Rule 21: Definitions
- “Masthead light” means a white light placed over the fore and aft centreline of the vessel showing an unbroken light over an arc of the horizon of 225 degrees and so fixed as to show the light from right ahead to 22.5 degrees abaft the beam on either side of the vessel.
- “Sidelights” means a green light on the starboard side and a red light on the port side each showing an unbroken light over an arc of the horizon of 112.5 degrees and so fixed as to show the light from the right ahead to 22.5 degrees abaft the beam on its respective side. In a vessel of less than 20 metres in length the sidelights may be combined in one lantern carried on the fore and aft centreline of the vessel.
- “Sternlight” means a white light placed as nearly as practicable at the stern showing an unbroken light over an arc of the horizon of 135 degrees and so fixed as to show the light 67.5 degrees from right aft on each side of the vessel.
- “Towing light” means a yellow light having the same characteristics as the “sternlight” defined in paragraph (c) of this Rule.
- “All-round light” means a light showing an unbroken light over an arc of the horizon of 360 degrees.
- “Flashing light” means a light flashing at regular intervals at a frequency of 120 flashes or more per minute.
What this means:
Rule 21: Definitions – Our plain English guide to help you understand & remember this rule.
Side & Stern light
Probably the best-known lights, the side and stern lights allow an onlooker to determine what aspect of a vessel they are looking at.
Standard across all vessels, they divide the water around the boat into three distinct arcs. This allows the onlooker to determine which aspect they are viewing, which is critical for collision avoidance.
Masthead light
Masthead lights are only shown by power-driven vessels and always in conjunction with sidelights and a stern light. They are sometimes referred to as steaming lights, harking back to when ships were powered by sail or steam engines.
Masthead lights cover the same arc of visibility as the combined sidelights and give the onlooker an indication of the size alongside its power-driven status. Masthead lights are visible from further away than sidelights, so you will see them before you see the sidelights.
All-round light
All-round lights are self-explanatory—visible from any angle to an onlooker. They expand the vessel types that can be displayed and, for sensible reasons, are used instead of sidelights and stern lights in smaller vessels.
Under IRPCS, all-around lights provide status. Different colours and combinations can be displayed to indicate a vessel’s current situation.
Flashing & towing lights
Flashing & towing are the remaining light types in the definitions. These are reserved for more specialist applications.
A flashing all-round yellow light is displayed by hovercraft, and, given they can go anywhere, they are expected to keep out of everyone’s way.
