How the COLREGs apply in fog and poor visibility: safe speed, sound signals, radar use and the special conduct rules that keep vessels safe when they cannot see.
Fog changes everything on the water. The comforting habit of watching other vessels and reading their intentions disappears, and you are left navigating a world you cannot see. The COLREGs recognise this and set out special rules for conduct in restricted visibility, most importantly Rule 19, which governs how vessels that are not in sight of one another must behave. Understanding these rules is a matter of safety and a regular feature of European boating exams, where fog scenarios test whether you know how the ordinary give-way rules change when you cannot see.
This guide covers what “restricted visibility” means, why the normal stand-on and give-way roles do not apply in the usual way, how to act on radar information, and the sound signals and speed adjustments that keep everyone safe when the horizon vanishes.
Restricted visibility means any condition in which visibility is reduced by fog, mist, falling snow, heavy rainstorms, sandstorms or similar causes. The key legal point is not the weather itself but whether vessels can see one another. The special conduct rules apply to vessels that are not in sight of each other and are navigating in or near an area of restricted visibility.
This “in sight or not” distinction matters enormously. The ordinary crossing, overtaking and head-on rules, with their stand-on and give-way roles, apply only when vessels are in sight of one another. The moment you lose sight of another vessel in fog, those rules step aside and the restricted-visibility rules take over.
In clear conditions, the give-way and stand-on system works because both skippers can see each other and predict each other’s behaviour. In fog you often know a vessel is near only from radar or sound, not from a clear visual picture, so you cannot rely on the other vessel behaving as a stand-on or give-way vessel would. There is no stand-on vessel in restricted visibility. Instead, every vessel must navigate with caution and be ready to take avoiding action.
Because nobody has the right to simply hold course and speed, both vessels share responsibility. This is why fog demands a more conservative, hands-on approach than open, clear water. You cannot assume the other vessel will keep clear for you, so you must be prepared to keep clear yourself.
If you have radar and detect another vessel, Rule 19 gives specific guidance on what to do, and it is worth learning because it is frequently examined. The rule tells you to determine whether a close-quarters situation is developing or a risk of collision exists, and if so to take avoiding action in good time.
The rule also identifies two alterations of course to avoid where possible when acting solely on radar information:
The safer choices are generally to alter course to starboard for a vessel ahead and to slow down. This guidance exists because in fog you cannot see which way the other vessel is turning, so you choose the manoeuvre least likely to cause a conflict.
Two practical duties dominate fog navigation: signalling your presence and slowing down. Sound signals in restricted visibility announce what type of vessel you are and are sounded at intervals of not more than two minutes.
On speed, Rule 19 requires a vessel to proceed at a safe speed adapted to the prevailing conditions, which in fog means slower, so you can stop within the distance you can assess. There is also a specific instruction: if you hear the fog signal of another vessel apparently forward of your beam, and you cannot avoid a close-quarters situation, you must reduce speed to the minimum at which you can keep on course, and if necessary take all way off, that is, stop, and in any case navigate with extreme caution until the danger is over.
Putting the rules together, a sound approach to entering fog looks like this: reduce to a safe speed, start sounding the correct signal at the required interval, post an alert lookout using sight, hearing and radar if fitted, and stand ready to stop. Keep plotting any radar contacts to judge whether a risk of collision is building, and make any avoiding action early, generous and, where possible, a turn to starboard or a reduction in speed rather than a turn to port. Above all, never assume another vessel will give way for you, because in fog no one is the stand-on vessel.
Fog rules reward calm, methodical practice, and they come up often on European exams. In the boatingexamine course you can drill restricted-visibility questions in exam format, including the radar and sound-signal details, so you can respond correctly when visibility closes in.
Do the give-way and stand-on rules apply in fog? Not in the usual way. The crossing, overtaking and head-on rules apply only when vessels are in sight of one another. In restricted visibility, where vessels cannot see each other, there is no stand-on vessel; every vessel must proceed with caution and be ready to take avoiding action.
Which way should I turn if radar shows a vessel ahead in fog? Where possible, avoid turning to port for a vessel forward of your beam, and avoid turning toward a vessel that is abeam or behind you. Generally the safer choices are to alter course to starboard for a vessel ahead and to reduce speed. If you cannot avoid a close-quarters situation, slow to minimum steerage or stop.
How often do I sound fog signals? Signals in restricted visibility are sounded at intervals of not more than two minutes when underway. A vessel at anchor sounds its bell signal at intervals of not more than one minute. Consistent signalling lets nearby vessels track your position and status through the fog.
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