Module 4 - Channels & Frequencies
Channel 13 Is a Navigation-Safety Fallback, Not a Second Channel 16
What you will learn
Apply the documented Channel 13 fallback only to an unanswered unknown-MMSI ship call and keep it within bridge-to-bridge navigation communication.
For an unknown-MMSI inter-ship call, MCA guidance says to try a low-power voice call on Channel 16, move to an inter-ship channel if answered, and try Channel 13 if there is no response.
Channel 13 is allocated in GMDSS for bridge-to-bridge communications. The MCA small-craft sequence is specific: the other ship's MMSI is unknown, the initial low-power voice call on Channel 16 has not been answered, and relevant bridge-to-bridge contact is still needed.
This fallback does not make Channel 13 a universal second calling channel for friends, marinas, radio checks or long routine conversations. It should remain available for relevant navigation communication and must not be treated as a convenient quiet channel.
Before escalating the call, recheck identity, channel, volume, squelch, power and whether the other vessel is within range and likely listening. A failed routine call alone does not create distress priority.
Worked example
You need to attract an unidentified ship for relevant bridge-to-bridge navigation communication and have no MMSI for it.
- 1Listen on Channel 16, use low power initially and make a brief identified call.
- 2Leave time for a reply; if contact is made, move to an appropriate inter-ship channel.
- 3If there is no response, try Channel 13 for the bridge-to-bridge communication described by the MCA procedure.
- 4If there is still no reply, reassess the radio path and the actual safety situation rather than transmitting continuously.
Sense check: The reason for using Channel 13 should be explainable as relevant bridge-to-bridge navigation communication, not merely 'Channel 16 was busy'.
Keep the fallback tied to navigation communication
An unidentified ship is converging near a traffic route, does not answer the initial Channel 16 call and relevant bridge-to-bridge contact is still needed.
1. Confirm the first attempt
Make sure the brief low-power Channel 16 call was correctly identified and left a real interval for reply.
2. Use the bounded fallback
Try Channel 13 for the relevant bridge-to-bridge communication, keeping the message concise.
3. Reassess
If there is still no reply, continue safe navigation and classify any changing risk on its actual facts.
Sense check: A failed call never transfers collision-avoidance responsibility to the radio exchange.
Common mistake or limitation
- Going straight to Channel 13 without the documented low-power Channel 16 attempt.
- Using Channel 13 for a marina request or social ship-to-ship conversation.
- Escalating a failed routine call to Mayday without grave and imminent danger requiring immediate assistance.
Recap
- Unknown MMSI: try the brief low-power Channel 16 voice call first.
- If unanswered, Channel 13 is the documented bridge-to-bridge fallback.
- Channel 13 is not a second general calling or chat channel.
Optional quick check
Section 9 of 9
When does the MCA small-craft guidance direct an operator to try Channel 13?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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