Module 5 - COLREGs
Rule 7 — Risk of Collision
What you will learn
Determine collision risk from repeated, systematic observations while recognising the limits of compass bearing, radar, AIS and a single CPA value.
Collision risk is a developing relationship, not a one-frame judgement. Use all appropriate means, allow enough time for a trend and deem risk to exist whenever doubt remains.
Rule 6 first requires a safe speed that lets the vessel take proper and effective avoiding action and stop within an appropriate distance. Visibility, traffic density, manoeuvrability, background lights, wind, sea, current, hazards and draught all affect that decision; operational radar adds its own detection, scale and performance factors.
Rule 7 then requires all available means appropriate to the conditions to determine whether collision risk exists. If there is any doubt, risk is deemed to exist. Proper use of operational radar includes long-range scanning and systematic plotting or equivalent observation; assumptions must not be based on scanty information, especially scanty radar information.
A compass bearing of an approaching vessel that does not appreciably change is strong evidence of risk when range is decreasing. It is not a complete clearance test: risk can still exist despite appreciable bearing change when the other vessel is very large, towing or already close. Continue bearings or systematic radar observations and assess CPA, TCPA, aspect, rate of closure and uncertainty together.
AIS can improve awareness or identification but MCA guidance says collision-avoidance decisions should be based primarily on systematic visual and/or radar observation. Trying to negotiate a non-COLREG passing plan by VHF can waste time, misidentify the vessel or produce misunderstanding; comply with the Rules and take observable action.
Worked example
A ship's relative compass bearing is 042°, then 043°, then 042° while its range visibly decreases over several minutes.
- 1Record the time, bearing and range or visual size at each observation instead of relying on memory.
- 2Treat the effectively steady bearing and closing range as collision risk; check the radar plot or other systematic observations if available.
- 3Identify the visibility and encounter rules, take the required early action, then continue the observations to prove a safe passing distance.
Sense check: A one-degree wobble can come from reading error, yaw or geometry; it is not evidence that the vessel will pass clear.
Turn observations into a decision
AIS predicts a comfortable CPA, but visual bearings remain nearly steady and the vessel is growing rapidly in the windscreen.
1. Distrust the conflict
Check the target association, own data and radar or visual observations rather than choosing the reassuring display.
2. Deem risk
Because doubt and closing evidence remain, work on the basis that collision risk exists.
3. Apply and verify
Use the applicable rule, act in ample time and continue systematic observations until the passing distance is safe.
Sense check: A digital CPA is only as sound as the target, sensor, inputs and observation history behind it.
Common mistake or limitation
- One bearing or one AIS CPA is scanty information; the decision needs a trend and an understanding of uncertainty.
- An appreciable bearing change can still conceal risk with a large vessel, tow or close-range contact.
Recap
- Safe speed creates the time and distance needed for proper assessment and effective action.
- If doubt about collision risk remains, Rule 7 requires the risk to be deemed to exist.
- Use compass bearings as part of systematic observation, with radar where fitted and AIS only as supplementary information.
Optional quick check
Section 3 of 9
The bearing changes noticeably, but the target is a very large vessel already at close range. What is the sound conclusion?
Apply Rule 7 to complete encounters
The COLREGs module links lookout, safe speed, risk assessment, avoiding action and vessel responsibilities.