Navigation lights serve a single purpose: to allow mariners to identify what type of vessel they are looking at, which direction it is traveling, and from which side they are viewing it — all without radio communication and in conditions of complete darkness. When you see a light on the water at night, the ColRegs require that you be able to interpret it. Misreading a light pattern, or failing to show the correct lights, can cost lives.
The rules covering lights are contained in Part C of the ColRegs (Rules 20–31), and every vessel is required to show the specified lights from sunset to sunrise and during restricted visibility regardless of the hour.
The Basic Running Light System
All vessels under way are required to show sidelights and a sternlight. The sidelights are:
Red light — port side. The red sidelight covers an arc of 112.5 degrees, from directly ahead to 22.5 degrees abaft the port beam. When you see only a red light, the vessel showing it has its port side facing you.
Green light — starboard side. The green sidelight covers the same 112.5-degree arc, from directly ahead to 22.5 degrees abaft the starboard beam. When you see only a green light, the vessel's starboard side is facing you.
White sternlight. The sternlight covers the remaining 135 degrees — the arc astern from 22.5 degrees abaft the beam on one side to 22.5 degrees abaft the beam on the other side. When you see only a white sternlight, you are looking at the back of a vessel that is moving away from you or crossing ahead. You are the overtaking vessel; under Rule 13, you must keep clear.
On small vessels (under 20 meters), the sidelights may be combined in a single lantern at the bow, and the sternlight is fitted aft. On larger vessels, sidelights are fitted on the bridge wings or in the hull sides, well separated so they are not confused with other lights.
The Steaming Light (Masthead Light)
Power-driven vessels under way are additionally required to show a white masthead steaming light — technically called the "masthead light" in the ColRegs. This light covers 225 degrees, from dead ahead to 22.5 degrees abaft the beam on each side, and must be carried forward and above the sidelights. Vessels over 50 meters must show a second, higher masthead light aft of the first.
The steaming light is the key distinguishing feature between a vessel under power and a vessel under sail. A sailing vessel under sail only does not show a steaming light. If you see a white light forward above red and green, you are looking at a power-driven vessel. If you see only red, green, and a stern light (or a single tri-color at the masthead), you are looking at a sailing vessel under sail.
The Sailing Vessel's Special Rule
A sailing vessel under sail alone (not using the engine, or using the engine with no sail) has two compliant options under Rule 25:
Option 1: Separate sidelights (red and green) plus a white sternlight. These may be combined into a single tri-color lantern fitted at or near the masthead on a vessel under 20 meters.
Option 2: The tri-color masthead light, which combines red, green, and white sectors in a single fitting at the top of the mast. The tri-color is popular on offshore sailing yachts because the elevation makes it visible at greater range. The tri-color must not be used simultaneously with the separate sidelights and sternlight — it is one or the other, not both.
The critical practical point: the moment a sailing vessel starts its engine and uses it to make way, it becomes a power-driven vessel for ColRegs purposes and must show the steaming light. Many sailors forget this and continue sailing under engine with no steaming light displayed — a ColRegs violation that also means other vessels cannot correctly identify them.
Vessels at Anchor
A vessel at anchor shows an all-round white anchor light. For vessels under 100 meters, one white all-round light visible from 360 degrees is required, placed where it can best be seen. Vessels over 100 meters must show a forward anchor light higher than a stern anchor light. Vessels over 50 meters must also illuminate their decks.
A vessel under 7 meters at anchor in a location not in or near a narrow channel, fairway, or anchorage, and not where other vessels normally navigate, is exempt from the anchor light requirement. In practice, if there is any doubt, show the light. The penalty for being invisible at anchor to a vessel entering an anchorage in the dark is severe.
Small Power Vessels
Power-driven vessels under 7 meters capable of no more than 7 knots may, instead of the full running light complement, show a single all-round white light plus sidelights if practicable. In practice, most small powerboats show at minimum a combined lantern with red, green, and white sectors. This is not technically compliant with the full scheme but is common enough that mariners should be aware of it.
Power vessels under 12 meters may combine their sidelights and carry a single white masthead light visible for two miles, plus sidelights, plus a sternlight — or they may show sidelights and an all-round white light in lieu of the masthead and sternlight combination. The practical result is that small powerboats have several compliant configurations, and a white all-round light combined with sidelights is often what you will see.
Fishing Vessels
Vessels engaged in fishing — specifically vessels using nets, lines, trawls, or other gear that restricts maneuverability — show distinctive all-round lights that identify them as fishing vessels and indicate which direction their gear extends.
Fishing vessels (not trawling): two all-round lights arranged vertically, red over white. The phrase used by many sailors to remember this is "red over white, fishing tonight." If the gear extends more than 150 meters from the vessel, an additional all-round white light is shown in the direction of the gear.
Trawlers (vessels dragging gear through the water): two all-round lights arranged vertically, green over white. The "Christmas tree" pattern — a green light over a white light — means a trawler with gear in the water. The vessel may be moving slowly and trailing gear astern or to the side; its ability to maneuver is restricted. It ranks above sailing vessels in the ColRegs hierarchy.
Both types also show running lights (sidelights and sternlight) when making way, so the complete picture of an underway fishing vessel includes the all-round fishing lights plus the standard running lights.
Reading Another Vessel's Lights to Determine Its Course
The most important practical skill with navigation lights is reading a light combination to determine another vessel's course relative to yours — specifically, whether you are on a collision course.
Red and green lights together, no sternlight visible: The vessel is approaching nearly head-on. Both sidelights are visible simultaneously only in the forward arcs. This means the vessel is coming toward you. If the bearing is steady, a collision is developing. Rule 14 (head-on situations) requires both vessels to alter course to starboard.
Green light only, no red: You are seeing the starboard side of the approaching vessel. The vessel is moving from your left to your right. From your perspective, the other vessel's bow is pointing to your right. Depending on the crossing angle and bearing change, you may or may not be on a collision course — but you are seeing its starboard side, which means it is crossing from port to starboard across your bow or ahead.
Red light only, no green: You are seeing the port side of the vessel. It is moving from your right to your left. The vessel is crossing from starboard to port from your perspective.
White sternlight only: You are abaft the vessel's beam. It is ahead of you or crossing away. You may be overtaking; if so, Rule 13 makes you the give-way vessel.
The crossing situation rule under ColRegs (Rule 15): when two power-driven vessels are crossing and a risk of collision exists, the vessel that has the other on its starboard side shall give way. The give-way vessel must avoid crossing ahead of the other vessel. Applying this requires knowing which side of the approaching vessel you are on — which is precisely what reading the sidelights tells you.