Operational Guide · Registers & Logbooks

# COLREG in Practice

The rules for preventing collisions have not changed substantively since 1972: yet Rule 10, on traffic separation schemes, remains the single most misunderstood rule of all.

COLREG collision regulations TSS navigation

## Operational Explanation

COLREG (the Convention on the International Regulations for Preventing Collisions at Sea, 1972) sets out the rules of conduct for avoiding collisions at sea. The last substantive amendments date back to Resolution A.1085(28), in force since 1 January 2016; since then IMO has chosen not to amend the text further, focusing instead on developing a regulatory framework for autonomous ships (the MASS Code), whose non-mandatory adoption is expected in May 2026.

Despite the stability of the text, the most recent studies confirm that Rule 10 (traffic separation schemes, TSS) remains the most misunderstood in practice, both by officers of the watch and by Masters, with a direct impact on safety in the most congested traffic areas.

## Regulatory Reference

COLREG 1972, as amended (last substantive amendment Res. A.1085(28), in force since 1 January 2016): Rule 10 (traffic separation schemes), Rule 5 (lookout), Rule 19 (conduct in restricted visibility), among others.

## Scope of Application

Every ship under way, in international waters and, with appropriate local adaptations, in many territorial waters that adopt equivalent rules.

## Procedure / How to Complete It

1. Correctly identify the crossing, overtaking or head-on situation according to the Part B rules (Steering and Sailing Rules).
2. In the presence of a traffic separation scheme (TSS), proceed in the general direction of traffic flow, avoiding crossing it unless at as nearly as practicable a right angle to the general direction.
3. Maintain a proper lookout using all available means (sight, hearing, radar, AIS), not relying on a single instrument.
4. Apply the specific rules of conduct in restricted visibility, regardless of the radar detection of the other vessel.
5. Communicate via VHF when useful to clarify mutual intentions, without substituting verbal communication for the application of the COLREG rules.

## Practical Example

Applied example: a ship that must cross a TSS on a course not parallel to the general direction of traffic must do so at as nearly as practicable a right angle to the general direction, as quickly as possible, in accordance with Rule 10(c).

## Real Cases

Recent studies on the understanding of Rule 10 have found a significant knowledge gap among officers of the watch, Masters and other certified personnel, with direct implications for safety in the busiest traffic areas: violation of the COLREG rules remains the most significant factor in the collisions analysed, with the human factor implicated in 81.1% of maritime casualties in the 2014-2021 period according to the available analyses.

## Common Mistakes Mistake Library

Mistake | Consequence | How to avoid it
--- | --- | ---
Crossing a TSS at an angle not close to 90° relative to the general direction of traffic | Violation of Rule 10, risk of collision with traffic in the lane | Always cross as close as possible to a right angle relative to the general direction
Exclusive reliance on radar/AIS for lookout, neglecting direct visual observation | Failure to detect obstacles not fitted with AIS or with a weak radar echo | Always maintain a lookout using all available means, not a single system
VHF communication used as a substitute for applying the COLREG rules rather than as a support | Uncertainty over mutual manoeuvres, especially if the communication is ambiguous or misunderstood | Always apply the COLREG rules as the primary reference, using VHF only as supplementary clarification

## PSC Observations

Compliance with COLREG is not typically the subject of direct PSC documentary verification, but officers' understanding of the rules may emerge during interviews, especially following a navigational incident.

## Operational Tips

- Periodically review Rule 10 with the bridge team: it is the most misunderstood rule according to the available studies.
- Never substitute VHF communication alone for the application of the COLREG rules.
- Always maintain a lookout using all available means, combining visual observation, radar and AIS.

## Checklist

- Crossing/overtaking/head-on situation correctly identified
- TSS crossing carried out at an angle close to 90°
- Lookout maintained using all available means
- Restricted-visibility rules applied when relevant
- VHF communication used as a support, not a substitute for the rules

## FAQ

Has COLREG been amended recently?
No: the last substantive amendments date back to Resolution A.1085(28), in force since 1 January 2016. IMO considered it premature to amend the text further, focusing instead on developing the MASS Code for autonomous ships.
Why is Rule 10 considered the most misunderstood?
The most recent studies find a significant knowledge gap among officers of the watch, Masters and other certified personnel regarding the practical application of the crossing and navigation rules in traffic separation schemes.
What does the MASS Code under development provide for, and when will it enter into force?
It is a dedicated regulatory framework for autonomous ships; the roadmap envisages adoption of a non-mandatory version in May 2026, development of a framework for an experience-building phase by December 2026, and the start of development of a mandatory code from 2028.
🎬 Additional photos, videos and interactive diagrams for this topic will be available in a future version of the platform.

## Related Topics

Watchkeeping Navigation in Restricted Visibility

### Leave feedback

Help us improve the platform during the beta -- it takes thirty seconds.

How useful do you find the platform so far?
Comment (optional)
Sign me up for the newsletter
Role