VHF Radio and DSC Distress Calling
MAYDAY is the most important two-syllable word a sailor knows — and most sailors have never practiced saying it.
VHF Radio Basics
Channel 16 is the international distress, safety, and calling frequency. Every vessel with a VHF radio must monitor Channel 16 at all times when underway. All distress calls are initiated on Channel 16. All coast guard communications begin on Channel 16. If you are not monitoring Channel 16, you are not maintaining a proper radio watch.
Marine VHF operates on line-of-sight propagation, meaning range is limited by the horizon — typically 15–25nm between vessels at sea level, and more when communicating with a coast guard station whose antenna is elevated. A fixed-mount VHF with a masthead antenna will significantly outperform a handheld radio. A handheld with a 1.5-metre antenna height provides perhaps 5nm of reliable range. In an emergency, range matters — install a fixed-mount radio connected to a masthead antenna.
Handheld VHF radios remain essential as a backup. Keep one charged, tested, and accessible from the cockpit at all times. In a knockdown or sinking situation, the fixed-mount radio may be submerged or inoperable. A handheld in a life jacket pocket or grab bag is the backup. Ensure it is waterproof rated (IPX7 minimum for immersion) — not merely splash-resistant.
The MMSI (Maritime Mobile Service Identity) is a nine-digit number programmed into your VHF radio's DSC controller and registered with maritime authorities. Without an MMSI, your radio cannot send or receive DSC calls — including distress calls — and you are invisible to the automated distress monitoring system. In the US, recreational boaters can register an MMSI free through Sea Tow, BoatUS, or the FCC. Do it before you go offshore.
Licensing: in the US, a ship station licence is required for vessels operating in international waters or communicating with foreign vessels. The operator is required to hold a Restricted Radiotelephone Operator Permit (RROP) for domestic operation or a Restricted Certificate of Competency (GMDSS-R or equivalent) for international use. Check the requirements for your passage.
Test your fixed-mount VHF before every offshore passage by calling Coast Guard on Channel 22A for a radio check. Confirm audio quality and that your transmitted MMSI appears correctly. A radio that can't be heard is not a safety tool.
What is the primary purpose of Channel 16 on marine VHF?
Digital Selective Calling (DSC)
Digital Selective Calling (DSC) is a feature built into all Class D VHF radios (the standard for recreational vessels) that allows a formatted digital distress signal to be transmitted with a single button press. When correctly set up and registered, it is the fastest and most information-rich distress signal a recreational sailor can send.
A DSC distress call transmits: your MMSI number (identifying your vessel to the rescue coordination centre), your GPS position (if the radio is connected to a GPS receiver), the nature of the distress (selected from a menu: sinking, fire, flooding, collision, MOB, etc.), and the UTC time of the transmission. This data arrives at receiving stations in digital form, instantly readable without transcription errors — far more reliable than a voice transmission in difficult conditions.
For DSC to include your position, the radio must be connected to a GPS source. This is a critical and frequently missed installation step. Connect the VHF to the boat's chartplotter or a dedicated GPS receiver via NMEA 0183 or NMEA 2000. Verify the GPS position is displayed on the radio's DSC controller screen. Without GPS integration, a DSC distress call still sends your MMSI and distress type — but without a position, the rescue services know you are in trouble without knowing where you are.
Registering your MMSI is mandatory for DSC to be useful. An unregistered MMSI triggers an alert but provides rescuers with no vessel information — no name, no description, no emergency contacts, no voyage details. Registration links your MMSI to your vessel record. In the US, register at beaconregistration.noaa.gov or through BoatUS/Sea Tow. Keep your registration current — if you sell the vessel, transfer or cancel the MMSI registration.
The Distress button on a Class D radio is typically red, protected by a cover. To send a DSC distress call: lift the cover, press and hold the button for five seconds. The radio will automatically switch to Channel 16, broadcast the DSC distress signal on the DSC frequency (Channel 70), and begin broadcasting a MAYDAY voice call. Do not transmit a DSC distress call and then immediately transmit a voice MAYDAY — the DSC transmission typically occurs on a separate digital frequency and the radio needs a moment to switch back to voice on Channel 16.
After installing or servicing your VHF, verify GPS integration by checking that a valid latitude/longitude appears on the DSC status screen. Do this check every pre-departure inspection. An unconnected GPS renders your DSC position capability useless precisely when you need it most.
Do not press the DSC Distress button accidentally. A false DSC distress alert requires immediate notification to the Coast Guard on Channel 16 — explain that it was transmitted in error, provide your MMSI, and confirm you are safe. False alerts consume SAR resources and carry legal consequences in many jurisdictions.
Your VHF radio has a valid MMSI registered, but the DSC controller shows 'No GPS' on the status screen. If you send a DSC distress call, what information does the rescue coordination centre receive?
The MAYDAY Call
A MAYDAY is declared when a vessel or person is in grave and imminent danger and requires immediate assistance. 'Grave and imminent' means now — the vessel is sinking, there is uncontrolled fire aboard, a crew member is in cardiac arrest, or you have a MOB you cannot recover. Do not wait until the situation is unrecoverable before calling. Call MAYDAY early.
The full MAYDAY call procedure: (1) Switch to Channel 16. (2) Key the microphone. (3) Transmit: 'MAYDAY MAYDAY MAYDAY. This is [vessel name] [vessel name] [vessel name], MMSI [nine digits]. My position is [latitude and longitude, OR bearing and distance from a known landmark]. I am [nature of distress — sinking, on fire, medical emergency, etc.]. I have [number of persons aboard]. [Any additional relevant information — taking on water, require immediate assistance, etc.]. Over.' (4) Release the mic and listen for acknowledgement on Channel 16.
The vessel name is repeated three times to ensure it is received clearly despite noise and interference. The MAYDAY prefix is repeated three times for the same reason — it is an internationally recognised distress signal and the repetition prevents any confusion. Speak slowly and clearly. Do not rush. Rescuers need to transcribe the information accurately. If you are breathing hard from exertion or stress, take a breath before transmitting.
If you receive no acknowledgement within two minutes, repeat the call. If no response after repeated attempts, try MAYDAY Relay: any vessel that hears a MAYDAY and cannot see a response may relay it: 'MAYDAY Relay MAYDAY Relay MAYDAY Relay. This is [your vessel name]. The following MAYDAY was received from [vessel in distress] at [time]…' and repeat the original message.
PAN PAN is the urgency call — one level below MAYDAY. Used when a situation is serious and urgent but does not yet involve grave and imminent danger. Examples: a crew member is ill and needs medical advice, an engine has failed and you are drifting toward a lee shore (if not immediately dangerous), you have lost steering but are not yet in immediate danger. Format: 'PAN PAN PAN PAN PAN PAN. All stations, this is [vessel name] [repeated three times]. My position is [position]. [Nature of urgency]. [Persons aboard]. Over.'
Memorise this and post it at the helm: MAYDAY x3, vessel name x3, MMSI, position, nature of distress, persons aboard. Practise saying the full call out loud at the dock. Under stress, you will default to what you have rehearsed — make sure you have rehearsed something accurate.
Your vessel is taking on water rapidly and you cannot control the flooding. You decide to call MAYDAY. Your vessel is at 34°15'N, 119°52'W with 3 persons aboard. Which of the following transmissions is correct?
Radio Discipline and Watch-Keeping
Monitoring Channel 16 when underway is not a courtesy — it is a legal requirement in most jurisdictions and a fundamental safety obligation. Every MAYDAY you hear may be from a vessel you can see. Every DSC alert received may require a relay. Vessels that are not monitoring Channel 16 have failed in their duty to other mariners.
Channel 16 must be kept clear of all non-distress, non-safety, and non-calling traffic. Working channel conversations — discussing anchorage plans, arranging a fuel stop, socialising — belong on a working channel (68, 69, 72, or others as locally designated). Initiate the call on 16, agree a working channel, then move. Do not hold extended conversations on Channel 16.
Radio checks before offshore passages confirm your radio is transmitting correctly. Call the Coast Guard on Channel 22A (the designated coast guard working channel in the US): 'Coast Guard [station name], this is [vessel name], radio check on Channel 22A, over.' A clear response confirms your radio is working. A weak response tells you something is wrong with your installation. No response is something to investigate before departure.
NOAA Weather Radio is available on channels WX1 through WX7 on US VHF radios. Continuous weather broadcasts, updated every 1–6 hours, include marine forecasts, wind and sea state, and weather warnings. Listen to the marine forecast before departure and during the passage. A radio with Weather Alert (WX Alert) will interrupt other audio to broadcast urgent weather warnings — enable this feature and leave it on.
DSC all-ships calls and DSC position requests between vessels are features of the DSC system that allow routine digital communications. Do not use the DSC distress function for non-emergency communication. The Distress button is for emergencies — never 'test' it at sea. Self-test functions exist for that purpose.
Set your VHF to dual-watch mode with Channel 16 and your preferred working channel (e.g., Channel 68 in busy anchorages). Dual-watch scans both channels and alerts you to any traffic on 16 even while monitoring another channel. Every modern fixed-mount VHF supports this feature.
Transmitting a false MAYDAY or PAN PAN call is a federal offence in the US (punishable under 47 USC 325 by fines up to $250,000 and imprisonment). If you accidentally press the DSC Distress button, immediately contact Coast Guard on Channel 16, identify your vessel by MMSI, and report the accidental transmission.
While offshore, another vessel hails you on Channel 16 to discuss where you are both planning to anchor tonight. What is the correct response?
Summary
Channel 16 is the international distress, safety, and calling frequency. Monitor it continuously when underway — this is a legal obligation, not a suggestion.
DSC sends a digital distress signal with MMSI, position, distress type, and UTC time at the press of a button. For it to work, the radio must be connected to GPS and the MMSI must be registered.
The MAYDAY call format: MAYDAY x3, vessel name x3, MMSI, position, nature of distress, persons aboard, 'over'. Speak slowly, clearly, and listen for acknowledgement.
PAN PAN is the urgency call for serious but not immediately life-threatening situations. Use it early rather than escalating unnecessarily to MAYDAY.
A fixed-mount VHF with a masthead antenna provides 15–25nm range. A handheld is essential as a backup — keep one charged, waterproof-rated, and accessible.
Radio discipline: keep Channel 16 clear of working traffic, conduct radio checks before offshore passages, and use NOAA WX channels for weather monitoring.
Key Terms
- Channel 16
- The international VHF frequency designated for distress, safety, and initial calling. All vessels with a VHF radio are required to monitor Channel 16 continuously when underway.
- DSC
- Digital Selective Calling — a feature of Class D VHF radios that transmits a formatted digital distress signal including MMSI, GPS position, distress type, and UTC time at the press of a single button.
- MMSI
- Maritime Mobile Service Identity — a unique nine-digit number assigned to a vessel and programmed into its VHF radio's DSC controller. Required for DSC functionality and must be registered with maritime authorities.
- MAYDAY
- The international voice distress call indicating grave and imminent danger requiring immediate assistance. Transmitted on Channel 16 and repeated three times, followed by vessel name (x3), MMSI, position, nature of distress, and persons aboard.
- PAN PAN
- The international urgency call, one level below MAYDAY. Used when a situation is serious and requires assistance but does not yet involve grave and imminent danger to persons.
- MAYDAY Relay
- A procedure used when a vessel receives a MAYDAY call but sees no response from shore or other vessels. The receiving vessel relays the original MAYDAY call on behalf of the vessel in distress.
- Class D VHF Radio
- The standard recreational marine VHF radio, incorporating DSC capability on Channel 70. Required to have a DSC controller and distress button to comply with international standards.
- NMEA 0183 / NMEA 2000
- Data communication standards used to connect marine electronics. A VHF radio connected to a GPS via NMEA 0183 or NMEA 2000 can include the vessel's position in DSC distress calls.
VHF Radio and DSC Distress Calling
You need to contact another vessel to co-ordinate a fuel stop. You call them on Channel 16. After they respond, what is the correct next step?
Your VHF radio's DSC controller shows 'No Position' on the status screen. You send a DSC distress call. What does the rescue coordination centre receive?
What does the MAYDAY call format require the vessel name to be repeated?
A crew member has developed a serious but non-life-threatening illness offshore and you need medical advice. You are not in grave and imminent danger. Which call do you make?
You accidentally press and hold the DSC Distress button and a distress alert is transmitted. What do you do immediately?
References & Resources
Related Links
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FCC — Marine Radio Licensing
Federal Communications Commission guidance on ship station licences, MMSI registration, and operator permit requirements for US vessels.
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NOAA — MMSI Registration (BoatUS)
Free MMSI registration for recreational boaters through BoatUS in the US.
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USCG — VHF Radio and DSC Information
US Coast Guard NAVCEN guidance on DSC operation, MMSI registration, and distress alert procedures.
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NOAA Weather Radio
NOAA Weather Radio All Hazards — continuous broadcasts on VHF WX channels. Coverage maps and frequency information.