Saturday, August 25, 2018

Morse Code by Light.

Morse code sent by flashing  light was once widely used at sea by both the merchant navy and naval warships. It was sometimes used by coastal stations/coast guards to communicate with ships at sea.   It is little used now and has largely been replaced by VHF.  

In the merchant marine it was  a requirement that deck officers were able to operate signal lamps (normally a hand held light), at a speed of around 10wpm. Warships normally had dedicated radio operators, 'signalmen,  capable of using morse by signal lights  at at anything between 12wpm - 15 wpm - about the maximum possible speed by which can be read by light.     

Like all radio operators in the Royal Navy we were however required to be able to operate light signalling equipment and I did so occasionally. This normally happened when we were short staffed or I'd gone on the bridge for a break when there happened to be no dedicated signalman present.

Larger warships were fitted with  powerful carbon arc lights (20" diameter lens) and  with these you could signal to the horizon even in strong tropical sunlight. It was also possible to signal well beyond the horizon by flashing the lamp onto a cloud base if it was overcast, or when it was dark.  Other signalling lamps were 10”, and  like the large  20” lamp, these were mounted on a swivel base fixed to the deck.  Other lamps included  hand held Aldis lights.  Also fitted were mast head signalling lights operated from a morse key from within the bridge or bridge wings. With the exception of mast head signal lights, the 20", 10" and hand held Aldis lights were not  switched on & off to send morse code. Once switched on the light remained 'on'.  The flashes were produced by  the operator using some kind of lever or trigger to send morse  and this action triggered various types of shutters which prevented or allowed light to escape from the lens or bulb.    

There were  other visual method of using Morse code apart from lights. Using a flag held in the hand and waved 180 degrees to signify a 'dash' and 90 degrees to signify the 'dit'. The ships bell could also be used and in this case two rapid rings were a dash, and one ring was a dit. Both these methods needed good spacing between letters and timing. Neither of  these two methods of signalling were  in use when I was at sea in the 1960’s and 70’s.  (Semaphore is not the same as cw as it is the position of the arms which directly indicate the individual letters.)

It was quite common to signal merchant ships carrying the flag of British merchant ships and ask them; "Where bound and from?", especially  if you were in some big lonely ocean. (A report of the ship and it's signalling standards  were sometimes sent to Lloyds of London) Rarely would a merchant ship initiate communications. Light signalling was also used in the Royal Navy for communicating manoeuvres such as turns/directions and so on. It was also used when ships were at anchor, approaching or within harbours, or  to signal to shore parties etc.  It was a quick, reliable and a  simple method of communications directly between ship's bridges and also had the advantage of being more secure if needed, in that it was difficult to intercept.  However most flashing lights can be read by anyone  as long as they are roughly in line between sender and receiver,  but the Royal Navy certainly had one or two smaller signalling lamps which had the flashing light directed down a narrow tube  over the lens and this could only be read by someone directly in front.  It required concentration to use and a steady hand, especially if the sea was not calm.

Like the different procedures used by the military, commercial and amateur users of radio communications there were also some procedural differences between merchant ships and naval ships in terms of calling up other ships. 

Many ham/amateur radio operators claim that morse is an entirely aural/sound method  of communicating.  However, I'm not aware that there were any discernible differences between operator ability to slip between using visual or sound methods to read what was being sent other than those  limitations imposed by the maximum speed achievable by light over morse sent by radio.   I never met a wireless/radio operator or who couldn’t read CW sent by light and all the dedicated signalmen could read CW sent at  14wpm or less by sound!!  As a test, I discovered the  few members of my radio club who can read and use CW, could also read my CW sent by torch - the first time most of them had ever done so. The few merchant ship deck officers I knew could also read CW sent by sound, provided it was sent slow enough. 

Practical differences in use? You can't blink much or look away using lights so either a good memory or someone to write for you was a good requirement, or you would  read out aloud what was being sent so the Officer Of the Watch could hear what you were reading.  Wet, cold  weather also made light signalling difficult for obvious reasons.  In terms of sending morse by light it was important to space out the characters of the letters much more than you might if you were sending morse by sound because it is harder by light to read quickly sent characters.  It was the normal procedure in both warships and merchant ships that each word  sent was acknowledged individually by the receiving operator sending a simple flash or 'T' to the sending operator.  If he failed to do so, the word was sent again until it was acknowledged.  At the end of the message a simple 'R' was send to acknowledge receipt of the whole message.

As an aside I was once in  one of a three vehicle group all occupied by experienced with  radio operators.  This was long before mobile/cell phones or small hand held radios were invented and we signalled driving directions between vehicles, using ordinary torches,  whilst the vehicles  were being driven. (it was on quiet country roads in Hampshire England!!!).

Here's the general procedure for signalling at sea.  (from the International Code of Signals)



1. A signal made by flashing light is divided into the following parts:
(a) The 
call.—It consists of the general call or the identity signal of the station to be called. It is answered

by the answering signal.

  1. (b)  The identity.—The transmitting station makes “DE” followed by its identity signal or name. This

    will be repeated back by the receiving station which then signals its own identity signal or name. This

    will also be repeated back by the transmitting station.

  2. (c)  The text.—This consists of plain language or Code groups. When Code groups are to be used they

    should be preceded by the signal “YU”. Words of plain language may also be in the text, when the

    signal includes names, places, etc. Receipt of each word or group is acknowledged by “T”.

  3. (d)  The ending.—It consists of the ending signal AR” which is answered by “R”.

2. If the entire text is in plain language the same procedure is to be followed. The call and identity may be omitted when two stations have established communications and have already exchanged signals.

3. A list of procedure signals appears in Chapter 1, Section 10, Pages 19 and 20. Although the use of these signals is self-explanatory, the following notes might be found useful:

(a) The General call signal (or call for unknown station) AA AA AA” etc., is made to attract attention when wishing to signal to all stations within visual signaling distance or to a station whose name or identity signal is not known. The call is continued until the station addressed answers.

  1. (b)  The Answering signal “TTTT” etc., is made to answer the call and it is to be continued until the transmitting station ceases to make the call. The transmission starts with the “DE” followed by the name or identity signal of the transmitting station.

  2. (c)  The letter “T” is used to indicate the receipt of each word or group.

  3. (d)  The Erase signal “EEEEEE” etc., is used to indicate that the last group or word was signaled incor-

    rectly. It is to be answered with the erase signal. When answered, the transmitting station will repeat the last word or group which was correctly signaled and then proceed with the remainder of the trans- mission.

(e) The Repeat signal “RPT” is to be used as follows:
(i) by the transmitting station to indicate that it is going to repeat (“I repeat”). If such a repetition does

not follow immediately after “RPT”, the signal should be interpreted as a request to the receiving

station to repeat the signal received (“Repeat what you have received”);
(ii) by the receiving station to request for a repetition of the signal transmitted (“Repeat what you have

Special Repetition signals “AA”“AB”“WA”“WB”, and “BN” are made by the receiving

station as appropriate. In each case they are made immediately after the repeat signal “RPT”. Examples:
“RPT AB KL”—“Repeat all before group KL”.
“RPT BN 'boats' 'survivors' ”—“Repeat all between words 'boats' and 'survivors' ”.

If a signal is not understood, or, when decoded, it is not intelligible, the repeat signal is not used. The receiving station must then make the appropriate signal from the Code, e.g., “Your signal has been received but not understood”.

(f) A correctly received repetition is acknowledged by the signal “OK”. The same signal may be used as an affirmative answer to a question (“It is correct”).

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  1. (g)  The Ending signal “AR” is used in all cases to indicate the end of a signal or the end of the trans- mission. The receiving station answers with the signal “R” = “Received” or “I have received your last signal”.

  2. (h)  The transmitting station makes the signal “CS” when requesting the name or identity signal of the receiving station.

(i) TheWaitingsignalorPeriodsignal“AS”istobeusedasfollows:
(i) When made independently or after the end of a signal it indicates that the other station must wait

for further communications (waiting signal).
(ii) When it is inserted between groups it serves to separate them (
period signal) to avoid confusion.

(j) The signal “C” should be used to indicate an affirmative statement or an affirmative reply to an in- terrogative signal; the signal “RQ” should be used to indicate a question. For a negative reply to an interrogative signal or for a negative statement, the signal “N” should be used in visual or sound sig- naling and the signal “NO” should be used for voice or radio transmission.

(k) When the signals “N” or “NO”, and “RQ” are used to change an affirmative signal into a negative statement or into a question, respectively, they should be transmitted after the main signal. Examples:
“CY N” (or “NO” as appropriate) = “(Boat(s) is(are) not coming to you.” “CW RQ” = “Is boat/raft on board?”

The signals “C”“N” or “NO”, and “RQ” cannot be used in conjunction with single-letter signals.

The requirement for signalling by light at sea is as follows:-

Please note that the STCW Code, Section A-II/1 states the mandatory minimum requirements for certification of officers in charge of a navigational watch on ships of 500 gross tonnage or more.


Every candidate for certification shall be required to demonstrate the competence to undertake, at the operational level, the tasks, duties, and responsibilities listed in column 1 of table A-II/1 of the STCW Code.


One of those competence is to Transmit and Receive information by visual signalling.


A knowledge, understanding and proficiency requires the ability to transmit and receive, by Morse light, distress signal SOS as specified in Annex IV of the International Regulations for Preventing Collisions at Sea, 1972, as amended, and appendix 1 of the International Code of Signals, and visual signalling of single letter signals as also specified in the International Code of Signals.


MCA approved training Colleges for the HNC programme have also an MCA approved examiner for the signal’s exams.


MCA Signal exam is split into 4 steps as follow:-


  • Receive Morse by light – 20 letters. A group of letters and numbers is sent as a block.
  • Transmit Morse. Students must be able to transmit a block of Morse code – 10 letters.
  • Flags. Students to do a test on flags. Know what they are and their single letter meaning.
  • General. Code and decode using INTERNATIONAL CODE OF SIGNALS (INTERCO) BOOK.


General Knowledge will be oral assessment.


I hope that the above information has addressed your query.




Best regards


Captain Reza Nosrati  (he/him)        Tel: (Direct) + 44 (0) 203 81 72447

Master Mariner, BA (Hons), Cert Ed             

Examiner of Masters and Mates          Email:

UK Seafarer Services



Maritime & Coastguard Agency

Spring Place, 105 Commercial Road,

Southampton, SO15 1EG

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Safer Lives, Safer Ships, Cleaner Seas


1 comment:

Royski said...

Wonderful stuff - thanks for posting and 73