How Do Ships Avoid Collisions? Rules That Decide Who Gives Way
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Two vessels approaching each other in open water have no lane markings, no traffic lights and no controller. What they have is an international set of rules specifying exactly which one must alter course and which must hold steady, plus a system of lights and shapes allowing each to identify what the other is and what it is doing, in the dark.
The structure of the rules
The international regulations for preventing collisions at sea, agreed in 1972 and universally applied, are organised around identifying a give-way vessel and a stand-on vessel in each situation:
- •Head on, where two power-driven vessels meet directly, both alter course to starboard and pass port to port, which is the only situation where both act
- •Crossing, where the vessel that has the other on its own starboard side gives way, and the other holds its course and speed
- •Overtaking, where the overtaking vessel keeps clear regardless of what type either vessel is, and remains the give-way vessel until well past and clear
- •A hierarchy by manoeuvrability, in which vessels not under command, restricted in their ability to manoeuvre, constrained by draught, engaged in fishing, sailing and finally power-driven give way in that order, so the least able to move has priority
- •The requirement that any action be positive, made in ample time and large enough to be obvious to the other vessel on radar or by eye, since a small gradual alteration is worse than none
- •The overriding obligation of the stand-on vessel to act if the give-way vessel is plainly not doing so, and the general requirement to do whatever is necessary to avoid collision regardless of the rules
How vessels identify each other
The rules only work if each vessel can determine what the other is, which at night or in poor visibility means reading a pattern of lights. Every vessel carries a red light to port, a green light to starboard and a white stern light, with arcs defined so that the combination visible tells an observer the other vessel's aspect: seeing only red means looking at its port side, seeing both red and green means end-on. Power-driven vessels add masthead lights, with two indicating a vessel over a certain length and the relative position of the two showing the heading. Additional configurations of lights and, by day, of shapes indicate specific conditions, so a vessel engaged in trawling, at anchor, aground, not under command or towing displays a defined pattern that identifies it precisely. Sound signals convey manoeuvres in restricted visibility and communicate intent in close quarters, with a specified number of short blasts meaning altering to starboard, to port or operating astern propulsion.
The traffic systems
Where density makes the basic rules insufficient, additional structure is imposed. Traffic separation schemes establish one-way lanes with a separation zone between them in congested straits and approaches, and vessels are required to follow the direction of flow and to cross lanes at right angles if crossing at all, which converts a crossing problem into a clearer one. Vessel traffic services operate like air traffic control in busy ports and waterways, monitoring and advising or directing. Automatic identification systems broadcast each vessel's identity, position, course and speed continuously, which appears on the screens of nearby ships and ashore and has transformed situational awareness, while creating a dependency and a spoofing vulnerability. Radar with automatic plotting calculates the closest point of approach and time to it for each contact, which is the number watchkeepers actually use. Pilotage is compulsory in many ports, placing a local expert on the bridge.
Why collisions still happen
Investigation reports show remarkably consistent causes. Failure to keep a proper lookout is the most frequently cited, with bridges undermanned at night, watchkeepers distracted by paperwork or fatigued, and radar and identification systems watched rather than the water. Over-reliance on automatic identification data, which does not show vessels that are not transmitting and can be wrong, appears repeatedly. Fatigue is systemic in an industry with small crews, long voyages and port turnarounds that consume rest periods. Misunderstanding of the rules is common, particularly the frequent error of a give-way vessel making a small alteration that the other cannot detect, or of two vessels agreeing a manoeuvre by radio that departs from the rules and then misunderstanding each other. Language barriers between crews of different nationalities contribute. The remedies are unglamorous and mostly concern manning levels, rest hours, bridge resource management training and enforcement of lookout requirements.
The takeaway
Collision regulations designate a give-way and a stand-on vessel in each encounter, with head-on meetings turning both to starboard, crossing decided by which vessel is on the other's starboard side, and an overtaking vessel always keeping clear. A hierarchy gives priority to whatever is least able to manoeuvre. Coloured lights with defined arcs let one vessel read another's aspect and activity at night. Most collisions trace to inadequate lookout, fatigue and alterations too small to be noticed.