Ships and submarines share a crowded ocean with cargo vessels, fishing boats, ferries, research ships, and naval traffic. Collision avoidance matters because large vessels need long distances to turn or stop, and underwater vehicles can be hard to detect. The International Regulations for Preventing Collisions at Sea, called COLREGs, give mariners a common set of rules for right of way, lights, sounds, and safe speed.
These rules reduce uncertainty so each vessel can predict what the other should do.
Key Facts
- COLREGs are the international navigation rules that help vessels avoid collisions at sea.
- Give-way vessel: the vessel required to take early and substantial action to keep clear.
- Stand-on vessel: the vessel that should maintain course and speed unless collision risk remains.
- Collision risk exists when the bearing to another vessel stays nearly constant while range decreases.
- Relative speed for vessels on crossing paths depends on their velocity vectors: v_rel = |v1 - v2|.
- Closest point of approach, CPA, is the smallest predicted separation distance if both vessels keep current course and speed.
Vocabulary
- COLREGs
- COLREGs are the international rules that tell vessels how to navigate safely and avoid collisions.
- Give-way vessel
- A give-way vessel is the vessel that must change course, speed, or both to avoid a dangerous approach.
- Stand-on vessel
- A stand-on vessel is the vessel that normally keeps its course and speed so the other vessel can maneuver predictably.
- Closest point of approach
- Closest point of approach is the predicted minimum distance between two vessels if they continue their current motion.
- Bearing
- Bearing is the direction from one vessel to another, usually measured as an angle clockwise from north or from the vessel's heading.
Common Mistakes to Avoid
- Assuming the stand-on vessel has absolute right of way is wrong because it may still need to act if the give-way vessel does not avoid collision.
- Waiting until vessels are very close is wrong because ships have large momentum and may need minutes or kilometers to turn or stop safely.
- Watching only distance and ignoring bearing is wrong because a constant bearing with decreasing range is a classic sign of collision risk.
- Making a small, unclear course change is wrong because COLREGs favor early and substantial maneuvers that the other vessel can easily detect.
Practice Questions
- 1 Two ships are 6.0 km apart and moving directly toward the same crossing point. Ship A is 3.0 km from the point and travels at 12 km/h. Ship B is 3.0 km from the point and travels at 18 km/h. How long does each ship take to reach the crossing point, and is there an immediate collision risk if neither changes course?
- 2 A vessel observes another ship at a constant bearing while the range decreases from 4.0 km to 2.5 km in 5.0 minutes. What is the average closing speed in km/h, and what does the constant bearing suggest?
- 3 In a crossing situation, one vessel is labeled give-way and the other stand-on. Explain why the stand-on vessel should usually maintain course and speed, and describe when it should take action anyway.