Select Page

How do navigators identify the different types of light around our coasts at night and what are their characteristics?

Navigating at night is made possible through an assortment of navigation aids, both ancient and modern. Lights are arguably the most important of these. Therefore, navigators need to be able identify the different types of light and what they signify. 

Electronic instruments help pinpoint a boat’s position, heading and speed. However, in order to be able to steer a safe and accurate passage, navigators must look out for and recognise the lights of navigation marks, beacons and lighthouses along their route.

How are lights identified?

Trying to identify every light you see can be difficult, especially when entering a harbour at night. In order to help with identification, each type has three main features:

  • Colour – green, red and white are most common. Yellow, blue, orange and purple are also sometimes used. 
  • Character – the rhythm of the light, or the flashing pattern it makes. 
  • Period – the length of time the light’s flashing sequence takes in seconds.

Learn all about buoyage and lights in our reference app IALA Buoys & Lights at Sea for all navigators at sea. The app includes a quiz to test yourself.

Light characteristics

Unless you are unfortunately colour blind, identifying the colour of lights is not too challenging. However, identifying a light’s character and the length of time its flashing sequence takes requires good night vision and concentration.

There are a number of light patterns which are used. Navigators need to learn the different types and their chart abbreviations in order to interpret charts correctly and to recognise lights. They include:

  • Fixed (F) – a single steady light. Often 2 together.
  • Occulting (Oc) – long periods of light with short periods of darkness. This has the effect of making the periods of darkness seem like flashes. 
  • Isophase (Iso) – equal periods of light and dark.
  • Flashing (Fl) – short periods of light with long periods of darkness. Eg Fl(3) flashes in groups of three and then a period of darkness.
  • Quick (Q) – quick flashes of light varying from 50 to 300 flashes per minute.
  • Alternating (Al) – a light with different colours of alternating light. Eg (Al RG) alternating red and green lights.
  • Morse code (Mo) – lights that flash a Morse code letter.

Tip: 

Time several light sequences with a stopwatch to confirm you have identified a light correctly.

Classes of lights and their abbreviations

Learn all about buoyage and lights in our reference app IALA Buoys & Lights at Sea for all navigators at sea.  The app includes a quiz to test yourself.

Light abbreviations

Charts, almanacs and pilot guides use abbreviations to describe lights A typical light characteristic, including its height above sea level and range is condensed into a few letters and numbers. Here is an example together with its meaning explained:

Fl(4) WRG 20s 23m 25M

FL(4)   Character of light: regularly repeating a group of 4 flashes.

WRG   Colours of light: white, red and green. If no colour is given then the light is white.

20s   Period of time taken to exhibit one full sequence: 20 seconds.

23m   Height of light above MHWS: 23 metres.

25M   Luminous range  the distance at which the light can be seen: 25 nautical miles.

Learn all about buoyage and lights in our reference app IALA Buoys & Lights at Sea for all navigators at sea.  The app includes a quiz to test yourself.

Happy boating and stay safe on the water!

ColRegs Rule 14 – Head-on Situation

  ColRegs Rule 14: Head-on Situation (a) When two power-driven vessels are meeting on reciprocal or nearly reciprocal...

Gybing a sailing boat

Gybing is the sailing manoeuvre used to change a boat's direction through a following wind. As with the tacking manoeuvre,...

Antifouling for leisure boats – Part 3

Antifouling is one of the least pleasant boat maintenance jobs to do, but it has to be done. The very worst job of all is removing the old antifouling as this can get seriously messy and is very hard work.

Safety Equipment Checklist for Boats

Safety Equipment Checklist for Boats   Liferaft line attached The liferaft will not work unless the trigger line is...

Medical Emergency at Sea

How to deal with a medical emergency afloat   If you are planning a boating trip, it is important to have at least one...

Peer to Peer yacht charter – How can you monetize your boat?

There is a growing trend in peer to peer yacht charter. How does it work? People already rent rooms, cars and bikes from one...

Boatyard Health and Safety

Boat storage facilities are potentially hazardous environments and it is the responsibility of both boat owners and boatyards to ensure that the...

Always have an emergency grab bag to hand when at sea…

  Grab bag: In the event of having to abandon ship, it is recommended to have a designated waterproof bag to carry...

Essential boat engine checklist

Boat engine checklist Engine oil level check Even if you have checked it previously, confirming the engine oil level is up...

Capsize – understanding the risks

A skipper should know how their boat will cope with rough seas. By working within known limits and understanding the risks,...

Rig check – how to prevent failure at sea

Regular rig checks prevent the risk of mast and rigging failure at sea. This includes regular rig inspections of the spars, ...

Dag Pike’s Boat Survey

We're really pleased to be working with Dag Pike on some great new apps for iPhone & Android. Here'a a bit more about the first app: Dag Pike's...

Passage planning and pilotage

Passage planning and pilotage help skippers navigate safely from one port to another. A passage plan takes into account all...

Fire safety advice at sea from the Marine & Coastguard Agency

Fire safety advice for boaters Top fire safety advice at sea: 1. Fit smoke alarms, carbon monoxide and gas detectors 2. Turn...

Antifouling for leisure boats – Part 4

Applying antifouling. Antifouling is best applied on a dry, calm day. It is best to apply the antifouling in the middle of the day to ensure the hull is dry and as warm as possible.

Cleaning & polishing gelcoat topsides

The gelcoat topsides of a GRP boat can be pampered and restored to their former glory relatively easily when it is ashore. Gelcoat is only a very thin outer layer of the hull, often less than 1mm thick, so you should avoid cleaning it with highly abrasive cleaners, or an-ything that could potentially damage its surface.

Hourly Checks when sailing or motoring

  Hourly Checks Get into the habit of carrying out these checks and both yourself, your crew and your boat will be...

Boat surveys

A full boat survey assesses the condition of the hull, mechanical gear and means of propulsion. The survey is carried out with the boat...

First Aid at Sea – strains and sprains

Strains and sprains respond well to rest and cooling. Wrap ice in a tea towel before applying. First Aid at Sea Strains and...

ColRegs Nav Lights & Shapes, Rules Of The Road and IALA Buoys Apps

ColRegs Nav Lights & Shapes, Rules Of The Road and IALA Buoys Apps Make Learning Rules on iPhone, iPad, iPod and Android...

A five day sailing cruise of the Solent, UK

Welcome to our virtual Solent sailing cruise – a five day sail in the south of England from Bosham Quay in Chichester...

Sail boat rig checks – Part 1

Sailing boat rigs need to be checked regularly to reduce the risk of rig failure at sea. In part one of Sail boat rig checks we run through a series of useful checks that owners and skippers can carry out.

ColRegs when sailing single handed

  Don’t neglect the Colregs when sailing single handed Sailing single-handed represents several challenges for skippers, not least how to...

Anchoring – getting it right is not always straightforward

If you can set an anchor correctly with confidence and know your boat will be safe in a secure anchorage, then you can rest...

Boat Improvements

My Boat - practical improvements Author - Mike Rossiter Most boat owners who have had their craft for any length of time...