At night, another boat is just a few points of colored light on a dark sea. Those lights tell you what kind of vessel it is, which way it is heading and what it is doing. Your own lights tell everyone else the same about you. Getting them right is a legal requirement and one of the most important safety habits after dark.

Quick answer: a sailboat under sail shows red and green sidelights and a white sternlight. Boats under 20 meters may combine all three in a single tricolor at the masthead. As soon as the engine is running and in gear, the sailboat is a power-driven vessel and must add a white masthead (steaming) light and switch off the tricolor in favor of deck-level sidelights and sternlight. At anchor, show an all-round white anchor light.

This guide follows the international rules (COLREGs Part C, Rules 20 to 31). National and inland rules, including the US Inland Rules, are almost identical for sailboats but have a few exceptions, noted below.

Why Navigation Lights Matter

Rule 20 requires lights to be shown from sunset to sunrise and in restricted visibility, such as fog or heavy rain. They do two jobs. First, they make you visible. Second, and just as important, they let other vessels work out your aspect: whether they are looking at your bow, your side or your stern. That tells them who is the give-way vessel under the right of way rules. Wrong or missing lights can mislead another skipper into turning the wrong way.

Sidelights, Sternlight and Masthead Light

Every navigation light has a fixed color and a fixed horizontal arc, defined in Rule 21:

Light	Color	Arc	Where it shines
Sidelights	Red (port), green (starboard)	112.5° each	From dead ahead to 22.5° abaft the beam on each side
Sternlight	White	135°	Aft, 67.5° either side of dead astern
Masthead (steaming) light	White	225°	From dead ahead to 22.5° abaft the beam on both sides
All-round light	Any (white, red, green, yellow)	360°	Visible from every direction

The arcs fit together like a jigsaw: the two sidelights plus the sternlight cover 360°, and the masthead light covers the same forward sector as the two sidelights combined. The 22.5° boundary is also what defines overtaking: if you can see only a vessel’s sternlight, you are overtaking it.

Minimum visibility

Rule 22 sets how far each light must be visible on a dark night:

Vessel length	Masthead	Sidelights	Sternlight / all-round
Under 12 m	2 nm	1 nm	2 nm
12 m to under 20 m	3 nm	2 nm	2 nm
20 m to under 50 m	5 nm	2 nm	2 nm
50 m and over	6 nm	3 nm	3 nm

Replacement lights should be certified for the range your boat needs. Many manufacturers state the length range or the nautical-mile rating on the housing.

Lights for Sailboats Under Sail

Under Rule 25, a sailing vessel underway shows sidelights and a sternlight. There are three legal ways to do it:

1. Separate sidelights and sternlight at deck level. On boats under 20 meters, the two sidelights may be combined in one bicolor lantern on the bow pulpit.
2. A masthead tricolor (boats under 20 meters only): one lantern at or near the top of the mast showing red, green and white in the correct arcs.
3. Deck-level lights plus optional all-round red over green at or near the masthead (“red over green, sailing machine”). This optional pair may not be used together with a tricolor.

The tricolor is popular on cruising boats for good reasons. It uses a single bulb, so it draws less power, and at the top of the mast it is clear of waves and headsail shadows, making it easier to see from a distance. Many sailors use the tricolor offshore and the deck-level lights near busy harbors, where other boats close by may find a light at deck height easier to judge.

Small sailboats

A sailing vessel under 7 meters should show sidelights and a sternlight if practicable. If not, it must have a white electric torch or lantern ready to show in time to prevent a collision. The same applies to rowing boats and dinghies. A dinghy out at dusk needs at least a bright waterproof flashlight.

Lights When Motoring or Motorsailing

A sailboat using its engine, with or without sails up, is a power-driven vessel. Under Rule 23 it must show:

• Sidelights (deck-level or bow bicolor).
• A sternlight.
• A white masthead (steaming) light, usually mounted on the front of the mast about two thirds of the way up, which must be above the sidelights.

This is the most common lighting mistake on sailboats: leaving the tricolor on while motoring, or switching on the steaming light together with the tricolor. Both are wrong. The tricolor tells other vessels you are under sail, and a white light below a tricolor looks like nothing in the rulebook. When the engine goes into gear, switch from tricolor to deck-level sidelights and sternlight, and add the steaming light.

By day, a sailboat motorsailing should hoist a black cone, point down, forward. The US Inland Rules exempt vessels under 12 meters from the cone; the international rules do not.

Power-driven vessels under 12 meters may combine the masthead light and sternlight in one all-round white light. That is common on small motorboats, but most auxiliary sailboats have the separate lights described above.

Tricolor and Anchor Lights

Many masthead fittings combine a tricolor with an all-round white anchor light, sometimes with a strobe. The anchor light is a single all-round white light where it can best be seen (Rule 30). Vessels over 50 meters show two.

• Masthead anchor light: visible from far off, but sailors in a dark anchorage often fail to notice a light 15 meters above them. Many cruisers add a second, lower anchor light or cockpit lantern at boom height so dinghies and approaching boats can see them.
• When it is required: vessels under 7 meters at anchor away from channels, fairways and anchorages are exempt. In the US, vessels under 20 meters in designated special anchorage areas do not need an anchor light, but showing one is still good practice.
• By day: show a black ball forward when at anchor.

Anchoring overnight is covered in more detail in how to anchor a sailboat.

Reading Other Vessels’ Lights at Night

Interpreting lights is a skill you build with practice. Start with aspect:

• Red and green together: the vessel is heading straight at you. Act early.
• Red only: you are looking at its port side. If you are a power-driven vessel crossing, it is on your starboard side and you give way. “If red you see, give way you must.”
• Green only: you are seeing its starboard side; it is crossing from your port side.
• White only: you are looking at its stern (or it is a small boat or a vessel at anchor).
• Two white masthead lights: a power-driven vessel, required on ships 50 meters or longer. The lower forward light and higher after light act like a range and show its heading: if they are nearly in line, it is heading straight toward you.

Then look for all-round lights that show a special status. A few combinations and memory aids:

Lights (in a vertical line)	Meaning	Memory aid
Red over green	Sailing vessel (optional)	Red over green, sailing machine
Green over white	Trawling	Green over white, trawling at night
Red over white	Fishing (other than trawling)	Red over white, fishing at night
White over red	Pilot vessel on duty	White over red, pilot ahead
Red over red	Not under command	Red over red, the captain is dead
Red, white, red	Restricted in ability to maneuver	Restricted
Three red	Constrained by draft (international rules only)	Deep ship
Two or three white masthead lights, yellow over the sternlight	Power-driven vessel towing astern	Look for the tow behind

Fishing and RAM vessels also show sidelights and a sternlight when making way through the water, and a towing vessel’s tow is lit too. A tow can be hundreds of meters long, so never pass between a tug and its tow. A quick reference card in the cockpit helps a lot on night watches. Keep a steady compass bearing check on any lights that come closer: if the bearing does not change, you are on a collision course.

Maintenance and LED Upgrades

Navigation lights live in a wet, salty, vibrating environment, so failures are common. Check them before every night sail, along with the tide times for your passage; it belongs on your pre-departure checklist.

• Switch each light on and walk around the boat (or ask someone ashore) to confirm it works and shows the right color in the right direction.
• Clean lenses; crazed or salt-crusted lenses reduce range.
• Look for green corrosion on bulb sockets and connectors; clean and protect with a dielectric grease.
• Inspect masthead wiring when the mast is down, especially where cables pass through the deck.
• Carry spare bulbs, fuses and a set of battery-powered emergency navigation lights.

LED lights are a worthwhile upgrade. They use a fraction of the power of incandescent bulbs, which matters on long night passages, and they last far longer. A few cautions: buy fixtures certified for the range your boat requires rather than LED replacement bulbs that may not produce the right color or arc in an old lens; check that the LEDs do not interfere with VHF reception, a known problem with some cheap drivers; and keep the old fixture as a spare if it still works. Navigation lights are part of the essential safety equipment on any boat, so they deserve quality parts. See also cruising sailboat equipment essentials.

Sailboat Navigation Lights: Summary

• Under sail: sidelights and sternlight, or a masthead tricolor on boats under 20 meters.
• Under power or motorsailing: sidelights, sternlight and a white masthead light. Never use the tricolor with the engine on.
• At anchor: an all-round white light (and a black ball by day).
• Sidelights cover 112.5° each, the sternlight 135°, the masthead light 225°.
• Learn the common all-round combinations to recognize fishing boats, tows, pilots and ships with restricted maneuverability.
• Check your lights before every night sail and carry backups.

Lights work hand in hand with charts and rules: the same characteristics that identify buoys are explained in how to read a nautical chart, and once you know who is who, the sailboat right of way rules tell you what to do about it.