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.
Understanding Ships and Submarines: Collision Avoidance
A navigator does not wait until another vessel looks close. They track its direction from their own ship over several observations. If the direction remains nearly unchanged, the two paths may meet even when the other vessel still appears small.
Radar can measure distance and direction in poor visibility. Electronic charts combine this with water depth, shorelines, traffic lanes, and restricted areas. A computer can predict the closest point of approach and the time until that point.
These predictions are useful warnings, not guarantees. They assume that every vessel continues at its present speed and course. Wind, waves, currents, steering delays, and a change made by another crew can quickly alter the result.
The action chosen depends on how the vessels are meeting. In a crossing situation, a vessel that sees the other on its starboard side normally keeps clear. A clear alteration of course is often easier for the other crew to notice than a small speed change.
In an overtaking situation, the vessel coming from behind must stay clear because it has the better view of the situation. When two power driven vessels approach nearly head on, both usually turn to starboard. These actions work because they create a predictable pattern.
A crew should avoid a series of tiny corrections. Small changes can be mistaken for drifting, while late actions leave little room for error. Keeping a proper lookout remains essential even when navigation equipment is working.
At night, coloured navigation lights show a vessel's direction of travel. Seeing a red sidelight means the observer is looking toward the other vessel's port side. Seeing a green sidelight means its starboard side is visible.
Masthead lights, stern lights, shapes in daylight, and sound signals give further clues about a vessel's type or activity. Fishing gear, towing lines, shallow water, and limited manoeuvrability can make normal avoidance harder. A large tanker may need several kilometres to stop.
A fast ferry can cover a surprising distance while its crew assesses a target. Safe speed therefore depends on visibility, traffic density, sea room, weather, and the ability to detect hazards.
Submarines add a different problem because a submerged boat may not be visible on radar or by eye. Passive sonar listens for propellers, machinery, and water flow. It can reveal the direction of a sound, yet estimating distance may be difficult because sound bends and reflects in layers of water.
Active sonar sends out a pulse and listens for an echo, which can give range, but it reveals that the submarine is transmitting. Near the surface, submarines may use a periscope, radar, visual checks, and carefully planned routes. Students should practise drawing velocity arrows to understand relative motion.
They should note that a calculated closest approach is only a model. Good collision avoidance combines physics, clear rules, observation, communication, and early judgement.
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.