Night sailing: reading lights, recognising vessels and arriving safely

You identify a light by its characteristic of rhythm, colour and period: Fl stands for flashing, Oc for occulting, Iso for isophase, Q for quick. “Fl(2) W 10s” means two white flashes every ten seconds. Time it with a stopwatch and compare it with the chart, and you will know which light you are looking at.

How do I read a light's characteristic?

From left to right: rhythm, colour, period, then elevation and range. Taking “Fl(2) W 10s 15m 12M” as an example:

You time the period from the start of one sequence to the start of the next. You often see a long-range light first as a loom above the horizon, before the light itself appears.

What types of light are there?

The rhythms have international abbreviations and are the same on every chart:

Numbers in brackets are groups: Fl(3) means three flashes, then a longer pause.

What do the sectors of a light mean?

A sector light shows a different colour depending on the direction you see it from. The limits are given on the chart and in the List of Lights as bearings from seaward towards the light.

Often a white sector leads through the channel, with red and green sectors either side showing which way you are straying; the chart tells you which colour is on which side. “Iso WRG 6s” stands for an isophase light with white, red and green sectors. Leading lights are two lights one behind the other, a front and a rear light: when they are vertically in line, you are on the leading line.

How do I recognise buoys and cardinal marks at night?

Lateral marks show red or green lights, cardinal marks white quick or very quick flashes in a fixed number. The number follows a clock face: 3 for east, 6 for south, 9 for west.

In Europe, Africa and large parts of Asia the IALA A buoyage system applies: entering from seaward, red is to port and green to starboard. In North, Central and South America including the Caribbean, in Japan, Korea and the Philippines, IALA B applies, where it is the other way round.

Special marks show yellow lights. Unlit buoys are invisible at night, so plan to keep well clear of them.

How do I recognise other vessels by their lights?

By their sidelights, masthead light and sternlight: red is port, green is starboard, and a white masthead light shows a vessel under power. If you see red and green at the same time, it is heading straight for you.

If the bearing of a light stays steady and it is getting closer, there is a risk of collision (COLREGs Rule 7). Under engine, a sailing yacht also shows a masthead light and counts as a power-driven vessel; the tricolour lantern at the masthead is only allowed under sail.

How do I plan a night passage?

With a fixed watch system, safety equipment within reach and a route whose lights you know in advance. Put critical stretches in daylight wherever possible.

• Watches of two to four hours; at night nobody goes on deck without a harness and a life jacket with a light.
• Reef before dusk, check torches, switch on the navigation lights.
• Red light in the cockpit and at the chart table: your eyes need around 20 to 30 minutes to adapt to the dark.
• Note the lights along the route with their characteristics and cross traffic separation schemes promptly and as close to a right angle as possible; more under Traffic separation schemes.
• Have a plan B for the arrival: an anchorage, or the option of staying out at sea until morning.

How do I get into harbour safely in the dark?

Only into harbours whose entrance you know from the chart and the pilot book, with the entrance lights noted in advance. If in doubt, wait outside for daylight.

Town lights drown out the pierhead lights, and unlit buoys, fishing floats and fish farms stay invisible. Go slowly, send someone with a torch to the foredeck and have fenders and lines ready before you enter. How to prepare the whole route is covered in the guide Passage planning step by step.

How does Sailseer help with night sailing?

If a leg falls in darkness, the weather check on the map lists the lights within 4 nm of the route in the direction of travel, major lights first, and, for an arrival after sunset, the lights at the entrance of your destination. The “Lights” layer shows around 28,100 lights with characteristic and range, and from zoom level 12 the sectors as arcs around the light; in Finland and Portugal they come, sectors included, from the official lists. From zoom level 12, tapping a seamark tells you what it is (for example “South cardinal buoy”), with its characteristic and how to pass it. Sailing in the dark feeds into the rating of every departure, and the sailing window alert can be limited to daylight departures. With an AIS receiver on board, Sailseer shows targets with CPA and TCPA via the boat instruments and warns if the passing distance is too small.

Sailseer is a planning aid, not for navigation and not for collision avoidance. The official chart and List of Lights take precedence.