GMDSS and VHF Distress Procedures

TL;DR — A DSC distress alert is transmitted on VHF Channel 70; the subsequent voice MAYDAY is transmitted on VHF Channel 16 (156.8 MHz). Know the complete sequence of a MAYDAY call, the recognized distress signals under federal regulation, and the difference between MAYDAY and PAN-PAN.

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What the Rule Says

Recognized Distress Signals

Federal regulation enumerates the signals that indicate distress and need of assistance. Any one of the following, used separately or together, constitutes a recognized distress signal 33 CFR §87.01:

• A gun or other explosive signal fired at intervals of about a minute
• A continuous sounding with any fog-signaling apparatus
• Rockets or shells throwing red stars, fired one at a time at short intervals
• The Morse Code group . . . — — — . . . (SOS), made by any method
• The spoken word "Mayday" sent by radiotelephony
• The International Code Signal N.C.
• A square flag with a ball (or anything resembling a ball) above or below it
• Flames on the vessel (e.g., from a burning tar barrel or oil barrel)
• A rocket parachute flare or a hand flare showing a red light
• A smoke signal giving off orange-colored smoke
• Slowly and repeatedly raising and lowering arms outstretched to each side
• A DSC distress alert transmitted on VHF Channel 70, or on MF/HF frequencies 2187.5 kHz, 8414.5 kHz, 4207.5 kHz, 6312 kHz, 12577 kHz, or 16804.5 kHz
• A ship-to-shore distress alert via Inmarsat or other mobile satellite service provider ship earth station
• Signals from an EPIRB
• Signals from radiocommunication systems including survival craft radar transponders meeting 47 CFR 80.1095
• A high-intensity white light flashing at regular intervals from 50 to 70 times per minute

Candidates frequently miss the DSC channel split: DSC alert goes out on Channel 70; voice communication follows on Channel 16. Both are part of the same distress sequence but operate on different channels 33 CFR §87.01.

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The VHF MAYDAY Procedure — Step by Step

The recommended emergency broadcast procedure, which satisfies the requirement for an emergency broadcast placard, is as follows 46 CFR §184.510:

1. Ensure the radiotelephone is on. 2. Select 156.8 MHz (VHF Channel 16) or 2182 kHz. Both Channel 16 VHF and 2182 kHz SSB are for emergency and calling purposes only. 3. Press the microphone button and, speaking slowly, clearly, and calmly, say: - "MAYDAY — MAYDAY — MAYDAY" for situations involving immediate danger to life and property, or - "PAN — PAN — PAN" for urgent situations where there is no immediate danger to life or property. 4. Say: "THIS IS [vessel name] [vessel name] [vessel name] [call sign], OVER." 5. Release the microphone button briefly and listen for acknowledgment. If no answer, repeat steps 3 and 4. 6. If no acknowledgment, or if the Coast Guard or another vessel responds, say: "MAYDAY [vessel name]" (or "PAN [vessel name]"). 7. Describe your position using latitude and longitude, or range and bearing from a known point. 8. State the nature of the distress. 9. Give the number of persons aboard and the nature of any injuries. 10. Estimate the present seaworthiness of the vessel. 11. Briefly describe the vessel: length, color, hull type, trim, masts, power, and distinguishing features. 12. Say: "I WILL BE LISTENING ON CHANNEL 16/2182." 13. End: "THIS IS [vessel name and call sign]." 14. If the situation permits, stand by the radio. If no answer, repeat, then try another channel.

The same procedural elements — position, nature of distress, persons aboard, vessel description — appear in the emergency instructions required to be posted aboard commercial fishing vessels 46 CFR §28.265.

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Distress Signaling Priorities

The USCG Boatswain's Manual establishes a priority order for distress signaling: initiate a VHF Channel 16 MAYDAY with DSC if equipped (DSC automatically transmits identity and position), activate the EPIRB if rapid response is critical, and use visual flares and signal mirror as backup. Continue transmitting MAYDAY until acknowledged USCG Boatswain Manual Ch. 7 §7.2.

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GMDSS Operator Endorsements

Vessels subject to SOLAS Chapter IV must carry personnel holding STCW endorsements as GMDSS radio operators. The applicable USCG credentialing subpart covers both radio officer endorsements and STCW GMDSS operator endorsements for service on vessels subject to SOLAS Chapter IV provisions 46 CFR §11.601. For the OUPV/Master 100 GT exam, you are not expected to administer GMDSS equipment at the SOLAS level, but you must know the DSC channel, the MAYDAY sequence, and the recognized distress signals.

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Why It Matters on the Exam

Exam questions on this topic fall into three predictable categories:

1. Channel identification. The exam will ask which VHF channel is used for DSC distress alerts versus voice MAYDAY. The answer is unambiguous: DSC alerts transmit on VHF Channel 70; voice MAYDAY transmits on VHF Channel 16 (156.8 MHz) 33 CFR §87.01 46 CFR §184.510. Confusing these two channels is the single most common error on this section of the exam.

2. MAYDAY call sequence. Questions may present a partial MAYDAY call and ask what element is missing or out of order. The vessel name is spoken three times in the initial call. Position is described by latitude/longitude or range and bearing from a known point — not a vague description like "near the inlet." The call ends with the vessel name and call sign 46 CFR §184.510 46 CFR §28.265.

3. Recognized distress signals. The exam may present a list and ask which item is NOT a recognized distress signal, or ask what color smoke is required. Orange smoke is the correct color for a smoke distress signal 33 CFR §87.01. A white flashing light is recognized only when it flashes at 50–70 times per minute 33 CFR §87.01 — a steady white light is not a distress signal.

4. MAYDAY vs. PAN-PAN. MAYDAY is reserved for immediate danger to life and property. PAN-PAN is for urgent situations with no immediate danger to life or property 46 CFR §184.510. Misusing MAYDAY when PAN-PAN is appropriate — or failing to use MAYDAY when it is warranted — is both a regulatory and a practical error.

5. Posting requirements. Emergency instructions, including distress call procedures, must be posted in conspicuous locations accessible to the crew. On a vessel operating with fewer than four individuals aboard, the instructions may be kept readily available rather than posted 46 CFR §28.265.

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Common Pitfalls

Pitfall 1 — Swapping Channel 16 and Channel 70. Channel 70 is DSC only. Channel 16 is voice only. The exam exploits this confusion directly. Memorize: 70 = DSC alert; 16 = voice MAYDAY 33 CFR §87.01.

Pitfall 2 — Stating position incorrectly. The regulation specifies latitude and longitude, or range and bearing from a known point. A description such as "two miles south of the marina" does not satisfy the regulatory standard 46 CFR §184.510 46 CFR §28.265.

Pitfall 3 — Omitting the vessel name repetition. The vessel name is spoken three times in the initial call. Candidates who write or recite it only once will miss exam questions that test this specific element 46 CFR §184.510.

Pitfall 4 — Treating 2182 kHz as a VHF frequency. 2182 kHz is an MF/SSB frequency, not VHF. Both 156.8 MHz (VHF Ch. 16) and 2182 kHz are designated emergency and calling frequencies, but they operate on entirely different radio bands 46 CFR §28.265 46 CFR §184.510.

Pitfall 5 — Assuming any white flashing light is a distress signal. The regulation is specific: the white light must flash at 50 to 70 times per minute. A light flashing at a different rate is not a recognized distress signal 33 CFR §87.01.

Pitfall 6 — Confusing MAYDAY with PAN-PAN. A vessel taking on water slowly with no immediate risk of sinking warrants PAN-PAN, not MAYDAY. A vessel actively sinking warrants MAYDAY. The distinction is immediacy of danger to life 46 CFR §184.510.

Pitfall 7 — Stopping transmissions after one unanswered call. The Boatswain's Manual is explicit: continue to transmit MAYDAY until acknowledged USCG Boatswain Manual Ch. 7 §7.2. If no answer is received, repeat the call, then try another channel 46 CFR §184.510.

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Quick Check

Q1 — On which VHF channel is a DSC distress alert transmitted?

VHF Channel 70. The subsequent voice MAYDAY is transmitted on VHF Channel 16 (156.8 MHz). 33 CFR §87.01

Q2 — How many times is the vessel name spoken in the initial MAYDAY call?

Three times. The format is: "MAYDAY — MAYDAY — MAYDAY. THIS IS [vessel name] [vessel name] [vessel name] [call sign], OVER." 46 CFR §184.510

Q3 — What is the correct color of smoke for a recognized distress smoke signal?

Orange. A smoke signal giving off orange-colored smoke is a recognized distress signal. 33 CFR §87.01

Q4 — What urgency signal is used when there is an urgent situation but NO immediate danger to life or property?

PAN-PAN (spoken three times). MAYDAY is reserved for situations involving immediate danger to life and property. 46 CFR §184.510

Q5 — At what flash rate must a white light flash to qualify as a recognized distress signal?

50 to 70 times per minute. A high-intensity white light flashing at regular intervals within that range is a recognized distress signal. 33 CFR §87.01

Q6 — A commercial fishing vessel operates with three persons aboard. Must emergency instructions be posted or may they be kept readily available?

They may be kept readily available. On a vessel operating with fewer than four individuals aboard, emergency instructions may be kept readily available as an alternative to posting. 46 CFR §28.265

Q7 — List the MF/HF frequencies on which a DSC distress alert may be transmitted.

2187.5 kHz, 8414.5 kHz, 4207.5 kHz, 6312 kHz, 12577 kHz, and 16804.5 kHz. 33 CFR §87.01

Q8 — After transmitting a MAYDAY with no response, what should the operator do?

Repeat the call, then try another channel. The operator should continue transmitting MAYDAY until acknowledged. 46 CFR §184.510 USCG Boatswain Manual Ch. 7 §7.2