When two vessels approach each other at sea, the risk of a collision depends on how their distance and relative bearing change over time. A collision could occur when the distance decreases and bearing between two vessels does not change, indicating that they are on a converging path.
Understanding this principle is essential for watchkeepers and bridge teams to interpret early warning signs and take timely action. The sections below explore how bearing behavior signals risk, how to assess collision likelihood, and how structured procedures reduce incidents.
| Bearing Trend | Distance Trend | Collision Risk | Recommended Action |
|---|---|---|---|
| Stable or changing very little | Decreasing | High | Take early and substantial avoiding action |
| Increasing | Increasing | Low | Maintain course and speed, monitor situation |
| Changing rapidly | Decreasing | Moderate to low | Continue to monitor, reassess if trend reverses |
| Unclear or oscillating | Decreasing | Moderate | Clarify relative motion, prepare maneuvering plan |
Relative Bearing Change as a Collision Indicator
Relative bearing change describes how the angle between the observer’s heading and the target vessel appears from the own ship. If the bearing remains steady while distance decreases, the vessels are likely on a collision course, and the risk escalates quickly.
Bridge teams should track both distance and bearing using radar, AIS, and visual confirmation. A collision could occur when the distance decreases and bearing between two vessels does not change, so constant monitoring is essential to detect any slow drift in bearing that may indicate a developing situation.
Rules of the Road and Early Warning Signs
Navigation rules emphasize early and decisive action to avoid close-quarters situations. A key early warning sign is a combination of decreasing range and an unchanging bearing, which often appears before visual acquisition is possible.
Watchkeepers should treat a steady bearing with reducing distance as a high-priority indicator. Understanding how a collision could occur when the distance decreases and bearing between two vessels does what allows the bridge team to interpret signals correctly and initiate appropriate maneuvering well in advance.
Assessment Techniques and Decision Making
Effective assessment combines quantitative data with qualitative judgment. Radar plotting, CPA and TCPA calculations, and trend analysis help determine whether a risk is evolving or resolving.
- Monitor radar and AIS plots for at least five minutes to confirm trend consistency.
- Use visual bearings in clear visibility to cross-check electronic information.
- Document changes in bearing and distance at regular intervals to build situational awareness.
- Escalate to the master or officer in charge when risk remains high despite corrective actions.
Operational Procedures for Safe Passage
Procedural discipline reduces ambiguity in decision making. Standard bridge team communication, clear articulation of intentions, and confirmation of actions support safe outcomes under pressure.
Training scenarios that simulate a collision could occur when the distance decreases and bearing between two vessels does not change help crews recognize the pattern, practice timely responses, and refine coordination between lookout, helmsman, and officer of the watch.
Technology Integration and Limitations
Modern radar, AIS, and ECDIS tools enhance detection and tracking capabilities. However, system latency, display settings, and human interpretation can introduce delays or misinterpretations.
Regular calibration, proper setup of safety alarms, and cross-verification between systems ensure that trends in bearing and distance remain reliable inputs for collision avoidance. Do not rely on technology alone; integrate it with traditional watchkeeping practices.
FAQ
Reader questions
What does it mean if the bearing stays the same while the distance to another vessel is decreasing?
The vessels are likely on a converging course, and a collision could occur if no corrective action is taken promptly.
How soon should I act when I notice decreasing distance and an unchanging bearing?
Initiate early and substantial avoiding action as soon as the trend is confirmed, rather than waiting for last-minute maneuvers.
Can rapidly changing bearing eliminate collision risk even if distance is decreasing?
Rapidly changing bearing generally indicates that the vessels are not on a direct collision course, but risk should still be assessed using CPA and TCPA values.
What tools are most reliable for confirming that distance is decreasing and bearing is stable?
Radar with target tracking, AIS track display, and verified visual bearings provide the most reliable confirmation of this situation.