Navigational Lights and Day Shapes

Why Lights and Day Shapes Exist

Before radar and AIS, the only information sailors had about a vessel they could see in the dark was the pattern, color, and position of its lights. Even now, lights are the primary passive identification system for every vessel at sea. COLREGS Part C (Rules 20–31) prescribes exactly which lights must be shown, in what color, in what arc, and to what minimum visibility — creating a system where any competent mariner can identify a vessel's type, direction, and status from its lights alone.

Why this matters for right-of-way: the lights and day shapes a vessel displays directly indicate its classification in the vessel hierarchy. A vessel showing red-white-red lights vertically is a RAM — it has greater right of way than a sailing vessel. A vessel showing a ball forward is at anchor — you must not collide with it. A vessel showing a cone pointing forward with sails set is using its engine — it's a power-driven vessel in the hierarchy, not a sailing vessel.

The general system: sidelights (red on port, green on starboard) indicate direction of travel — you can tell which way a vessel is heading from its colors. The masthead (steaming) light shows forward, indicating a power-driven vessel is underway. The stern light shows aft. Additional lights indicate special status. Day shapes communicate the same information during daylight when lights would be invisible.

When lights must be shown: from sunset to sunrise, and in restricted visibility at any hour. In broad daylight with no visibility concerns, lights are optional (except that vessels not under command, engaged in fishing, etc., must show their shapes by day).

Lights: Power Vessels and Sailing Vessels

Power-driven vessels underway must show: a white masthead (steaming) light forward (visible 225° from dead ahead to 22.5° abaft the beam on each side), sidelights (red port, green starboard, each 112.5°), and a white stern light (135° centered on dead astern). Vessels over 50 meters must show a second masthead light aft and higher than the forward light. The combined effect: from any direction, an observer sees a combination of lights that indicates the vessel's heading and confirms it is power-driven.

Understanding the arc system: each light illuminates a specific sector. The 225° white masthead light means a vessel approaching from any direction from dead ahead to just past the beam on either side sees the white masthead light. The sidelights (112.5° each) mean an observer seeing both red and green simultaneously is directly ahead, in a head-on situation. The 135° stern light means an observer directly astern sees only white.

Sailing vessels underway under sail only show sidelights and stern light but no masthead (steaming) light — the absence of a white masthead light is what distinguishes a sailing vessel from a power-driven vessel at night. Small sailing vessels under 20 meters may combine sidelights and stern light in a single tricolor lantern at or near the top of the mast, but only when under sail — not when motoring.

Vessels at anchor show a single white all-round light forward (vessels under 50m) or a white all-round forward plus a lower white all-round aft (vessels 50m and over). No sidelights — the vessel is not underway.

Note on binoculars: at sea, lights can be at the limits of naked-eye detection. Always have binoculars on the watch at night. The sequence of lights visible as a vessel approaches tells you its heading and status long before you can resolve the hull.

Day Shapes: Cones, Balls, and Diamonds

Day shapes serve the same function as lights but in daylight — a vessel's shape configuration tells observers its status and classification. They are black geometric forms displayed where they can best be seen.

Ball (sphere): a single black ball displayed forward indicates a vessel at anchor. A vessel not underway but also not at anchor — drifting, for example — does not display a ball.

Cone, point downward: displayed by a sailing vessel under power, forward where visible. This signals the loss of sailing vessel status in the hierarchy. A sailing vessel with sails set but engine running is legally required to display this shape during daylight hours.

Two balls vertically: displayed by a vessel not under command — the day shape equivalent of two red lights vertically at night.

Ball-diamond-ball vertically: displayed by a vessel restricted in ability to maneuver (RAM). The day shape equivalent of red-white-red lights.

Diamond (rhombus): displayed by a vessel being towed (when the tow exceeds 200m, both the towing vessel and the tow display a diamond). The towing vessel also displays a black diamond when the tow exceeds 200m.

Cylinder: displayed by a vessel constrained by draft (CBD). International rules only.

Three balls vertically: displayed by a vessel aground (in addition to anchor shapes, a grounded vessel adds a second ball below the anchor ball, making three balls vertically — or per some interpretations, a vessel aground uses three balls).

Day shapes must be displayed from sunrise to sunset in conditions of good visibility. They are typically made from rigid plastic or canvas and are hoisted on a forestay, on a designated halyard, or placed in a visible location forward.