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Every sailor knows that communication equipment on board can be vital and Maritime communication is an essential part of offshore sailing.

Global Maritime Distress and Safety System (GMDSS)

GMDSS is the Global Maritime Distress and Safety System. The system consists of technical installations, authorities and rules concerning global marine distress and safety assistance. The GMDSS is mandatory for all commercial vessels over 300 tonnes and for several passenger and fishing vessels. These vessels are required to carry the GMDSS equipment on board, depending on the area where they sail. In any case, they are required to have on board a VHF radio with DSC, NAVTEX, EPIRB, SART/AIS-SART, 2 waterproof GMDSS walkie-talkies. Depending on the area, they must also have INMARSAT in the A3 area. Or a MF / HF-SSB radio in A4 areas. Also the mandatory books. These include, for example, the List of Shipstations, List of Coaststations, List of Callsigns, Manual for Use by the Maritime Mobile and Maritime Mobile Satellite Services, International Aeronautical and Maritime Search and Rescue Manual (IAMSAR), INMARSAT Maritime Communications Handbook. In addition, they must keep a Ship’s Diary. There must also be provision for emergency power and lighting and an emergency procedure card with the ship’s call sign and MMSI.

Download a free Template for a DSC / VHF Emergency Procedure Card below. You can customise this card for your own DSC / VHF and enter your own MMSI number and ship name. You can print the card in colour, plasticise it and keep it with the VHF in a highly visible place. You can download the Powerpoint file by going to File – Download as – Download a copy. The downloadable file is not “read only”, you can modify it with Powerpoint or a similar programme.

VHF Emergency Procedure Card for Standard Horizon GX1500E
VHF Emergency Procedure Card for B&G V90S Blackbox VHF

GMDSS sea areas

Which equipment you need depends on the sea area where you sail. There are the following 4 GMDSS sea areas:

  • A1 Within continuous Coast Guard VHF coverage, approximately 20 – 30 nm
  • A2 Outside sea area A1, but within continuous MF (Medium Frequency) – coverage, about 100 NM from shore
  • A3 Outside sea areas A1 & A2, but within INMARSAT – coverage. That is between 70 degrees North and 70 degrees South.
  • A4 Outside sea areas A1, A2 & A3, i.e. the polar areas. That is above 70 degrees North and South. There is only range there with an MF / HF-SSB radio.

Not all coastal waters in the world are A1 area and equipped with shore stations every 30 Nm. You may find yourself sailing along the coast in, say, Africa, South America, but have no coverage with the coastguard. So in those coastal areas, you would need to have Inmarsat on board, for example.

Operating certificates

In one of the next lessons, we will discuss in detail the communication equipment on board. Below you can see which operator’s certificate you need if you have certain equipment on board. For recreational boating or rather all non-SOLAS vessels, the RYA’s SRC Short Range Certificate gives almost the same powers as Marcom-B. The exception herein is INMARSAT C, which requires the LRC Long Range Certificate. This is because SRC is intended for A1 area and LRC for outside the A1 area, where you need an HF or MF transmitter or satellite equipment.

 No operating certificateDutch Basic VHF certificateMarcom-B / SRCMarcom-A / LRC
PLB X   
Yacht radar X   
Inland marine radio X   
AIS  X  
VHF or handheld radio X  
Maritime or combination radio  X  
DSC Class D  X  
Satellite EPIRB 406 MHz  X  
GMDSS walkie-talkies  X 
INMARSAT A, B, C, F, M and Mini M with GMDSS alarms  MARCOM-B or SRC for pleasure craft 
MF / HF-SSB radio   X

RCC: Rescue Coordination Centres

Coastguard Centre Den Helder (or Den Helder Rescue) is the Dutch Communication and Coordination Centre. It is designated as the Netherlands Maritime and Aeronautical Rescue Coordination Centre. Coastguard Centre Den Helder is therefore both MRCC and ARCC, which is referred to as Joint Rescue Co-ordination Centre (JRCC). In large countries, there are Maritime Rescue Sub-Centres (MRSC). The area an RCC is responsible for is called an SRR: Search and rescue region. Coastguard Centre Den Helder is therefore tasked with coordinating SAR (Search and Rescue) activities. VHF channel VHF 67 is intended for SAR VHF traffic, for example between the RCC and the OSC (On-scene coordinator), which is usually the first ship at the scene to relay communication to the coastguard.

Global regulations and authorities

The IMO (International Maritime Organisation) is a part of the United Nations focused on safety and environment at sea. The IMO set up the GMDSS under the SOLAS Convention. SOLAS is the “International Convention for the Safety of Life at Sea, 1974”. The first version of SOLAS dates back to 1914, in response to the sinking of the Titanic. SOLAS lists things like mandatory equipment, controls and radio procedures. Ships required by the IMO to comply with the HSSC, which stands for “Harmonised System of Survey and Certification”, must have, among other things, a Radio Certificate including equipment list. The International Aeronautical and Maritime Search and Rescue Manual (IAMSAR) is a manual of the IMO that describes the organisation and operation for search-and-rescue (SAR) for aviation and shipping.

The ITU is International Telecommunication Union is globally responsible for the airwaves and the Radio Regulations (RR). Under the Radio Regulations, for example, it is not allowed to intercept or use radio communications over the VHF, MF, HF as there is a duty of secrecy. In the ITU’ s Maritime mobile Access and Retrieval System (MARS), you can search for ship or shore stations by ship name, call sign, MMSI, EPIRB Hex Id code, IMO number or national registration number. The ITU also publishes the “List of Coast Stations and Special Service Stations” and the “List of Ship Stations and Maritime Mobile Service Identity Assignments”.

European regulations and bodies

Relevant European directives are the RED (2014/53/EU) formerly the R&TTE Directive (99/5/EC) for radio equipment and MED. The Marine Equipment Directive (MED – Maritime Equipment Directive) sets out what, among other things, life-saving appliances and navigation equipment on board an EU vessel must comply with. The directive covers types of maritime equipment covered by the following International Conventions of the International Maritime Organisation (IMO):

  • SOLAS 1974: Life-saving appliances/navigation equipment/radio equipment
  • MARPOL 1973: Marine pollution
  • COLREGS 1972: Prevention of collisions

The Wheelmark logo (a steering wheel) is the sign that the device complies with the MED directive.

Moreover, from the end of 2018, a new European Directive will apply to all pleasure craft longer than 20 metres (and/or with a length x width x depth of more than 100 cubic metres). These vessels now have a mandatory Certificate of Inspection. This also applies to tugs, passenger ships (for 12 people), ferries, inland vessels carrying hazardous substances, among others. The mandatory inspection must be carried out by a recognised classification society/inspection body, under the responsibility of the Ministry of Infrastructure and Water Management’s Environment and Transport Inspectorate.

Dutch regulations and authorities

The Dutch laws that apply include the Ships Act, the Ships Decree (those Dutch laws contain the SOLAS), Fishing Vessels Decree and the Regulation on the use of frequency space without a licence 2008 and Telecommunications Act. In the Netherlands, it is the Telecom Agency (AT) that oversees enforcement, together with the KLPD (water police). The AT is part of the Ministry of Economic Affairs and Climate. The AT mainly focuses on the proper use of frequencies, for example for VHF. The Inspectorate for the Environment and Transport (ILT) is the supervisory body of the Ministry of Infrastructure and Water Management and consists of the sectors Fisheries Sea Shipping and Inland Navigation.

For ships above 500 gross registered tonnes, which are thus covered by the Ships Act, a Safety Certificate and therefore GMDSS is mandatory. For passenger and cargo ships over 300 GRT on international voyages, a Radio Safety Certificate is already mandatory (See SOLAS Chapter 4: Radiocommunications). The mandatory equipment then consists of a DSC VHF radio, NAVTEX, SART, EPIRB and INMARSAT. Depending on the area, additional equipment must be on board, an HF transmitter in A4 area. Explanations of this equipment will follow later. The equipment has to be tested annually and an inspection every 5 years to renew the safety certificate.

CCVs or Commercial Cruising Vessels also have certain mandatory Radiocommunication Equipment on board. CCVs are seagoing vessels longer than 12 metres designed for recreational use by passengers (i.e. charter vessels). CCV motor vessels may carry a maximum of 12 passengers and CCV sailing vessels a maximum of 36 passengers.

According to the Dutch law on inland waterways, a VHF radio is mandatory on motor vessels >20 metres and even 2 on some busy waterways, as 2 channels must be listened to and dual watch is prohibited on inland waterways. A VHF radio is also mandatory on any vessel with an approved inland waterway radar. If you have a VHF radio on board, the following documents are also mandatory:

  1. Registration certificate; (Ship Station Licence)
  2. Operating certificate;
  3. Handbook of marine telephony in inland navigation.

If you have a VHF radio on board then you are obliged to listen. Especially in fog on all waters, on some busy waters and at locks and bridges. Large ships also have a duty to report, for example in block areas. These are areas with busy shipping monitored by a radar station. The responsibility for the correct use of the VHF radio always lies with the skipper, and therefore not with the holder of the operating certificate (if this is someone else) or the yacht charterer.

Purpose of VHF radio

The main purpose of an on-board VHF radio is to enhance safety in general and people in particular. A secondary purpose of an on-board VHF radio is participation in public radio traffic. The latter means using the VHF radio to contact the phone of a person ashore via a shore station. Even though, practically speaking, this component has of course been replaced by mobile phones.


The Maritime mobile-satellite service MMSS at sea or the MMS on inland waterways is a system set up by the International Telecommunication Union regulated by the Radio Regulations (RR).

Types of communication

The VHF radio allows us to conduct the following types of communication:

  • Emergency, urgent and safety communications
  • Public communication: This means using the marine telephone to make a telephone connection with someone ashore via a shore station. This is almost non-existent after the advent of mobile phones, but it is still possible in some countries.
  • Port operations and vessel movements: For example, when designating a berth or navigating in ports, with locks, bridges or a Vessel Traffic Service (VTS).
  • Communication between ships: Also called intership or ship-ship. For example, to make course agreements.
  • Communication on board (intraship): internal connections on board 1 ship, e.g. between the bridge and foredeck when mooring or docking, or between the pusher/tug and the tow.
  • Automatic communication: Ships send data to each other such as their name, course, speed, etc. via a special VHF channel so that you can see those ships and their data on a screen. This system is called AIS (Automatic Identification System).

Types of stations

  • Ship station: An installation for radio communication in inland navigation. A ship station can consist of one or more radio installations (e.g. Inland AIS and/or VHF systems).
  • Maritime marine station: This refers to a marine radio and it always transmits at high power. A marine marine VHF radio is prohibited on inland waterways. Therefore, we often see a combi-marifone on board sailing and motor yachts, which can be manually set to an inland mode.
  • Aircraft stations, such as a Coast Guard helicopter.
  • Shore stations:
    • Coastal stations: for public traffic
    • Coastguard stations
    • Rescue coordination centres (RCC)
    • Vessel traffic service station (VTS)
    • Pilot stations


You can compare radio waves to waves at sea. These also have a certain wave height (Amplitude) and length (Frequency). To transmit radio traffic by means of a radio signal over the ether, Amplitude Modulation (AM) varies the height of each wave and Frequency Modulation (FM and VHF) varies the length of each wave. Radio waves have a speed of 300,000 km per sec.


This is the way radio signals travel through the airwaves. VHF signals move rectilinearly. So you can never transmit beyond the radio horizon. Therefore, the height of the antenna is also essential.

There are also radio signals that follow the curvature of the earth, like that of the Navtex. As a result, the range is much further than the horizon.

There are also straight-line radio signals that bounce against an ionised layer in the atmosphere (sky-wave), such as the signal from an HF. An HF transmitter is intended for long distances and it is not possible with an HF transmitter to contact station that is a short distance from you (i.e. in the dead zone). With the HF transmitter, you have a range of thousands of nautical miles. The higher the frequency, the greater the range that can be obtained, but the properties of the ionosphere depend on sunlight, so frequencies above about 12 MHz make for less reliable communication during the night. In HF communication, it is necessary to take into account the time of day, season and the required communication distance to choose the frequency. The power requirement for HF communication is around 400 W. The maximum power of ship stations is 1500 W.

In MF, it is ground-wave or night sky-wave formed by successive reflection between the earth and the ionosphere. The range depends mainly on the transmitter power and is on the order of 1 nautical mile for every 2 W of transmitter power. A 250 W transmitter at 2182 kHz provides a range of about 125 nautical miles. At night, the range increases to about 1,000 nautical miles due to sky-wave. FM radio can be used in sea areas A2, A3 and A4.

Range VHF

VHF signals move rectilinearly. As a result, you never have much more range, than the horizon as seen from the antenna height. So if there is a mountain or island between your ship and the shore station, you are unlikely to be able to reach that shore station directly. You may be able to do so via another ship that can relay your message for you. This is only possible if that ship is in a position where it has coverage with both you and the shore station.

There is a rule of thumb to calculate range and that is 3 x √ antenna height, where the result is in Nautical miles. For example, the VHF horizon of a sailing ship with an antenna height of 16 metres is 3 x 4 = 12 Nm. A shore station with an antenna height of 100 metres has a VHF horizon of 30 Nm. If you want to calculate the range between this sailing yacht and the shore station, you may add it together, i.e. 12 + 30 = 42Nm.

Frequencies and channels

Every VHF channel e.g. channel 16 meant for Emergency, Urgency and Safety traffic, has 1 or sometimes 2 frequencies. 156.800 MHz is the frequency of VHF 16. Such a channel (VHF 16) is of course much easier to remember and more practical, than 156.800 MHz.

Simplex and (semi-)duplex

First, in a nutshell. With Simplex, you have to speak alternately on one frequency, by switching manually with the Push-to-talk (PTT) button. While transmitting with your own ship station, receiving another station is not possible. Therefore, we always say “over” to give the other the opportunity to respond. An example of full-duplex is the telephone. You can talk with duplex simultaneously and through each other. A VHF channel sometimes has 1 frequency, then it is called a simplex channel and sometimes a channel has 2 frequencies, then it is a Duplex channel.

On a simplex channel, all ships transmit (and receive) in turn on the same single frequency. See the illustration below.

Semi-Duplex is a mode of transmission with alternating speaking on a duplex channel. Transmitting and receiving are then done on two separate frequencies. Transmitting is only possible alternately in both directions of the radio link, e.g. by switching manually with the talk button. The PTT button is pressed only when transmitting. Semi-duplex is thus NOT like a telephone call, there will still be a need to take turns speaking. Moreover, Ship stations cannot always hear each other. In the example below, ships transmit on frequency y and receive on frequency x. The traffic centre receives on frequency y and transmits on frequency x. Ships can therefore communicate directly with the traffic centre, but the ships cannot hear each other. The shore station in this example can hold down the PTT button and transmit and receive at the same time.

With a relay system, ships can also hear each other. A shore station then re-transmits the signal received at frequency Y on frequency X, so that all ships in range of the traffic centre can receive it. In case of an urgent situation, the shore or coastal station can switch off the relay, so that everyone can hear the shore station, but without being disturbed by other ships talking through it.

VHF channels

Below is a list of the main VHF channels in the Netherlands. Please note that channels may vary from country to country. For example, if you cross over to England, Channel 80 or M2 might be the marina channel. We don’t have those channels in the Netherlands and sometimes they are not on the VHF radio you buy in the Netherlands. On other VHF radios, you can select different “channel banks”, such as INT=International; USA=USA; CAN=CANADA.

VHF 0 KNRM Intraship traffic
VHF 1 Central reporting point IJsselmeer and also Maas approach
VHF 2 Brandaris traffic control centre
VHF 3 Sector Maasmond and also Traffic control centre Terneuzen
VHF 4 Central Wadden Sea Control Centre and also Radar Kruisschans
VHF 5 Schiermonnikoog maritime traffic centre
VHF 6 Intership traffic
VHF 7 IJmuiden traffic control centre
VHF 8 Intership traffic
VHF 10 This is the safety and recall channel on the inland waterways. In poor visibility, also on the IJsselmeer.
VHF 13 Diversion channel for channel 10 on inland waterways. According to the GMDSS, wherever the skipper deems it necessary, the skipper must listen on channel 13 for safe navigation and the rest is done via DSC.
VHF 14 Flushing traffic centre
VHF 15 Intraship traffic for onboard communication
VHF 16 Emergency, urgent, safety and call channel VHF 17 Intraship traffic
VHF 18 Bridge and lock channel
VHF 20 Bridge and lock channel
VHF 21 Scheveningen traffic centre and also Saeftinge radar
VHF 22 Bridges and locks channel
VHF 23 Safety and weather reports Netherlands Coastguard working channel
VHF 25 Ouddorp traffic post
VHF 31 Marinas channel
VHF 60 Radar Walker Approach
VHF 62 Den Helder traffic centre
VHF 64 Steenbank traffic control centre
VHF 65 Hansweert traffic centre and also Wandelaar traffic centre
VHF 66 Radar Zandvliet
VHF 67 Coastguard working channel (reserved for SAR rescue operations)
VHF 68 Wemeldinge traffic centre – Oosterschelde VHF 69 Zeebrugge traffic centre
VHF 70 DSC – Digital selective call
VHF 72 Social traffic and salvage and towing operations
VHF 77 Social traffic
VHF 82 Provisioning and bunkering
VHF 83 Dutch Coastguard safety and weather messages
VHF 87 MSI messages AIS 1 Automatic Identification System
VHF 88 Water sports events channel and Maritime radio traffic AIS 2 Automatic identification system

Block channel

For example, if we come from the sea and sail up the North Sea Canal towards the Markermeer at IJmuiden, we pass through several areas, each with its own block channel. We can see which channel applies by looking at the sea chart. This block channel applies, within a given area, simultaneously to the categories ship-ship (e.g. course agreements) and nautical information.

VHF 7 IJmuiden Approach
VHF 74 Seaport Marina
VHF 3 North Sea Canal Traffic Service
VHF 4 Port of Amsterdam
VHF 60 Sector Schellingwoude
VHF 18 Oranjesluizen


Ship name

Every ship has a mandatory ship name. This is also very useful in VHF radio traffic, as it is easier to remember that a long number or code. The only drawback is that the ship’s name is usually not unique and thus cannot serve to really identify you.

Call sign

To identify you on the VHF radio, each ship is given a unique call sign. The call sign of a seagoing vessel consists of four letters and ships on inland waterways two letters and four numbers, the first letter of which in the Netherlands is a P. For example, PA1234.

For the Netherlands, the following combinations apply:

  • PA to PI (plus four digits): barges and yachts;
  • PC (plus four digits): seagoing yachts;
  • PAAA to PIZZ: large seagoing vessels subject to the Ships Act.


Maritime identification for mobile services – Maritime Mobile Service Identity (MMSI) is a unique nine-digit identification number that authorities assign to their sea and inland waterway vessel stations. This number is used by the DSC.

The first three digits represent the maritime identification digit (MID, Maritime Identification Digit), which identifies the authority concerned. For the Netherlands, the MID is: 244, 245 or 246, for Belgium 205, For the UK 232, 233, 234, 235 and for France 226, 227, 228.

A ship is identified by an MMSI without leading zeros (e.g. 244111111), a group of ships is identified by one leading zero (e.g. 011111111) and a shore station by two leading zeros followed by the MID number and the station number (e.g. 002441111).

An MMSI number of an AIS unmanned marine station starts with 99 and of SAR aircraft with 111.

The MMSI is mandatory for the use of Inland AIS. Vessels operating on inland waterways subject to the regulations of the Regional Regulation on Radio Communications Service on Inland Waterways must have an MMSI to generate their individual ATIS code.


ATIS stands for Automatic Transmitter Identification System. An identification system, in other words. The transmission of the ATIS code follows automatically after releasing the talk button. In continuous transmission, the ATIS signal is broadcast every five minutes. ATIS must also be programmed in walkie-talkies. The ATIS code consists of:

  1. the number 9
  2. the MID code
  3. a 0
  4. the second letter of the callsign converted to a digit. The letter A becomes digit 1, the letter B becomes digit 2, etc.
  5. the last 4 digits of the callsign

So a total of 10 digits. All VHF systems and walkie-talkies on inland waterways must be equipped with ATIS. At sea, ATIS must be switched off and identification is via DSC. Public authorities can permit equipment for VHF radio installations that allows suppression of the reception of the ATIS signal in the loudspeaker or handheld device by appropriate technical measures. This is the so-called ATIS killer.

VHF radio

A VHF radio is a transceiver or a combination of a transmitter and receiver for communication by radio telephony at sea and on inland waterways. No two types are the same but virtually all VHF radios have the following features.

Channel switch

Selects the channel. Sometimes it is a rotary knob, sometimes up/down buttons, sometimes number buttons.


Squelch suppresses noise. First let the VHF radio squelch and set the appropriate volume. Then turn until the hiss is just gone, but no further, otherwise weak signals you might want to receive will not get through either.


Controls the volume of the speaker on your own ship.

Dual Watch

It allows you to listen to two channels “simultaneously”. The radio scans the two channels and as soon as something is received on a channel, the radio remains tuned to it. However, if the signal is interrupted for more than 5 seconds, scanning starts again. Officially, you should actually use two VHF radios to listen out two channels. 3CH scan also exists, then the VHF radio scans 3 VHF channels including channel 16. In ALL SCAN mode, the radio scans all channels.

Talk button

This button is also known as the PTT (Push-to-talk) button. To switch on the transmitter, the talk button must be pressed and to switch on the receiver, the button must be released.

Hi / Low Power

Selects high (6 – 25W) or low power (0.5 – 1W). Except for emergency, urgent and safety traffic on channel 16, always try to make contact on low power first so as not to unnecessarily disturb other traffic. For VHF radios, the output power should be set to a value between 0.5 W and 25 W.

NMEA 2000 (N2K)

NMEA 2000 (N2K) is a communication network standard used for connecting nautical electronic devices. It allows devices to work together and allows a display unit to display information from different sources, for example. By no means every marine radio has this, but it is interesting to know what modern equipment can do additionally. If the AIS is linked to the VHF radio, for example, it is possible to see a list of ships (with AIS) in the vicinity via the menu on the VHF radio. This makes contacting them much easier. If the chartplotter is also linked, it becomes possible to click on a ship with AIS on the chartplotter and call it with DSC, without the need to type the MMSI number into your DSC. This is obviously much faster and less error-prone.

Inland or maritime shipping

Modern VHF radios are combi-marifones and the software determines whether it is an inland marine radio complying with the Basel agreement or a marine radio. So you can switch a (combi) radio from a seagoing to an inland navigation radio.

The features of an inland marine radio are:

  • Automatic reduction on different channels
  • no ‘dual watch’
  • DSC (digital messaging) disabled
  • ATIS on
  • for inland navigation

A Marine VHF radio has the following features:

  • Self-switching between high and low power
  • dual watch
  • DSC
  • No ATIS
  • for at sea


The antennas should be free, at least 4 metres from other things placed higher than the antenna. Moreover, as high as possible. On sailing yachts, this is at the top of the mast. On inland water, the maximum height is limited to 12 m. At sea, this maximum height does not apply. The length of the antenna should be half or a quarter of the wavelength. The wavelength is the speed of the radio signal / frequency. For 160MHz, this is 1.87m. A quarter-wave antenna is 1/4 of that, say half a metre. A half-wave antenna is twice as long, say 1 metre. The longer (1m10) antenna has a longer range than the shorter (0m50) antennas and they have a different radiation pattern. That of the long antennas is quite flat, while the short antennas radiate more spherically. On a sailing yacht that is often inclined, the spherical beam pattern is better. Heavy motor yachts, lying flat on the water, often choose the flatter beam behaviour of a longer antenna because it gives them more range. Polarisation indicates the direction of the electric field. VHF uses vertical polarisation, so the antenna should always be set up vertically. A Coaxial antenna cable of at least Ø6mm and an impedance of 50 ohms connects the VHF radio to the antenna.


The fuse or melting fuse protects the radio and prevents wires from melting if the current is too high. In general, the fuse of a VHF radio will not exceed 6 a (Ampere). Fast fuses contain the abbreviation F (Fast) and slow fuses the abbreviation T (Time delayed). The latter therefore does not burn out immediately but can withstand excessive current for a short time. If a slow fuse burns out, it should never be replaced by a heavier fuse.


12-Volt batteries consist of six cells, each with a voltage of 2.1 volts. The cells are connected in series so together deliver 12.6 V. With continuous charging, the voltage is maximum at 13.8 V. When connecting the radio to the battery, the polarity must be taken into account: the plus wire must be connected to the plus of the battery and the minus wire to the minus of the battery.

Because the cells in most batteries have a liquid electrolyte, overcharging creates boiling gas, an explosive mixture of oxygen and hydrogen. If you smell the odour of rotten eggs on board, a battery may be “cooking”. Stop charging the battery by switching off the engine (alternator) or battery charger and ventilate the room to vent the popping gas to prevent explosion.

Sulphuric acid

Sometimes batteries leak sulphuric acid, which is very corrosive. For example, if you have a steel boat, sulphuric acid can rust holes in the hull very quickly. Therefore, place the batteries in a battery box to prevent this.

Difference between starter and light battery

Starter batteries can deliver high current (Ampere) for a short time, for starting the marine engine. Starter batteries have thin lead plates and should therefore not be discharged beyond 20%, otherwise their capacity will deteriorate sharply due to sulphation. Sulphation creates a hard layer on the electrodes that is not electrically conductive. Normally, a starter battery is not discharged much, because immediately after the marine engine starts, the alternator will recharge the battery.

The light battery is another type of battery, made to provide long-term power for, for example: lights, refrigerator, marine radio, etc. These batteries have thicker lead plates, allowing these batteries to be discharged much deeper. These batteries are more resistant to sulphation than starter batteries.

For both starter and light batteries, if you don’t use the battery for a long time, then you should charge it every month or put it on a charger.

Semi-traction battery

Also called recreational batteries. Can be discharged up to 50 – 70%.

Traction batteries

Also called Deep cycle batteries. Are extra robust and can therefore be discharged up to 80%.

Gel batteries

Are made to provide power for long periods of time. The Gel battery is gastight and leakproof and can even be side-mounted. These batteries can be deeply discharged. Gel batteries are maintenance-free: no water needs to be added. Sulphation is less likely to occur. Self-discharge is less than with liquid batteries. These batteries are prone to overcharging, cavities forming that cannot escape, reducing capacity. Of equal size, these batteries can deliver less current and have a lower capacity. These batteries are also more expensive than liquid batteries.

AGM (Absorbing Glass Material) battery

Maintenance-free, gas-tight and leakproof, and can even be side-mounted. These batteries can be 80% discharged.


Cold Cranking Amps is the amperage a battery delivers for 30 seconds at 0 degrees Fahrenheit, or -18 degrees Celsius, and a minimum of 9.6 volts. This value is important to see if a battery is strong enough to be used as a starter battery.


Battery capacity is expressed in ampere-hours, or Ah. This means that a certain current can be drawn from the battery for a number of hours. A battery’s capacity is specified for a discharge time of, say, 10 hours. For example, a 45Ah (10h) battery can give off a total of 45Ah, measured over 10 hours. So that’s an average of 4.5 amps per hour for 10 hours. Capacity decreases as the battery discharges faster. For example, if you were to connect a 9-ampere device to a 45Ah (10h) battery, you might expect the battery to fully discharge in 5 hours. In reality, it will be shorter because a relatively high demand is placed on the battery.

In electricity theory, P = U x I or Watt = Volt x Ampere

P is power in watts (W)
U is voltage in volts (V)
I is current in amperes (A)

U x I


Volt x Ampere

From the above triangle, you can extract the following formulas:

P = U x I
Power (Watts) = Voltage (Volts) X Amperage (Amps)

U = P / I
Voltage (Volts) = Power (Watts) / Current (Amps)

I = P / U
Current (Amps) = Power (Watts) / Voltage (Volts)

The power consumption of the VHF radio is roughly as follows. Rx is receive. Tx is transmit:

  • Standby ≤0.35 A
  • Rx ≤1.5 A
  • Tx High power ≤6 A
  • Tx Low power ≤1.5 A


A scramblerdevice makes you unintelligible except to the person who has previously given you the ability to understand you. This is allowed on public traffic and scrambler channels and should never be possible on VHF 16. Fishing vessels that belong together like to use the scrambler so as not to betray to the competition where the fish are.

Portable radios on board

For walkie-talkies, the output power should be set to a value between 0.5 W and 6 W. The walkie-talkie should only be used on board that ship, i.e. not between shore and ship. For that, use a smartphone. The walkie-talkie must be listed on the licence. The person using the VHF radio must hold an appropriate VHF operating certificate. The use of walkie-talkies is limited to channels 15 and/or 17.

GMDSS walkie-talkies

A walkie-talkie is a portable VHF radio including antenna and battery pack. Portophones have a limited battery capacity and a smaller range because the antenna is not mounted high. The purpose is mainly to carry it in the life raft or if the mast with antenna is knocked overboard. These walkie-talkies are waterproof and yellow in colour. VHF walkie-talkies should have at least channel 16 and one other channel. In addition, the walkie-talkie should be easy to operate, possibly with gloves. The battery and emergency battery must be replaced before the expiry date.

GMDSS walkie-talkie

Digital Selective Calling (DSC)

DSC is a digital calling system to call other vessels or shore stations by entering their MMSI number over the VHF radio. This includes a working channel on which the traffic will be handled via VHF radio. It is possible to call 1 station or a group of ships or only the ships in a certain area. It is part of the Global Maritime Distress and Safety System (GMDSS). The red distress button allows you to send a DSC distress alert, automatically identifying it with your MMSI number and sending its position along with it. A DDA is much faster and more accurate than a MAYDAY via VHF radio. Moreover, on channel 70, which is used for DSC, no squelch is used and therefore the range is also slightly better than the regular VHF radio. The use of DSC is not allowed on inland waterways. Combi radios automatically switch ATIS on and DSC off on inland waterways. On board yachts, DSC is usually class D and requires Marcom-B or the SRC. DCS can also be linked to an MF or HF transmitter. You can read more about the DSC functions in the next Procedures section.


An EPIRB is an Emergency Position-Indicating Radio Beacon. It is a transmitter that can be activated in an emergency and then sends a distress signal on 406-MHz via satellites, possibly including the GPS position. COSPAS-SARSAT is the organisation that manages this global system. COSPAS-SARSAT satellites consist of GEOSAR satellites (GEostationary Orbit Search and Rescue Satellites standing at fixed point over the equator) and LEOSAR satellites (Low Earth Orbit Search and Rescue – satellites) flying in low orbit over the poles. If no GPS is built into the EPIRB, Doppler measurement can be used to determine the position (to an accuracy of 5km). Doppler measurement is based on the same principle of what happens to sound when a car drives by: As the car approaches you, the sound gets higher and higher; as the car moves away from you, the sound gets lower and lower. Overflying satellites that receive the EPIRB signal calculate the EPIRB’s position in the same way. Doppler measurement is only possible with Low-Earth Orbiting Search And Rescue (LEOSAR) Satellites that fly in orbit around the earth and therefore not using Geostationary Orbiting Search And Rescue (GEOSAR) Satellites that appear to be stationary relative to a fixed position on earth. A LEOSAR satellite receiving transmissions from a 406 MHz COSPAS-SARSAT EPIRB transmits those transmissions to a LEOLUT, a Local User Terminal. It can take up to 90minutes for a LEOSAR satellite to detect an activated EPIRB in the polar regions. The EPIRB also transmits your identification number along with it. On 121.500 MHz, EPIRBs also transmit a homing signal, allowing rescue ships, helicopters and aircraft to probe the radio signal with a radio direction finder (homingdevice). The EPIRB must be carried in the life raft and attached to the raft with a line. The EPIRB is often in a hydrostatic release unit on the bridge of ocean-going vessels (outside). That means, when it goes 3 metres underwater, it activates automatically. On sailing yachts, the EPIRB is often in the grabbag, which is taken along when one should board the life raft. You must manually activate the EPIRB in that case. You should test the EPIRB monthly with a test functionality, where it connects to a satellite. The EPIRB should be checked at least once a year by an authorised body. Should you accidentally activate the EPIRB, switch it off and contact the Coast Guard to cancel the distress signal.

A COSPAS SARSAT Satellite picking up an EPIRB distress signal will send it back to Earth to a LUT, a Local User Terminal. The MCC, Mission Control Centre manages the COSPAS-SARSAT system and forwards the message to the relevant RCC.


A PLB has more or less the same functions as the EPIRB, but of lesser quality/reliability. The PLB can also send a distress signal via satellites to the coastguard and sends along the GPS position that is built in. The PLB this also has a homing signal on 121.500 MHz. As the PLB is much smaller, you can easily attach them in your lifejacket. Still, a real EPIRB is much more reliable than a PLB because:

  • The EPIRB has a much larger battery
  • The much smaller antenna on a PLB has to be manually extended
  • The antenna will not automatically point straight up when the PLB floats in the water
  • The PLB does not activate automatically with a hydrostatic release unit, such as an EPIRB


A SART (Search and Rescue Transponder) should be carried in the life raft, activated and mounted on a special stick/stem as high as possible. If the SART is mounted 1 metre high it will be visible at a distance of 5nm on radar. If the SART is then beamed by an X-band radar, it will send back its own signal consisting of 12 dots from 150 metres from the SART’s position towards the edge of the radar screen. The SART is visible only on the X – band Radar (wave length 3cm, frequency 9 GHz). It is intended for long distances, such as at sea. This is in contrast to the S-band Radar with wave length 10cm, on which the SART will not be visible. The SART will also show a light and sound signal when it is approached by a radar beam. As the ship approaches, the dots on the radar screen become arcs and, on arrival, circles. The SART is equipped with a battery, which of course has to be replaced before the replacement date. Ships can see the SART up to about 8 nautical miles, helicopters up to about 30 nautical miles. You will not usually find a RADAR-SART on recreational vessels. The SART’s battery should have sufficient capacity to operate for 96 hours of standby and 8 hours of active operation. The SART should be tested every month using the test function and a 3cm radar. Should you ever observe a SART on your radar screen, the first thing you should do is plot its position. On your radar screen, you can determine the direction to the sart with an EBL (electronic bearing line) and the distance with a VRM (variable range marker). By plotting the line in the chart from your position, you will find the position, or coordinates, of the SART.


The AIS SART transmits a radio signal with a GPS position so that other ships can see it on their Automatic Identification System (AIS). AIS SARTs are brightly coloured yellow, are 25cm high and weigh about half a kilo. These are, of course, ideal for attaching in a life jacket of any crewmember of a small sailing yacht. In case of man overboard, the chances of the drowning person being found are of course much higher if he is wearing an AIS-SART. There are systems where the AIS-SART also activates a DSC alert on board the vessel after some time. So even a solo sailor who goes overboard could be helped by this system. Below is a Man Overboard position in relation to the ship. The top picture is of the radar screen showing the circle with a cross in it.


The reception of maritime safety information (MSI) such as weather reports, ice reports, piracy, navigation warnings, among others, takes place via the NAVTEX that stands for navigation telex. On 518 kHz, MSI are broadcast in English and on 490 kHz by local stations MSI in the local language. The Netherlands is in Navarea 1: North Atlantic, North Sea, Baltic Sea. A total of 21 Navareas cover the whole world. Each Navarea has stations with a range between 100 and 650 nautical miles. Each Navarea is in turn divided into Navtex areas. Each station has an identification code, for example the Dutch Coast Guard has the letter P. In the Netherlands, navtex messages are sent at the following fixed times 02:30, 06:30, 10:30, 14:30, 18:30, 22:30. These fixed times prevent stations from interfering with each other by transmitting at the same time.

Reading Navtex Messages

See below two examples of Navtex messages as they appear on your navtex screen on from the little printer. Of course, a printer is slightly more reliable than a digital screen, as the waterproof paper can still be read even in case of power failure.


A: 52-26.9N 004-12.7E B: 52-25.3N 004-17.2E
C: 52-14.5N 004-08.2E D: 52-09.8N 003-59.7E
E: 52-11.8N 003-56.2E F: 52-19.0N 003-53.8E
G: 52-23.1N 003-54.5E


ZCZC indicates the start of the navtex message
P indicates that it originates from the Dutch Coast Guard
A indicates the type of message, in this case a Navigation Warning. See below, where A,B,D, L should not be switched off.
The number indicates the sequence number
Time and date in UTC (z). (UTC + 2 is Dutch summer time, namely 1 hour because we are in a different time zone and 1 hour for daylight saving time in summer).
NNNN indicates that the navtex message has been terminated.

A Navigation warning
B Meteorological warning
C Ice messages
D SAR (Search And Rescue) and piracy
E Weather forecast
F Pilotage
G AIS (Automatic Identification System)
H LORAN (LOng RAnge Navigation system) this is a forerunner of GPS
I additional
J SATNAV (Satellite Navigation Systems with additional info on GPS, GLONASS, GALILEO
K Other Navaid messages
L Navigation warnings – in addition to A
Z There are no messages

NAXTEX abbreviations

North(erly) = N
Northeast(erly) = NE
East(erly) = E
Southeast(erly) = SE
South(erly) = S
Southwest(erly) = SW
West(erly) = W
Northwest(erly) = NW
Backing = BACK
Becoming = BECMG
Building = BLDN
Cold Front = C-FRONT / CFNT
Decreasing = DECR
Deepening = DPN
Expected = EXP
Forecast = FCST
Filling = FLN
Following = FLW
From = FM
Frequency = FRQ
HectoPascal = HPA
Heavy = HVY
Improving/Improve = IMPR
Increasing = INCR
Intensifying/Intensify = INTSF
Isolated = ISOL
Km/h = KMH
Knots = KT
Latitude/Longitude = LAT/LONG
Locally = LOC
Metres = M
Meteo… = MET
Moderate = MOD
Moving/Move = MOV or MVG
No change = NC
Nautical miles = NM
No significant change = NOSIG
Next = NXT
Occasionally = OCNL
Occlusion Front = O-FRONT / OFNT
Possible = POSS
Probability/Probable = PROB
Quickly = QCKY
Quasi-Stationary = QSTNR
Quadrant = QUAD
Rapidly = RPDY
Scattered = SCT
Severe = SEV / SVR
Showers = SHWRS / SH
Significant = SIG
Slight = SLGT or SLT
Slowly = SLWY
Stationary = STNR
Strong = STRG
Temporarily/Temporary = TEMPO
Further outlooks = TEND
Veering = VEER
Visibility = VIS
Variable = VRB
Warm Front = W-FRONT / WFNT


The International Mobile Satellite Organisation (Inmarsat), formerly the International Maritime Satellite Organisation, was established in 1979 by the IMO to operate satellite maritime communications systems and is now a private company. The Inmarsat Network Control Centre (NCC) in London, manages all ground stations, mobile stations and satellites. In the ‘INMARSAT Maritime Communications Handbook’, you can find details on Coast Earth Station ID Codes, among others.

Inmarsat uses geostationary satellites in four different positions, from which it serves four different regions of the world. The satellites transmit at a frequency of 1.6 GHz. The satellites cover the entire world except the polar regions (above 75 degrees north and 75 degrees south). The Inmarsat satellite coverage areas with Network Co-ordination Stations (NCS), are:

  • AOR-E – Atlantic Ocean Region – East
  • AOR-W – Atlantic Ocean Region – West
  • IOR – Indian Ocean Region
  • POR – Pacific Ocean Region

Inmarsat offers a variety of services. Firstly, SOLAS-compliant systems:

  • Inmarsat C is a two-way store-and-forward communication system that sends messages in data packets in the ship-to-shore, shore-to-ship and ship-to-ship directions. It thus provides you with e-mail, telex and fax, in addition to emergency and safety messages. Inmarsat C consists of a small omnidirectional antenna, compact transceiver (transmitter and receiver), message unit, and if GMDSS-compatible, with Distress button to activate a distress alert.
  • Inmarsat Fleet 77 provides full support for GMDSS, including advanced features like call priority and pre-emption, and offers voice and super-fast data up to 128 kbit/s. Emergency calls are made by telephone. The procedure for sending an emergency signal is to select the phone mode, Emergency Priority and the required LES access code. When the call request is initiated, the call is immediately forwarded to the RCC associated with the LES. If no response is received within 15 seconds, the emergency call is repeated. When contact is established, normal MAYDAY procedures are followed.
  • It was joined in 2018 by Fleet Safety, the next-generation GMDSS-compliant satellite service.

The following (cheaper) Inmarsat systems do not meet SOLAS specifications:

  • Inmarsat Mini C terminals are the smallest models available, depending on the model, supporting the same communication services as Inmarsat C terminals.
  • Mini M, Fleet 55 and Fleet 33

Inmarsat C

Inmarsat C terminals are equipped with a distress signal function that automatically generates and sends a distress signal to an RCC, including position and other information. The procedure for sending a distress signal is similar to that for DSC. The dedicated emergency button is pressed and held. If there is time to enter information, a menu allows you to enter the “nature of distress”. The LES should acknowledge the alert, but if no acknowledgement is received within 5 minutes, the distress notification is repeated. After the distress alert is sent and an acknowledgement is received, more detailed information about the incident and/or help can be sent via the same LES and is then also automatically sent to the same RCC.

Enhanced Group Calls (EGC)

Inmarsat C terminals can receive broadcast messages known as EnhancedGroup Calls (EGC). Enhanced Group Calls (ECG) is the broadcasting of maritime safety and SAR-related information messages over the Inmarsat C, Mini C and Fleet Safety terminals, via SafetyNET, SafetyNET II, RescueNET and FleetNET services. A special header is added to the text to indicate the group of vessels or geographical area to which the message is to be sent. There are two types of EGC:

  • SafetyNET that provides a way to transmit maritime safety information to ships at sea and is used by hydrographic, search and rescue, meteorological and coastguard coordination authorities. Messages can be forwarded to ships in specific regions such as one of the IMO NAV AREAs / METAREAs or the sea area around a search and rescue incident.
  • FleetNET that allows commercial information to be sent simultaneously to a number of pre-designed vessels. It is suitable for use in the distribution of news, stock market reports, sports results, weather analysis and road/port information.

SafetyNET plays an integral role in the GMDSS. The information provider forwards the SafetyNET MSI for a given area to an Inmarsat C LES, for broadcast via the satellite network over an entire Inmarsat ocean region. Ships can therefore receive SafetyNET MSI anywhere in that ocean region, regardless of their distance from the LES/information provider. MSI for a particular area is usually broadcast via NAVTEX or SafetyNET. When a coastal region is not covered by the international NAVTEX service, for example around Australia, MSI will be broadcast on SafetyNET. The SafetyNET EGC receiver is programmed for the relevant NAVAREA / METAREA (and coastal MSI area) for the voyage, just like a NAVTEX receiver. The ship’s electronic position confirmation system ensures that the receiver accepts messages addressed to a relevant geographical area. From shore to ship, ships are alerted by automatic receipt of distress relays originating from an RCCs sent via the SafetyNET service. Inmarsat terminals on board ships can be used for communication with other ships involved in distress incidents and for communication with RCCs. When multiple ships are involved, the EGC system is advantageous for operational updates and planning actions by RCCs.

Inmarsat C and Mini C terminals have an integrated Global Navigation Satellite Services (GNSS) receiver, which automatically transmits position, heading and speed along with a distress call. Distress alerting is a service on SOLAS-compliant maritime Inmarsat C and Mini C terminals and also on some non-SOLAS models. When there is no time to enter information manually into the terminal, the crew can simply hold down the Distress button for about five seconds to send the distress call. An INMARSAT satellite receiving a distress signal from an INMARSAT terminal will transmit it to a LES, a Land Earth Station, which will relay the distress signal to the RCC (Rescue Coordination Centre).

If you accidentally sent a distress alert with the INMARSAT-C, you can switch off the transmitter to prevent it from repeating. You should then contact the distress priority RCC to cancel the alert.

Single-sideband (SSB)

The MF / HF-SSB radio is a combined transmitter and receiver similar to the marine radio. The main difference is the frequency range in which they operate. MF / HF-SSB radios transmit in the frequency range of 1.6 MHz to 30 MHz. The user must choose a frequency based on atmospheric conditions to establish communication over different distances. Unlike VHF, MF / HF radios can transmit over much longer distances from ship to ship or ship to coast.

Along with the wide range of HF-SSB radio, these sets can be easily adapted to provide the user with a number of other services. E-mail, NAVTEX and weather fax are among the many things that can be easily accessed. These services can be easily set up by using a computer, software and a modem.

Unlike VHF VHF radios, which operate on frequencies between 150 and 160 MHz, (MF) and (HF) radios use lower frequencies (1.6 to 30 MHz). These lower frequencies are an important factor in allowing HF radios to communicate over much longer distances. In fact, given the right conditions, global communication is possible. Because of this and its relatively low cost, HF SSB is extremely popular for those setting sail around the world.


General rules

  • First listen out if the channel is free, then transmit;
  • Speaking concisely, slowly and clearly;
  • No broadcast without identification;
  • Limit to necessary calls;
  • Acting in accordance with procedural rules;
  • Do not hold down the talk button longer than necessary;
  • Keep transmit power in the nautical information category as low as possible to avoid interference with other links;
  • Receipt of a notification addressed to a ship station must be acknowledged.

Note: To avoid interfering with emergency calls, a broadcast on channel 16 should never exceed 60 seconds.

Lead radio traffic

In ship-ship radio traffic control lies with the called vessel. This means that the called station therefore selects the working channel. If a ship calls a shore station, the shore station is in charge of radio traffic.

Directions from the shore station

When communicating by VHF radio with a shore station, the latter’s instructions should be followed. If this creates a dangerous situation for the ship, the shore station’s instructions may be deviated from. The shore station should be informed accordingly. Examples of instructions are:

  • area a radio silence;
  • reduce the transmission power of the ship’s station;
  • listening out on a particular channel.


In VHF communications between a ship station and a shore station, the language of the country in which the shore station is located must be used. In VHF communication between ship stations the language of the country in which the ship station is located has to be used. In case of communication problems in radiotelephone traffic between ship stations or between ship stations and shore stations, the language specified in the relevant Shipping Policy Regulations has to be used. In the RPR area German in case of language problems.

IMO Standard Marine Communication Phrases (SMCP)

You should have good language skills in English for Marcom-B, both written and verbal, for good communication related to the safety of life at sea. You should study the IMO Standard Marine Communication Phrases (SMCP) well. In particular, learn these sections that are also very applicable for skippers of sailing yachts at sea:

AI/1 Distress Communications
AI/1.1 Distress traffic
AI/1.3 Requesting medical assistance
AI/2 Urgency traffic
AI/3 Safety communications
AI/3.1 Meteorological and hydrological conditions
AI/3.2 Navigational warnings involving
Appendix to AI – External Communication Phrases
Standard GMDSS Messages: Distress, Urgency, Safety

And especially the spelling alphabet:

A Alfa
B Bravo
C Charlie
D Delta
E Echo
F Foxtrot
G Golf
H Hotel
I India
J Juliett
K Kilo
L Lima
M Mike
N November
O Oscar
P Papa
Q Quebec
R Romeo
S Sierra
T Tango
U Uniform
V Victor
W Whiskey
X X-ray
Y Yankee
Z Zulu

Ranking of VHF traffic

Stations (on board ship and ashore) must give priority to all messages concerning the safety of human life on the waterway, on land and in the air. To ensure this priority, stations must give special priority announcements to VHF traffic. The order of precedence of VHF traffic is as follows:

  1. Emergency traffic Distress MAYDAY
  2. Urgency PAN
  3. Security traffic Safety SECURITY
  4. Other traffic Routine —


An emergency arises if immediate danger to a person or ship is imminent and assistance must be provided immediately. Whether an emergency exists is decided by the person responsible for the ship. This also applies when preventing danger on land. Emergency signals and emergency alarms are therefore only issued by order of the skipper. No regulation may prevent anyone from taking measures to save life and prevent danger. Before starting rescue measures, shore stations in the nautical information category should be called if possible. Through the ship station in distress, shipping may also be informed on a channel in the ship-ship category. During distress traffic, non-participating vessels should observe radio silence.

Emergency channel

At sea and spacious inland waters where the Coast Guard listens, such as the IJsselmeer, dial channel 16. On inland water the block channel or if there is no block channel, on channel 10.

Start of emergency traffic

Emergency traffic starts with an emergency call:

  • emergency signal “MAYDAY, MAYDAY, MAYDAY”;
  • the words “THIS IS”;
  • name of ship in distress, repeated three times;
  • call code or other identifying mark.
  • The rest of the communication shall be in a language authorised on the waterway concerned.

The emergency announcement on inland waterways that follows the emergency call should be as follows:

  • emergency signal “MAYDAY”;
  • name of ship in distress;
  • call code or other identifying mark;
  • position at km;
  • kind of emergency;
  • type of help needed;
  • further useful information.

This is a handy mnemonic to remember what all should be pronounced in the emergency call and in what order: MIPDANIO:

M ayday
I dentity
P osition
D istress
A assistance
N number
I information
O about

The relay of a distress report via VHF radio from a station that is not itself in distress should be as follows:

  • If the coastguard broadcasts a mayday relay, it may be preceded by the radiotelephony alarm signal: a for 30 seconds two pitch-changing signals followed by a tone lasting 10 seconds.
  • the words “ALL STATIONS” or the name of the shore station, repeated three times;
  • the words “THIS IS”;
  • the name of the relay station, repeated three times;
  • The call code or other identification of the relay station.

The rest of the communication is in a language permitted on the waterway concerned. This call is followed by an emergency announcement containing, as far as possible, the same information as in the emergency call or original emergency announcement.

Reçu: Confirmation of an emergency notification

In the nautical information category, the emergency notification is confirmed by the shore stations. In the ship/port authority category, confirmation by the port authority must be awaited. If no confirmation follows within one minute, a ship station should take over the emergency traffic. In the ship-to-ship category, the distress alert should be acknowledged by a ship station in the vicinity.

  • Emergency signal “MAYDAY”;
  • name and call code or other identification of the calling station;
  • the words “THIS IS”;
  • name and call code or other identification of the confirming ship station;
  • the word “RECEIVED”;
  • emergency signal “MAYDAY”.

Please note that an emergency call should never be followed by a receipt. A receipt of course only follows the full emergency message! A receipt and supplement to receipt may only be given by order of the skipper.

Radio silence during emergency traffic

The traffic post or centre or the body coordinating salvage operations, the station in distress or another station may order stations interfering with these communications to radio silence. Reporting will then be as follows:

  • the words “SILENCE MAYDAY”;
  • name of interfering ship or call code of interfering station or “ALL STATIONS”;
  • the words “SILENCE MAYDAY”.

As long as each station that has received the radio silence notification has not received a notification informing it that usual traffic can restart, broadcasting by those stations on frequencies where emergency traffic is taking place is not allowed. During emergency traffic, non-participating stations must not interfere with emergency traffic, which includes observing radio silence.

Termination of emergency traffic

The station that requested radio silence should have it reported that emergency traffic has ended.

  • Emergency signal “MAYDAY”;
  • the words “ALL STATIONS”, repeated three times;
  • the words “THIS IS”;
  • name of the calling station, repeated three times;
  • call code or other identification of the calling station.

The rest of the communication shall be in a language authorised on the waterway concerned.

  • Time the notification takes place;
  • the words “SILENCE FINI”

Urgent traffic

Urgent traffic is conducted when messages need to be exchanged that concern the safety of the crew or the ship, such as illnesses without immediate danger to life, or damage to the ship without immediate danger (e.g. anchored without cargo entering the water).

Emergency procedure

Emergency traffic is handled as follows:

  • emergency signal “PAN PAN, PAN PAN, PAN PAN”;
  • name of the called station or the call “ALL STATIONS” repeated three times;
  • the words “THIS IS”;
  • name of the calling station, repeated three times;
  • call code or other station identification.

The rest of the communication shall be in a language authorised on the waterway concerned.

  • Emergency notification (content of emergency traffic with position, etc.).

Safety Traffic

A safety message is a message containing an important nautical warning or an important weather warning.

The security message proceeds as follows:

  • security signal “SECURITE, SECURITE, SECURITE”;
  • name of the called station or the call “ALL STATIONS” repeated three times;
  • the words “THIS IS”;
  • name of the calling station, repeated three times;
  • call code or other identification.

The rest of the safety traffic should continue in a language permitted on the waterway concerned.

Other traffic

Direction ship-wall and ship-ship

  • Name of the called station (repeated up to three times);
  • the words “THIS IS” or “THIS IS”;
  • type of ship and;
  • name of the calling vessel (repeated a maximum of three times);
  • position of the ship;
  • sailing direction (in port possibly not required);
  • topic of conversation.

Shore-ship direction

  • Name of called station (repeated up to three times) or
  • the words “ALL STATIONS” or “CALL TO ALL STATIONS” (repeated a maximum of three times);
  • the words “THIS IS” or “THIS IS”;
  • name of the shore station (repeated a maximum of three times);
  • topic of conversation.

On a good connection, when calling, the name of the called station should be repeated once and the calling station or type of ship and the name of the calling ship twice. On established connection, calling the name of the ship station or shore station once is sufficient.

Test broadcast

If it is necessary to test a VHF system, this should be kept to a minimum and the transmission should not exceed 10 seconds. The test broadcast should include the call sign of the ship’s station followed by the word ‘test’. The call sign and the word ‘test’ should be spoken slowly and clearly.


Type of call

A DSC call proceeds by selecting the type of DSC call (type of call) in the menu, such as:

  • Distress call
  • Call to all ships (all ships call)
  • Call to individual station (selective call) You can also make a (selective) call to 1 single ship or shore station by entering the MMSI number or selecting from a “buddy list” containing your favourite stations.
  • Geographical area call. Area is indicated by number of degrees in southern and eastern direction relative to a position).
  • Group call It is also possible to request a group number for e.g. a group of vessels that will sail a regatta or rally.
  • Automatic/semi-automatic service
  • Dialphone (a telephone call to a shore subscriber via a coastal station)

Calling and work channel

VHF channel 70 is the calling channel and is intended only for the digital calling system “Digital Selective Calling”. New VHF radios should also not even be able to transmit on channel 70. On confirmation, the VHF radio automatically switches to the working frequency (working channel), which is the channel on which the voice message will then be spoken. You should therefore always transmit a working channel when calling another vessel via DSC.

A selective call may not be “picked up” by the ship or shore station. You will then get the message unable to comply with the reason, for example: busy, no reason given, equipment disabled.


On board the other ships, a visual and an acoustic signal always goes off in category/priority distress and urgency. For category/priority safety and routine, the acoustic signal can be switched off.

format specifier

You choose from the menu on your VHF radio to whom you want to send the call. Messages are structured according to a standard format (format specifier).

TX stands for transmitted message and refers to a sent message. RX stands for received message and thus refers to a received message. EOS stands for “end of sequence” and means “end of message”.


One of the advantages of DSC is that with 1 press of the red emergency button, a DSC distress call or VHF-DSC distress alert a DDA can be transmitted. This automatically includes the MMSI number and position. If no acknowledgement on a DDA is received, the distress alert is automatically repeated. A menu allows you to select what is going on, which is called the nature of distress (N OR D), for example:

  • Fire, explosion
  • Flooding
  • Collision
  • Grounding
  • List – danger of capsizing
  • Sinking
  • Disabled and adrift
  • Armed attack / piracy
  • Undesignated distress
  • Abandoning vessel
  • Person overboard

The position sent along comes from the GPS linked to the DSC. If it is not, an alarm goes off every 4 hours, so you have to enter the position manually. If the GPS no longer transmits position, you will get the message saying EPFS (Electronic Position Fixing System).

The acknowledgement is in principle only given by the Coast Guard. The VHF radio automatically switches to channel 16 where emergency traffic continues to be handled by radiotelephony on the VHF, where you must identify yourself with your MMSI and call sign. If you receive a DDA, you should immediately listen out on channel 16 and note what is said. Only on a retransmission and/or if you are able to render assistance can you acknowledge receipt via channel 16, as this does not disable the DDA but the DSC of the vessel in distress will continue transmitting until the coastguard acknowledges. A DSC Acknowledgement is only given after permission from the RCC.

If a DDA is sent by mistake, immediately restart the VHF radio with DSC if the DDA has not yet been acknowledged and cancel the DDA on channel 16.

Distress alert relay call is also possible if you want to send a DDA for another ship in distress that is unable to do so itself.

Questions & Answers

Question 1: The GMDSS is mandatory for:

a: Vessels longer than 20 meters
b: Commercial vessels over 300 tonnes
c: All seagoing vessels

Question 2: What is the A3 sea area?

a: Inside INMARSAT – coverage
b: In VHF coverage
c: In Navtex coverage

Question 3: Which operator certificate do you need for a DSC Class D VHF radio?

a: Marcom-A
b: Basic certificate VHF telephone service
c: Marcom-B

Question 4: Which operating certificate do you need for an Inmarsat C terminal?

a: Marcom-B
b: Marcom-A
c: Basic Certificate of VHF Telephone Service

Question 5: What is an MRCC?

a: Maritime Radio Coordination Centre
b: Maritime Radio Coordination Centre
c: Maritime Rescue Coordination Centre

Question 6: What Operation Certificate do you need for HF-SSB Radio?

a: Marcom-A
b: Marcom-B
c: Short Range Certificate

Question 7: Which authority in the Netherlands supervises the enforcement of the Telecommunications Act?

a: Police
b: IMO
c: Radiocommunications Agency

Question 8: What was the reason for setting up the SOLAS?

a: The creation of the EU.
b: The founding of the UN.
c: The sinking of the Titanic.

Question 9: Which authority in the Netherlands is in charge of managing the use of the radio frequencies?

a: Police
b: Agentschap Telecom (AT)
c: Military

Question 10: Which VHF channel is intended for SAR VHF traffic?

a: VHF 70
b: VHF 67
c: VHF 16

Question 11: What is Public Traffic?

a: GSM telephony
b: A VHF telephone connection to the shore
c: All VHF communication

Question 12: On-board communication is called?

a: Intraship
b: Intership
c: ship-ship

Question 13: What is a Maritime Shipping Station?

a: VHF with ATIS
b: Inland VHF
c: Nautical VHF

Question 14: What is modulated in VHF?

a: The frequency
b: The amplitude
c: Propagation

Question 15: What is the propagation of VHF radio signals?

a: Follows the curvature of the earth
b: Straightforward
c: Bounces off an ionized layer in the atmosphere

Question 16: What is the range between a sailing yacht with an antenna height of 16 meters and a coastal station with an antenna of 49 meters high?

a: 33Nm
b: 45Nm
c: 10Nm

Question 17: Can two sailing ships hear each other on a Duplex canal out of range of a coastal station?

a: Yes
b: No

Question 18: What is VHF 23 for a channel?

a: Coast Guard Work Channel
b: A channel for DSC
c: A channel for ports

Question 19: What kind of channel is VHF 6?

a: Public traffic
b: Intraship traffic
c: Intership traffic

Question 20: Which channel is intended for social interaction?

a: 77
b: 70
c: 69

Question 21: What is the MMSI number of a Dutch ship?

a: 002441111
b: 244111111
c: 231111111

Question 22: With how many watts do you transmit at high power with a fixed VHF radio?

a: 1 Watt
b: 5 Watt
c: 6 to 25 Watts

Question 23: Why is a shorter (0m50) antenna often chosen on sailing yachts?

a: They always have more range
b: They have a spherical beam pattern
c: They have a flat beam pattern

Question 24: Which batteries should not be discharged beyond 20%?

a: Starter batteries
b: Light batteries
c: Gel batteries

Question 25: After how long is a 45Ah (10h) light battery 50% discharged, if a 6 amp device is connected to it?

a: After 3:45 hours
b: after 5 hours
c: Faster than after 3:45 hours

Question 26: What will be the amperage (Ampere) if we connect a 60 Watt device to a 12 Volt battery?

a: 15 Amps
b: 5 Amps
c: 10 Amps

Question 27: Is a PLB a good alternative to an EPIRB?

a: Yes
b: No

Question 28: Can you use a SART to alert the Coast Guard if you are in the middle of the Atlantic?

a: Yes
b: No

Question 29: What does 171025 UTC stand for in a NAVTEX message?

a: 25th day of the current month at 5:10 p.m. (UTC)
b: 17th day of the current month at 10:25 a.m. (UTC)
c: This is the NAVTEX tracking code of the message.

Question 30: What does DECR stand for in a navtex message?

a: Declining
b: Increasing
c: Variable

Question 31: Is it permissible to broadcast without identification?

a: Yes
b: No

Question 32: If you want the message to be repeated, say?

a: Repeat
b: Correction
c: Say again

Question 33: If you submit a position using an oil platform, you give the bearing:

a: From the platform
b: Towards the platform

Question 34: What is meant by 1 “cable”?

a: 185.2 meters
b: 160 meters
c: 500 meters

Question 35: How do you spell “berth”?

a: Bravo, Echo, Romeo, Tango, Hotel
b: Bello, Echo, Romeo, Tango, Hotel
c: Bravo, Echo, Rosso, Tango, Hotel

Question 36: What is emergency traffic?

a: If there is imminent danger to a human being and assistance must be provided immediately.
b: If there is imminent danger from a person or ship and assistance must be provided immediately.
c: Always when there is immediate danger from a person or ship.

Question 37: What does the Coast Guard call out when someone disrupts emergency traffic?


Question 38: What term is used to end emergency traffic?


Question 39: What term do you use in an urgent procedure?

a: Pan Pan, Pan Pan, Pan Pan
b: Mayday, mayday, mayday
c: securitee, securitee, securitee

Question 40: What is the maximum duration of a test broadcast?

a: 20 seconds b: 10 seconds
c: 30 seconds