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.
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
- Correctly identify the crossing, overtaking or head-on situation according to the Part B rules (Steering and Sailing Rules).
- 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.
- Maintain a proper lookout using all available means (sight, hearing, radar, AIS), not relying on a single instrument.
- Apply the specific rules of conduct in restricted visibility, regardless of the radar detection of the other vessel.
- 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
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
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