The equipment in everyday use aboard a lifeboat has developed to the point where it has evolved its own jargon, almost as arcane as the language of the sea itself. In this, the first of an occasional series on the technicalities of lifeboats, Mike Floyd attempts to explain the mysteries of marine communication in everyday language.

To those lifeboatmen, seamen and yachtsmen to whom VHFDF is no more mysterious than their wrist watch- please bear with us in our attempts to simplify the subject!In this age of instant communication it is difficult to comprehend the situation, only tens of years ago, when a lifeboat put to sea and immediately became totally cut-off from the shore - unable to pass messages to the coastguard, boathouse or casualty.

Indeed lifeboats were lost for the simple reason that no-one could tell them their services were no longer required and they carried on into unnecessary danger.

Modern communication systems have changed the situation completely, and the interchange of information has now become a vital aspect of modern search and rescue operations.

Bulky valve sets have gone the way of cat's whisker radios, and modern transceivers can now be the size of a car radio with an aerial only some 4.5ft long. So inexpensive and compact have these radios become that they can now be fitted to virtually any size of boat with a source of 12v electrical power, and even smaller hand-portable versions with their own internal batteries are also available.

Very High Frequency Lifeboats now carry two different types of radio, enabling them to communicate with the shore and casualty, and often to locate a casualty's position from its radio transmissions.

The most commonly used radio system aboard a lifeboat is 'VHP' (Very High Frequency - a very short wavelength). Because of the way these short radio waves behave VHP is very clear when close-up but they travel in straight lines and cannot follow the rounded surface of the earth. This limits the range, to what is often called 'line of sight' because the transmissions go roughly as far as the horizon before going off into the earth's atmosphere. In the same way that the visual horizon extends * •' k - 5 - •O.

with increasing height, placing the VHP aerial as high as possible maximises the range. This is why lifeboats always have their radio aerials high on the superstructure and coast radios and coastguard stations have tall aerials mounted as high as possible.

A VHP radio has a maximum range of about 30 to 35 miles in normal conditions, but there can be pockets of poor reception.

Radio waves behave rather like waves in a pond, moving out in a circle from where a stone landed, becoming smaller and smaller until they are difficult to see and then finally disappearing.

If there is an obstruction on the pond the waves may not be able to get behind it, and this can also happen with radio waves.

If a boat is behind a high headland or other tall obstruction she may not be able to transmit or receive radio messages on VHP.

Medium Frquency In this situation, and when operating out near the lifeboats' 50-mile range of coverage the other type of radio can be used.

This is 'MF' (Medium Frequency). With MF radio the waves are longer, and because of the way they behave they can follow the curve of the earth out of sight over the horizon. They can also go upwards to bounce off a section of the atmosphere, increasing range and penetrating blind spots. However MF radio sets are larger, more expensive and are more complicated to use than VHP so many small boats, such as yachts and fishing boats, which usually operate within VHP range of the coast, only have VHP on board.

Direction Finding The position from which a radio set is transmitting can be located in much the same way as the position of a stone dropped into a pond - by locating the centre of the circles of waves spreading outwards from the source. Special 'DP' (Direction Finding) radios are fitted to lifeboats and these have an aerial which is sensitive over a very narrow angle rather than through the full circle of a conventional radio. An analogy would be a telescope - which clarifies an image, but only over a very narrow field of view. Just asa telescope 'receives' its visual image over a very small area so a radio DF aerial receives its radio signal over a similar narrow angle - and the direction of the strongest signal is the direction in which the casualty's transmitter lies. At one time the DF aerial was swung around until it received the strongest signal but now the whole process is electronic and automatic. By taking a RDF (Radio Direction Finding) bearing the lifeboat can home in on the vessel or, if another bearing can be taken from another point - perhaps a Coastguard radio ashore - the two can be used to fix the casualty's position.

DF can be used on both VHP and MF radios, although MFDF is not carried on the latest generation of boats as MF is largely confined to commercial vessels which are moving over to an 'active' radar transponder - one which can transmit a signal identifying the vessel on other ships' radar screens.

Radio Operation If every vessel could hear every radio transmission she could not identify her own messages, so each type of radio has different 'channels' on slightly different frequencies (wavelengths).

One of these frequencies is always kept free so that one boat can call another before switching to a different channel to pass the required message. Various channels are allocated for ship-toship, ship-to-shore and other specialised traffic such as harbour control. The calling channel is monitored by all shipping and the coastguard, and is therefore also used for distress calls.

On VHP Channel 16 is allocated for this purpose, but on MF the actually frequency in kilohertz, 2182, is used to identify it.

Because of the limited number of radio channels allocated to marine use it is necessary to keep messages short and also to ensure that the meaning of a message is entirely clear. As a result special procedures and 'keywords' have been evolved and are used internationally. These arehelmsdesigned to standardise the content of a message, shorten the transmission time and to provide maximum clarity. For example there are many ways of saying that a ship is in distress, and this could be multiplied by the number of languages in use, so the single word 'Mayday' is recognised internationally as a 'keyword'.

This one word conveys a great deal of information to any vessel which hears it.

Similarly the phonetic alphabet has also been standardised and the RNLI, for example, would always be spelt out as Romeo November Lima India whatever language was in use.

Anyone using a marine radio has to be licenced and is examined on their radio procedure and knowledge of the system.

To avoid interference from other users the Coastguard has a 'private' channel on the VHF frequencies, Channel Zero, to communicate with the lifeboat and rescue helicopters. No other vessel is permitted to have a radio capable of transmitting on this channel without the permission of the Coastguard.

Crew Paging This same VHF channel is used for some lifeboat crew pagers - when requested by the Honorary Secretary the Coastguard transmits a coded signal which activates the specially programmed pagers. Due to the 'blind spots' mentioned earlier the RNLI has installed repeaters near several lifeboat stations to ensure that the pager signal covers the required area.

Other stations in the UK are covered by the standard British Telecom 'Rescuepage' paging system, again radio activated but on entirely separate frequencies.

The RNLI has been operating its own, local radio paging system but now that the Irish Marine Emergency Service (very roughly equivalent to the UK's Coastguard) has a system similar to the VHF Channel Zero pager the RNLI is moving over to it.

Internal Communication Internal communication on a lifeboat can be as important as external communication and modern all-weather lifeboats have a sophisticated intercom system both above and below deck.

In addition to normal microphones and loudspeakers at strategic points many of the crews' helmets have built-in headphones and microphones which can be plugged in to a number of sockets. Waterproof sockets are installed on deck so that crew members on the foredeck, for example, can pass and receive messages from the coxswain. An additional feature is that key members of the crew, for example the coxswain and radio operator, can use the helmet headsets to broadcast directly on one of the lifeboat's VHF radios (there is usually a duplicate set installed at the outside steering position). Trials are underway with a short-range radio intercom system - a voiding the need for wires, plugs and sockets..