Dealing with restricted visibility - What rule when?

31 May 2024 The Navigator

Twenty years ago, The Nautical Institute published a survey that showed that there is often confusion over the sections of the Collision Regulations (Colregs) that apply in restricted visibility. The answer may seem straightforward enough – Rules 4-10, which always apply, plus Rule 19. However, the introduction of modern technology and autonomous vessels has added further layers to the question. The Nautical Institute has recently worked with the UK MCA to update their guidance. Here are some of the key points

Restricted visibility’ is broadly defined as when what you can see by eye is obstructed by any meteorological condition. It does not mean simply that it is dark – it means that visibility is restricted in some other way. Fog is perhaps the most obvious condition, but it is not the only one.

Causes of restricted visibility include:

  • Fog
  • Mist
  • Falling snow
  • Sandstorms
  • ‘Similar causes’ (e.g. smoke)

Applying the rules

Rule 19 applies when a vessel is in or near an area of restricted visibility. If you are aware of a fog bank up ahead but have not yet entered it – the rule still applies! Rules 1-10, which include requirements for safe speed, lookouts, etc. apply at all times.

The Colregs themselves do not state at what distance visibility should be considered to be restricted. This may vary from vessel to vessel. Your Safety Management System (SMS) should provide guidance on this. If there is nothing in the SMS, and you believe that visibility is restricted, then you should behave as if it is. You can also find more advice on how to judge visibility below.

Eyes up

It can be easy to assume that with modern aids to navigation, including radar and AIS, navigating in restricted visibility should not be a problem – but it still requires great caution. Yachts and other small vessels may not show up on radar and may not have AIS, so it is important to keep a good lookout by all available means.


The Rules require the vessel be at a ‘safe speed’ but offer little specific guidance.
There is no obligation to slow down, unless you hear a fog signal forward of your beam – in which case you should reduce speed to the minimum. The SMS should provide guidance on suitable speed, but if in doubt, remember that the slower you are going, the more time you will have to react.

Lights and sounds

You should always show navigation lights in restricted visibility, no matter what time of day. Likewise, you should always sound the appropriate sound signals when navigating in or near an area of restricted visibility. In this situation, only fog signals can be sounded.

In restricted visibility, fishing vessels and those that are restricted in their ability to manoeuvre sound the same signal (one prolonged, two short blasts) whether underway or at anchor.


It sounds like a trick question: why might you hear a fog signal in completely clear conditions? An uncrewed vessel with a remote operator where all cameras have failed will sound a fog signal to let other vessels know that it is, effectively, in restricted visibility – it can’t see you, even if you can see it, and will be operating according to Rule 19. This might sound unlikely, but as remotely operated vessels become more common, it is not impossible!

Determining risk

Determine if there is a risk of collision by carrying out systematic plotting and observations of detected targets. Relative motion trails on a radar provide a basic visual indication of a potential collision risk. However, this visual indication does not remove the requirement for the systematic plotting of targets.

On an ARPA display, a risk of collision with a tracked target exists if the relative vector of the target points at own ship’s position on the screen. Observing the compass bearing of a target is one means of determining whether risk of collision exists. If visibility allows, this should be used to supplement the systematic plotting of targets.

In restricted visibility, when use of visual compass bearing assessment is not possible, another method to observe the compass bearing of a target is to use a compass stabilised radar to take bearings of the observed target. An electronic bearing line (EBL) fixed to own ship is a convenient way of observing changes to the compass bearing of a target.

Even if the compass bearing does appreciably change, there may still be a risk of collision when approaching large targets, a tow or targets at close range.

You may already have CPA and TCPA alarms to alert you to the risk of collision, or a potential close-quarters situation with tracked targets. Be aware whether these alerts originate from radar or AIS and the differences between each system, especially if AIS data is being overlaid onto a radar image.

Take action!

You have determined that a risk of collision is developing – what now?
In restricted visibility there is no ‘standon’ or ‘give-way’ vessel. Neither vessel has ‘right of way’ and both vessels are expected to manoeuvre to avoid risk of collision. Take action early and make it obvious – expect the other vessel to act too. A substantial alteration of course will be more readily apparent than a change of speed.

Even in restricted visibility, with both vessels obliged to take avoiding action, vessels shall ‘not impede’ vessels which are restricted to navigating in a narrow channel or following a traffic seperation scheme. It is important to take early action and leave sufficient space.

Fishing vessels may look small, but may have trailing nets; bear this in mind when considering what action to take. Similarly, tugs may have another vessel in tow.

Where to turn?

If the target posing the risk of collision or a close-quarters situation is forward of your beam, try to avoid altering to port for that vessel unless you are overtaking it. If the target posing the risk is abeam or abaft of your beam, try to avoid altering course in a direction that would take you towards that vessel.

This article is based on the UK MCA’s Marine Guidance Note 369 (M&F). It is worth reading the whole note.