud it is fired at about 45°, so the parachute ignites below the cloud base and stays visible. Two prohibitions always apply: never into the wind, and never if a helicopter is approaching.

The red hand flare is a different thing: it is not seen from far away but shows exactly where you are, which is why it is used inshore or when other vessels are in sight. It is held at arm's length downwind and you do not look at the flame. Orange smoke is for daytime, especially in bright sunlight. Which flare by day and by night, and how long it burns, is in the card “Flares and smoke” of “Distress and rescue”.

The distress signals are listed in Annex IV of the Collision Regulations. The easiest way to remember them is to sort them by how they reach the rescuers: some are seen, some are heard, some go by radio. They are used only to ask for help, never for anything else.

Among those that are seen there are red star rockets or shells, the parachute rocket or hand flare showing a red light, flames on the vessel, orange smoke, the arms, the square flag with a ball and the flags N over C. Among those that are heard, a gun or other explosive signal at intervals of about a minute and the continuous sounding of a fog-signalling apparatus. SOS can be made with light or with sound. By radio: the word Mayday, the DSC alert on channel 70, the satellite alert (Inmarsat or another service), the EPIRB and the other approved radio signals, such as those of the SART. To be seen from the air, the Annex adds the orange canvas with a black square and circle and the dye marker.

The radar SART is a transponder that answers the radar of whoever is searching for you: you switch it on and raise it about a metre above the sea. On the screen of an X-band radar, at 9 GHz, it appears from far away as a line of 12 dots; as the ship gets closer the dots become arcs and, very close, concentric circles. Look at the table: the range at sea is about 5-6 miles, much more from an aircraft.

The AIS-SART works in a different way: it sends its position by VHF about every minute, with a sound alarm, and the battery lasts about 96 hours. The searcher sees it on the electronic chart as a red circle with a cross. Everyday AIS identifies the ship, not a person in distress. AIS and AIS-SART use the same two AIS channels (87B and 88B, 161.975 and 162.025 MHz), not channel 70.