Sea Rules of the Road: The Display of Light & Audio Signals

There exists specific rules and responsibilities between vessels at sea, as dictated by the International Regulations for Preventing Collisions at Sea 1972 (COLREGs).

For example, a sailing vessel, must keep clear of a vessel constrained by her draft. But how does she know that this is so? It becomes easy once we realize that the vessel constrained by her draft and the sailing vessel must both display typical lights and shapes. It then becomes easy to determine which is the ‘give way’ vessel, and action is taking accordingly.

The heights and arcs of visibility of each light are prescribed under the rules, and are subject to survey and approval during the construction of the ship. The distance at which each light can be seen at sea is also fixed; for example, a masthead light must be visible at six nautical miles. This is true for all lights, shapes and sound signals for all ships worldwide.

Normal Lights & Shapes for Power Driven Vessel

Power driven vessel

A normal power driven vessel displays (click on any image to enlarge)

  • A mastlight high on her forward mast
  • A second mastlight even higher on her after mast if the length of the vessel is more than 50 meters.
  • A sternlight on her stern
  • Two sidelights, a green one on her starboard (right) side and a red one on her port (left) side

At anchor, a power driven vessel shows an all round (360 degree) light. When she is more than 50 meters in length, she shows two all round lights, with the forward one being higher than the one near her stern.

Lights on Other Vessels

Typical lights

Click on the diagram on the left; it gives you some typical lights that are displayed by different vessels at night.

It is important to realise that most ‘special lights’ are shown in addition to the normal running lights of a power driven vessel when a vessel is under mechanical power. For example, the vessel constrained by her draft shows three all round red lights in addition to the normal masthead light(s), sidelights and sternlight.

Note also that a vessel aground (where she has run into the seabed and is fast there) shows two red lights in addition to her anchor lights.

A sailing vessel that is moving shows the same sidelights and sternlight as a powerboat, but does not show a masthead light. A vessel with sails up, but also being propelled by machinery must show the power-driven vessel ‘running lights’

A fishing vessel is defined as one engaged in fishing with nets, lines, trawls, or other gear that restricts her ability to manoeuvre. The diagram shows a trawler.

A tug towing a barge is not normally considered to be restricted in her ability to manoeuvre. A tug towing another vessel does not show normal masthead lights, but must show two masthead lights instead one vertically above the other. (normal masthead lights are horizontally displaced, but not in the case of a tug). If the length of the tow is more than 200 meters, it must display three masthead lights vertically disposed. In addition, it displays sidelights and a sternlight. It also must display an all round towing light at the stern, above the sternlight.

A vessel being towed does not display masthead lights, but sidelights and a sternlight.

A towing vessel seen from far away can be confusing to a first time mariner, and care must be exercised.



What lights indicate by night, shapes indicate in the daytime. Shapes are, typically, ‘a ball’ (spherical), a cylinder, a diamond, a cone etc. As with lights, their shape, colour and size is prescribed under the rules. These are normally hoisted on a mast so that they can be seen from afar by other vessels.

Some typical shape configurations:

  • A normal power driven vessel shows nothing special at daytime
  • A vessel at anchor shows a black ball at her forward end on the forecastle.
  • A vessel not under command shows two black balls in a vertical line on her main (highest) mast
  • A vessel restricted in her ability to manoeuvre shows two black balls separated by a diamond.

Some other typical shape configurations can be seen in the diagram below, along with the kind of ship they represent.

Sound Signals

There are only three types of sound-signal equipment mentioned in the Rules – whistle, bell, and gong, with the latter only required on vessels 100 meters or more in length. As with everything else, the specifications of the equipment are prescribed in the rules. The whistle and gong are used by larger vessels when anchored in poor visibility in addition to the normal sound signals.

  • A short blast is a signal on the foghorn (or whistle) of about one second duration.
  • A long or prolonged blast is of four to six second duration
  • When moving, a ship may use the following basic signals to ‘contact’ other ships
  • One short blast- I am turning to starboard (my right)
  • Two short blasts- I am turning to port (my left)
  • Three short blasts- I am going astern (backwards)
  • One long blast- I am moving (usually used in fog or at blind turns)
  • One long blast followed by two short ones: I am not under command
  • Five short rapid blasts: Please declare your intentions (also used to say, “what are you doing?, or get out of the way!”

Other kinds of vessels have similar signals identified in the rules, which must be followed.

Conclusion and Resources

Important: This series has been a basic primer on the Colregs. The rules can be very complex and a navigator is well advised to devote considerably time to this safety-critical issue. For further reading, search for Colregs on the internet, where the full text of the rules is available.


This post is part of the series: Marine Navigation: The Rules of the Road

The first article will explain basic concepts and definitions. Subsequent ones will go in some detail in the nitty gritty and explanation of the Rules.
  1. The Basics of the Sea Rules of the Road in Navigation
  2. Basics of the Sea: Rules of the Road – In any Condition of Visibility.
  3. Basics of the Navigation Rules: When ships can see each other, and when they can’t
  4. Basics of Sea Collision Regulations: Lights, Shapes, and Sound Signals