Marine VHF (Very High Frequency) radio operates between 156.000 MHz and 174.000 MHz and is the primary means of short-range maritime communication. It is used for distress signaling (Mayday calls), safety announcements, ship-to-ship communication, port operations, bridge-to-bridge navigation, and contacting coast guard or marine rescue services. VHF marine radios are essential safety equipment required on most commercial vessels and strongly recommended for all recreational boaters operating beyond sheltered waters.

Channel 16 (156.800 MHz) is the International Distress, Safety, and Calling Frequency. It is continuously monitored by coast guard stations, maritime rescue coordination centers, and other vessels worldwide. All vessels equipped with VHF radios are required to maintain a listening watch on Channel 16 when underway (unless actively communicating on another channel). It is used for:

• MAYDAY calls — life-threatening emergencies
• PAN-PAN — urgent situations (not life-threatening)
• SÉCURITÉ — safety messages and navigation warnings
• Initial contact (hailing) before switching to a working channel

Channel 70 (156.525 MHz) is reserved exclusively for Digital Selective Calling (DSC) — a digital data system that allows vessels to send automated distress alerts, make direct calls to specific vessels (using their MMSI number), and exchange position information when integrated with GPS. Voice communication is strictly prohibited on Channel 70. A DSC-capable VHF radio with a registered MMSI (Maritime Mobile Service Identity) number is required to use this feature. In a DSC distress situation, pressing the red distress button automatically transmits the vessel's identity, position, and nature of distress to all DSC-equipped stations.

Licensing requirements vary by country:

• USA: Recreational vessels not traveling to foreign ports do not need a station license, but operators should comply with FCC rules.
• UK: A Ship Radio License (free, from Ofcom) is required. Operators must hold a Short Range Certificate (SRC) or equivalent.
• Canada: A station license is required; operators need a Restricted Operator Certificate (Maritime) — ROC-M.
• Australia: A station license is required; operators need a Marine Radio Operator's Certificate of Proficiency (MROCP) or Short Range Operator Certificate (SROCP).
• Europe: Generally requires operator certification (SRC) and a station license from the flag state authority.

Always check with your national maritime communications authority for current requirements.

Simplex (S): Both parties transmit and receive on the same frequency. Communication is direct (ship-to-ship). Most recreational and ship-to-ship channels are simplex. Only one party can transmit at a time (half-duplex operation).

Duplex (D): The ship transmits on one frequency and receives on a different frequency. This allows full-duplex communication (like a telephone call) when communicating through a coast station or repeater. Duplex channels are typically used for port operations, public correspondence, and marina communications where a shore-based station relays signals.

VHF marine radio operates on line-of-sight principles. Typical range depends on antenna height:

• Small boat (antenna at 2m / 6.5ft): ~5–8 nautical miles
• Sailboat mast (antenna at 15m / 50ft): ~15–20 nautical miles
• Large ship (antenna at 30m / 100ft): ~25–30 nautical miles
• Coast station (antenna at 100m+ / 330ft+): ~40–60 nautical miles

Atmospheric conditions can occasionally extend range via tropospheric ducting, but this is unreliable. For long-range offshore communication, MF/HF (SSB) radio or satellite communication systems (Inmarsat, Iridium) are required.

Recommended recreational channels by region:

• USA: Channel 9 (hailing), Channels 68, 69, 71, 72 (ship-to-ship, non-commercial)
• Canada: Channel 9 (hailing), Channel 68, 69, 71, 72 (recreational)
• UK: Channel 72 (leisure ship-to-ship), Channel 80 (marinas)
• Europe: Channel 72 (leisure), Channel 77 (ship-to-ship), Channel 80 (marinas)
• Australia: Channel 72, 73 (recreational), Channel 77 (ship-to-ship)

Always: Hail on the designated calling channel (Ch 16 or regional equivalent), then immediately switch to an agreed working channel. Never use Channel 16 for casual conversation.

In the United States, NOAA Weather Radio (NWR) broadcasts continuous weather information on seven dedicated VHF frequencies between 162.400 MHz and 162.550 MHz (channels WX1 through WX7). These are receive-only channels — no transmission is permitted. Most modern marine VHF radios include these weather channels. They provide:

• Current marine weather forecasts and conditions
• Severe weather watches and warnings
• Hazardous conditions alerts
• 24/7 automated broadcasts

Similar services exist in Canada (Environment Canada's Weatheradio on the same frequencies) but are not universally available in Europe, UK, or Australia, where marine weather is typically obtained via NAVTEX, coast radio stations, or satellite services.

Certain channels (notably Channels 13, 15, 17, 75, 76) are designated for low-power operation (1 Watt maximum) to minimize interference with nearby vessels and shore stations. These channels are intended for:

• Bridge-to-bridge navigation (Ch 13): Short-range communication between vessels maneuvering in close proximity
• On-board communications (Ch 15, 17): Internal ship communications (deck to bridge, etc.)

Using high power (25W) on these channels can cause harmful interference over a wide area. Most modern VHF radios automatically reduce power when these channels are selected or have a dedicated low-power switch.

The United States uses a modified channel plan that differs from the international (ITU) standard in several important ways:

• Channel 22A: US-only simplex channel (157.100 MHz) used for Coast Guard liaison. In the international plan, Channel 22 is a duplex channel for port operations.
• Channels 24–28, 84–88: In the US, many of these are simplex or have different duplex pairings compared to the international plan, primarily for public correspondence and telephone calls (increasingly obsolete).
• Weather Channels (WX1–WX7): Only available in the US and Canada; not part of the international VHF marine band.
• Channel 9: Designated as the recreational boater calling channel in the US; in the international plan, it is a general simplex channel.

Always set your radio to the correct channel plan for your region. Most modern VHF radios allow switching between "US," "International," and "Canada" modes in the settings menu.