Operational Guide · Registers & Logbooks

# Navigation in Restricted Visibility

COLREG Rule 19 applies regardless of what the radar shows: why instrumental detection of another ship alone is not enough to decide the correct manoeuvre.

restricted visibility COLREG Rule 19 radar fog

## Operational Explanation

Navigation in conditions of restricted visibility (fog, heavy rain, snow, darkness combined with adverse weather) requires an approach distinct from navigation in normal visibility: COLREG Rule 19 applies specifically to these situations, with conduct criteria different from those set out for vessels in sight of one another.

A frequent conceptual error is to assume that radar detection of another ship allows the same conduct rules for vessels in sight of one another (Rules 11-18) to be applied: Rule 19 instead requires a specific approach, one that accounts for the inherent uncertainty of instrumental detection alone.

## Regulatory Reference

COLREG Rule 19 (Conduct of vessels in restricted visibility): every ship must proceed at a safe speed adapted to the circumstances and conditions of restricted visibility, with engines ready for immediate manoeuvre, and must avoid, where possible, an alteration of course towards a vessel detected only by radar forward of the beam, unless it is an overtaking situation.

## Scope of Application

Every ship navigating in conditions of restricted visibility, regardless of whether other ships are detected by radar.

## Procedure / How to Complete It

1. Reduce speed to a safe value appropriate to the visibility conditions, expected traffic and the ship's manoeuvring characteristics.
2. Have engines ready for immediate manoeuvre, informing the engine room of entry into an area of restricted visibility.
3. Intensify the use of radar and every other available means of detection, maintaining systematic plotting of contacts.
4. Apply the fog signals required by COLREG according to the type of ship and the operational situation.
5. Avoid, where possible, an alteration of course towards a radar contact forward of the beam, except in an overtaking situation, in accordance with Rule 19(d).
6. Assign additional resources to the bridge team (extra officer, dedicated lookout) during the most critical visibility phases.

## Practical Example

Applied example: a radar contact detected forward of the beam in thick fog; instead of altering course towards the contact (a breach of Rule 19(d)), the ship reduces speed and monitors the evolving CPA, ready to manoeuvre as necessary with engines already on standby.

## Real Cases

Collisions in restricted visibility have repeatedly been traced back, in investigations, to an incorrect application of the rules for vessels in sight of one another to a situation that instead required the application of Rule 19, often due to an overly optimistic interpretation of the reliability of radar detection alone.

## Common Mistakes Mistake Library

Mistake | Consequence | How to avoid it
--- | --- | ---
Applying the rules for vessels in sight of one another (Rules 11-18) to a restricted-visibility situation | Manoeuvre not compliant with Rule 19, risk of collision | Always recognise the condition of restricted visibility and apply Rule 19 specifically
Speed not adequately reduced in conditions of restricted visibility | Insufficient reaction time in the event of a close-quarters contact | Always reduce speed to a safe value proportionate to the real conditions
Alteration of course towards a radar contact forward of the beam | Direct breach of Rule 19(d), a possible cause of collision | Avoid altering course towards the contact, preferring speed reduction and monitoring

## PSC Observations

Conduct in restricted visibility is not typically the subject of direct PSC documentary verification, but it frequently emerges in post-collision investigations as an element in assessing the bridge team's competence.

## Operational Tips

- Do not wait for visibility to be already compromised before reducing speed: anticipate the reduction as soon as conditions begin to deteriorate.
- Always remember that Rule 19 applies regardless of how reliable the radar detection appears.
- Assign additional resources to the bridge team during the most critical visibility phases, do not face them with minimum manning.

## Checklist

- Speed reduced to an appropriate safe value
- Engines ready for immediate manoeuvre
- Systematic radar plotting of all contacts maintained
- Fog signals correctly applied
- No alteration of course towards a contact forward of the beam except in overtaking

## FAQ

Do the rules for vessels in sight of one another apply if another ship is visible only on radar?
No: in conditions of restricted visibility, COLREG Rule 19 applies specifically, with conduct criteria distinct from those for vessels in sight of one another (Rules 11-18).
What does Rule 19(d) provide regarding radar contacts?
It provides that, where possible, an alteration of course should be avoided towards a vessel detected only by radar forward of the beam, unless the situation is one of overtaking, precisely because of the inherent uncertainty of radar data alone.
Who decides when a condition is 'restricted visibility' for COLREG purposes?
It is an operational judgement of the bridge team based on real conditions (fog, heavy rain, snow, darkness with adverse weather) that reduce the ability to sight visually, not a fixed numerical value set by the rule.
🎬 Additional photos, videos and interactive diagrams for this topic will be available in a future version of the platform.

## Related Topics

COLREG in Practice Watchkeeping

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