Port sidelight: colour, sector and viewpoint

Port is the vessel’s left side when looking towards her bow. Her port sidelight is red.

The reference belongs to the vessel displaying it, not the observer. Facing another bow, that side appears on the image’s right.

The colour does not swap; the relationship between viewpoints does. Rule 21(b) defines colour and sector.

Orienting the drawn vessel comes before recognising the light.

The sector begins right ahead and ends twenty-two and a half degrees abaft the port beam. In plan, bow to beam is ninety degrees; adding twenty-two and a half gives one hundred and twelve and a half.

The starboard sector is green on the other side with equal extent. Neither is a semicircle or all-round.

The red light is not designed to be visible from every position around the hull.

The drawing marks the light and defined nominal horizontal limits. The figure presents no real distance or range scale.

An observer within the nominal sector lies in that light’s intended direction; this does not guarantee seeing it. Intensity, height, obscuration, lantern condition, atmosphere and background contrast affect observation.

Near boundaries, real installation has additional Annex I specifications that a coloured area does not reproduce.

The sector calculation can be checked without deciding an encounter: 90° to the beam plus 22.5° abaft it. The sternlight covers a different 135° arc and the starboard sidelight its own 112.5° arc.

These values describe functions rather than making every dot equally visible in a photograph.

Rule 22 separates minimum ranges by light and length. Sidelights below twelve metres have one mile; from twelve to below fifty, two; from fifty, three.

Required intensities relate to Annex I. Regulatory minimum range is neither guaranteed recognition at that distance in fog nor a protective radius.

It is also not the length of a coloured sector in an unscaled diagram.

Below twenty metres, 21(b) permits combining sidelights in one lantern on the fore-and-aft centreline. A single housing neither merges the sectors nor turns red and green into alternating colours.

Rules prescribing sets for sail, machinery, fishing or other status are separate from this definition. Rule 20 addresses when lights are shown and other lights that must not confuse or impair them.

One isolated red light does not alone establish a port sidelight, vessel type, course, speed, risk or give-way duty. It may belong to another set or have no navigation function.

Identification needs aspect, other lights and observed development. Distinguish vessel side, image side, nominal sector and limited conclusion.

The diagram teaches those relationships rather than deciding a manoeuvre from a colour.