Onboard Communications

A means of calling for help in the event of an onboard emergency is essential for all boaters.

Although in many instances a mobile phone may work, it is not designed for the marine environment and coverage may be non-existent just when you need it most. Another means of communication is therefore essential.


The carriage of at least a handheld VHF is in most cases practical, and a handheld will suffice if only a limited communications range is required. A fixed VHF set can allow communications to be made over a range of up to 20 – 40 miles depending on the installation and conditions. VHF also allows search and rescue (SAR) authorities to locate you using Radio Direction Finding (RDF) equipment.  

VHF antenna

To maximise the communications range the antenna must be positioned as high as possible. The antenna should also be fitted in accordance with the manufacturer's instructions, to ensure the installation is safe for the user.

On yachts the VHF antenna will usually be positioned at the top of the mast, which means that communications will be lost in the event of a dismasting. The positioning of the VHF antenna on a motor cruiser requires particular consideration to prevent damage. Locations which leave the antenna susceptible to damage, especially on smaller motor boats where it is prone to being grabbed for balance, or when people are getting on and off the boat should be avoided.

Should you lose your antenna, your VHF will be of little use to you unless you carry an emergency VHF antenna, which you can then fit to re-instate your communications. 

Routine VHF traffic

A VHF is however more than just a means of requesting assistance. For example, Maritime Safety Information which includes both weather information and navigational warnings is broadcast on VHF by the Coastguard every three hours. Within a port or harbour area it is useful (and sometimes a requirement) to monitor port operations frequencies or the local Vessel Traffic Service (VTS) frequencies. 

A cockpit loudspeaker will ensure that you can hear your VHF above the noise of the wind and / or your engines, or you may choose to keep a handheld set on deck for this purpose. Keeping a handheld VHF (and preferably a waterproof one) to hand or even on your person, is also worth considering as a way of being prepared for the worst (sudden sinking, capsize or finding yourself in the water).  Be aware however, if you are relying on a handheld VHF, that the range of a handheld VHF is significantly less than that of a fixed set. You should also be aware of the detrimental effect a handheld VHF carelessly positioned can have on other equipment such as a compass.  

Sea Areas

VHF communications are sufficient for boating around the UK coastline, which has been categorise as Sea Area A1 – within VHF coverage of a coast station.

The remainder of the world’s oceans are categorised as areas A2 – A4. Longer range communications such as MF / HF SSB and satellite communications equipment should be carried aboard vessels venturing outside sea area A1.

Is it mandatory?

Owners of UK pleasure craft of 13.7m in length and over that go to sea, should be aware that communications equipment is mandatory under the Merchant Shipping Regulations.


The Global Maritime Distress and Safety System (GMDSS) introduced by the International Maritime Organisation (IMO) reached full implementation on 1 February 1999. It is a system which using satellite and terrestrial radio communications should ensure that wherever a vessel is in the world, it can shout for help, alerting both Search and Rescue authorities ashore and other suitably equipped vessels in the vicinity, thereby greatly increasing the possibility of assistance being provided.

GMDSS is however only compulsory for passenger vessels on international voyages and for other vessels over 300GT. For these vessels there are a variety of different types of message deemed as essential communications which they must be able to transmit and receive, which vary with the GMDSS Sea Areas A1 – A4.

Digital Selective Calling

With the introduction of the Global Maritime Distress and Safety System (GMDSS) came digital selective calling (DSC) which enhanced distress alerting across many of the communications media including VHF.

A VHF DSC radio allows you to transmit a distress message in seconds, which can include the nature of your distress if you select it. If the set is connected to a GPS, this message will also include the GPS position of your boat. Presuming another station is within range, they become aware of your predicament, without you spending time transmitting your Mayday voice message.

The DSC alert does not, however, replace the requirement for a voice call. Once your DSC alert has been acknowledged, the acknowledging station will automatically be ready to receive your voice call. This is identical to the standard Mayday message, except that you include you MMSI (Maritime Mobile Service Identification) number prior to giving your position.

The DSC alert maximises the possible audience for your Mayday call by making sure any station within range, with DSC equipment, that acknowledges your digital alert, is listening to the channel you are going to transmit on. Should the nature of your distress make the follow-up call impossible, you also have the comfort that you have at least made an initial request for assistance.

For routine calling, as long as you know the MMSI number of the station you are calling you can contact them, without at any point "cluttering up" Channel 16. In your routine DSC alert you designate the working channel you wish to speak to them on and once you have had an acknowledgement to your DSC alert, your own set will also change to this channel. You then initiate a voice call on the selected working channel.

Calling the Coastguard on routine matters follows the same pattern except that when responding to the DSC alert, the shore station will designate the working channel. Once the acknowledgement has been received, it is again the station which initiates the DSC alert which also initiates the voice transmissions. A common mistake is to send the DSC alert and then wait for a voice call from the Coastguard - they will only send a DSC acknowledgement.

Monitoring of Channel 16

Although since February 2005, the Coastguard has not been required to maintain a dedicated headset watch on Channel 16, they have the facility to do this if required and will be maintaining a "loudspeaker watch" on Channel 16 for the foreseeable future.

Although, there is no legal requirement for vessels (including SOLAS ships) to maintain a continuous listening watch on Channel 16, the MCA and IMO guidance is that vessels fitted with VHF should maintain a listening watch on CH16 where practicable.

It is however mandatory for commercial vessels of 300 gross tonnage to have DSC communications. DSC is the primary means for these vessels to now receive emergency, safety and routine messages.

Ship to ship

Contacting a ship in the event of a potential collision situation has always been an area of concern to the skipper of a small sail or motor yacht. Generally the name, callsign and MMSI number of the ship is unknown making it difficult to raise the officer of the watch on either Channel 16 or Channel 13 (the bridge to bridge communications channel).

The skipper of a yacht with DSC communications equipment, now has the option of sending an All Ships Safety DSC alert to get the attention of ships within range, to establish communications with the vessel concerned. Vessels fitted with AIS equipment may be able to establish the MMSI number of the ship, allowing one to one communications to be established using DSC.

BUT at no point do the COLREGs suggest that bridge to bridge communications should be used for collision avoidance. In fact, such communications have been known to cause collisions instead. Best practice is to avoid a close quarters situation developing in the first place and in the event that one does react in accordance with the collision regulations.

The complete global distress and communications system

Digital Selective Calling is just one of a number of elements of the Global Maritime Distress and Safety System (GMDSS), which aims to be a complete global distress and communications system for the mariner. In addition to DSC, the GMDSS incorporates Navtex, EPIRBs and satellite communications.

Related document

Does your VHF comply with the appropriate relevant international standards?

Related pages

Licencing Onboard Electronics
Short Range Certificate


MGN 324: https://www.gov.uk/government/publications/mgn-324-mf-navigation-watchkeeping-safety-use-of-vhf-radio
GMDSS: benefits of digital selective calling (DSC): https://www.gov.uk/government/publications/gmdss-benefits-of-digital-selective-calling-dsc