VHF Channel 13
Bridge-to-bridge channel for navigational coordination, especially in US, Mexican, and some Asian waters.
In practice
Required watch in many US ports under USCG 33 CFR 26. Bridge officer keeps Ch.16 and Ch.13 simultaneously, switching attention to the calling channel.
Regulatory detail & full definition
VHF Channel 13 (156.65 MHz) — the bridge-to-bridge channel for navigational coordination, used for primary collision-avoidance communication between vessels in the United States, Mexico, parts of Canada, and several Asian and European waters. ITU Radio Regulations Appendix 18 designates the channel for inter-ship safety navigation; USCG 33 CFR 26 makes Ch.13 watch mandatory for many US-port-bound vessels. Ch.13 is used for one-whistle / two-whistle passing arrangements, anchorage coordination, and traffic-pattern queries — distinct from Ch.16, which is reserved for distress alerting and initial calling.
Authoritative source: ITU Radio Regulations ↗
Related
Source: ITU RR Appendix 18; USCG 33 CFR 26
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