Flashing Morse at sea: signal lamps and light signals
Ships flash Morse code with signal lamps: bright, aimed lamps whose beam is cut into short and long flashes, a short flash for a dit and a long one for a dah, read by eye on another ship in sight. Light is still used at sea because it needs no radio and no frequency, it works when radios cannot or should not be used, and it reaches only the ships that can see it. The code is the same International Morse that was sent by radio; only the medium changes, from sound to light.
Here is how a signal lamp works, how Morse timing turns into flashes, and how to read one.
How a signal lamp works
A signal lamp is a powerful lamp with a narrow beam, aimed by hand at the ship you want to reach. Many have a sight, so the signaler can keep the beam on that one ship.
Many signal lamps do not switch the light on and off at all. The lamp stays lit, and the signaler works a handle that opens and closes a shutter in front of it, often a set of slats like a venetian blind. Cutting the beam this way gives each flash a clean start and end, so a dit looks clearly shorter than a dah. Lamps with LEDs, which switch on and off almost instantly, can key the light itself.
Whatever the hardware, the signaler’s job is the same as a telegraph operator’s: dits, dahs and the right gaps between them.
Why ships still use light
Radio does most of the work at sea now, and Morse by radio stopped being the distress system when the GMDSS came fully into force on 1 February 1999 (When did ships stop using Morse code? tells that story). Light kept a place for reasons radio cannot match:
- It needs no radio. No frequency, no power from the radio room, no working antenna. A lamp and a battery are enough.
- It reaches only who can see it. A radio call spreads in every direction. A narrow beam goes to one ship, and anyone out of its line of sight never knows a message was sent. Navies have long valued that: a lamp can pass a message without putting a signal on the air.
- It crosses languages. Morse letters are the same for every crew, and the International Code of Signals, published by the IMO, gives fixed meanings to short letter groups so that ships with no language in common can still understand each other, by flag, light, sound or radio.
Light has limits too. It needs a clear line of sight, it is slow, and haze, rain or bright sun can wash it out. That is why it is a tool for ships close enough to see each other, not a way to reach the shore from over the horizon.
Morse timing in light
A flashing lamp keeps the same timing rule as Morse by sound, the 1, 3, 7 rule. Only the medium changes: a dit is a short flash, a dah is a long flash, and the gaps are darkness.
| Part | Length | In light |
|---|---|---|
| Dit | 1 unit | Short flash |
| Dah | 3 units | Long flash |
| Gap inside a character | 1 unit | Brief dark |
| Gap between characters | 3 units | Longer dark |
| Gap between words | 7 units | Long dark |
So the letter A, .-, is a short flash, a brief dark and a long flash. The word SEA is ... . .-: three short flashes, a pause, one short flash, a pause, then short and long.
Light is usually sent much more slowly than skilled radio Morse, slow enough for a reader to follow every flash by eye. That slowness is a gift when you are learning: at 5 WPM, for example, a dit is 240 milliseconds, long enough to see as a definite blink of light, and a dah is 720 milliseconds, unmistakably long. Morse code timing: the 1, 3, 7 rule for dits, dahs and gaps explains the arithmetic.
A common fault in flashing, as on a key, is short dahs. A dah that is only twice as long as a dit is hard to tell from one, especially at a distance. When you send by light, lean on the dahs and leave generous dark between letters.
How to read a flashing lamp
Reading light by eye is a skill of its own, but the method is simple:
- Fix your eyes on the lamp. Do not glance away to write. Every flash you miss is a letter lost.
- Say the rhythm, do not count dots. Call each letter under your breath as you see it: di-dah is A, dah-dit is N. If you know the letters by sound, you are most of the way to reading them by light, because the rhythm is the same.
- Work in pairs if you can. One person watches and calls out the letters; the other writes them down. It is the easiest way to keep your eyes on the light.
- When unsure, write the elements. If a letter does not come to you, jot its dots and dashes and keep watching. Decode it afterwards, with a chart if you need one.
- Use the darkness. Letters are separated by darker pauses than the gaps inside them, and words by longer still. If the letters seem to run together, it is usually the sender’s gaps, not your eyes.
One rule when you practice with a flashlight at home or on a boat: aim the beam at a wall, a sail or a target, never into anyone’s eyes and never at cars, boats under way or aircraft. How to signal Morse code with a flashlight covers home practice in more detail.
Lights that flash Morse on their own
Not every Morse light at sea has a person behind it. Some navigation marks show a Morse letter as their light pattern. A safe water mark, which means navigable water all around, may flash the letter A, .-, short then long, and charts write that as Mo (A). Some buoys and lighthouses also carry a racon, a radar beacon that answers a ship’s radar with a short pattern on the screen spelling a Morse letter, so the mark can be identified on radar.
The idea is the same as for aviation beacons: a short Morse identity, sent over and over, so whoever sees it can check it against the chart. Why aviation beacons still identify in Morse code tells the version in the air.
SOS by light
SOS, ...---..., sent by any signaling method, light included, is among the recognized distress signals at sea. Remember that it is one character: three short flashes, three long, three short, with no extra pause between the groups.
It is a last resort, not a plan, and never a substitute for calling for help. A boat in trouble should first use its radio’s distress alert, its beacon or a call to the emergency services; on land, call the emergency services. A light only helps if someone is close enough to see it and knows what it means.
Learning the rhythm before the light
Because the rhythm is identical, a good way to read lamps is to learn the letters by ear first, then carry them over to light. Dot & Dash does not flash a light, but it is a good place to build that rhythm on a Mac: its course teaches each letter by its rhythm, which you hear and then key, and Text to Morse turns a message you want to flash into its dots and dashes to send from.
Questions
Do ships still use signal lamps?
Yes. Signal lamps that flash Morse are still used between ships within sight of each other, even though Morse by radio is no longer part of the distress system at sea.
Is flashing Morse the same code as radio Morse?
Yes. It is International Morse with the same timing proportions: a dah three times as long as a dit, and gaps of one, three and seven units. It is usually sent much more slowly.
Why do signal lamps use shutters?
Many signal lamps keep the light burning and open and close a shutter in front of it, which gives each flash a clean start and end without switching the lamp itself. Lamps with LEDs, which switch almost instantly, can key the light itself.
Can you flash SOS with a light?
Yes, SOS by light is a recognized distress signal at sea, sent as one character: three short, three long, three short. It is a last resort, not a replacement for a radio distress alert, a beacon or a call to the emergency services.