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!

First Aid Afloat – fish spine injury

First Aid Afloat - Here is what to do if somebody stands on a fish spine: • Check for dangers. Is it safe for you to enter...

Essential Knots: Figure of eight

Essential Knots: Figure of eight Use: Stopper knot, prevents a rope from being pulled through a hole e.g. through a block or...

Stern gear maintenance

The stern gear of a boat needs to be checked carefully when the boat is ashore as this is something that can only be done when it is out of the water. The same applies for any maintenance and repairs that may need doing, so it is best to check it all over as soon after an end of season lift out as possible.

Boat engine basics

Boat engines come in all shapes and sizes and include inboards, outboards, petrol, diesel, electric and hybrid systems. Some engines are...

Learn ColRegs: Traffic Separation Schemes

Learn ColRegs Rule 10: Traffic Separation Schemes. (c) A vessel shall, so far as practicable, avoid crossing traffic lanes...

Finding your way at sea: waypoints

Any sea voyage needs a certain amount of planning before it is undertaken. It makes sense to think about where you are going, how you will get there and what factors might influence your plan. Planning the route itself is also critical. One of the essential parts of modern navigation is the use of waypoints.

Cleaning & polishing painted 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.

An explanation of the IALA maritime buoyage systems – IALA A and IALA B

What are the differences between the two IALA buoyage systems, IALA Region A and IALA Region B, and where are they used?   As recently as the 1970s...

Boat Security: Protecting your vessel from theft & vandalism

Boats are susceptible to theft and vandalism. Protecting your vessel from these risks requires a proactive approach to boat security. In this comprehensive guide, we’ll explore the various threats to boat security and provide practical strategies and technologies to safeguard your investment and enjoy peace of mind on the water.

Preparing for sailboat cruising

Preparing for a sailing trip entails a lot of planning. In this blog, we take a look at some of the many safety aspects that a skipper needs to consider before heading off on a cruise.

Boat Improvements

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

How to operate a winch

Winches are drum shaped mechanical devices used to handle halyards, sheets and control lines. One of the important crew...

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, ...

DIY boat upgrades: Budget-friendly projects to enhance your (older!) boat

DIY projects that can enhance both the functionality and aesthetics of your boat. In this article, we’ll explore a variety of DIY boat upgrades that won’t break the bank but will significantly enhance your boating experience.

Understanding your boat’s compass

Article submitted by Mike Rossiter, Certificated Compass Adjuster. Since the magnetic compass was first used by the Chinese...

How to improve a yacht’s upwind performance

There are several ways to improve the upwind performance of a sailing yacht. Read on for some useful tips including headsail reefing, heavy weather jibs and motor sailing.

Steel and Aluminium Hulls

The two metals used for hull construction are steel and aluminium. These are both very strong materials and will last a long time as long as they are cared for, which primarily means protecting steel boats from rust and aluminium boats from electrolytic action.

Safety at sea principles

Safety at sea is not as simple as just spending money adding shiny new emergency equipment such as liferafts, danbuoys, distress flares, EPIRBs and so on.

Sterndrive maintenance

Sterndrives are a popular form of propulsion in the powerboat market, but require a fair amount of care and maintenance. The main factors to be aware of are salt water corrosion, lubrication and regular inspection of the bellows, the condition of which is vital to prevent water from entering into the hull.

Repairing chips and dings in gelcoat

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.

Crew Overboard Drill – Updated

How to respond to crew overboard A Man Overboard (MOB) drill is a crucial safety procedure that every boater should practice regularly. It simulates...

Antifouling for leisure boats – Part 1

Boats that are kept afloat can very quickly become a home for small marine organisms such as barnacles, weed and slime. Applying an antifouling paint to your hull is necessary to protect it from these micro-organisms, as a fouled hull can cause problems and will slow down a boat’s maximum speed considerably if left unchecked.

Estimating your position at sea

Navigators use a combination of techniques to estimate their position at sea. The primary method used today is GPS (Global Positioning System), a network of 24 satellites that became fully operational in the 1990s and was originally restricted for use by the United States military.

Learning about diesel engine maintenance

Marine diesel engines are internal combustion engines that are designed specifically for use in maritime applications. These engines are commonly used in a variety of watercraft, ranging from small boats and yachts to large ships and vessels. Learn about marine diesel engine maintenance courses.

Cutless bearing replacement

Cutless bearings can last for many years but if the propeller shaft is out of alignment they will wear through more quickly. If you have noticed a clunking sound when motoring then it could be a worn cutless bearing that is causing the problem.