Risk of Collision and Action to Avoid Collision

Rule 7 — does risk of collision exist?

In short: Read Look-out and Safe Speed first. Rule 7 tells every vessel to use all available means to decide whether risk of collision exists. If in doubt, risk exists.

• Rule 7(a) — every vessel uses all available means appropriate to the prevailing circumstances and conditions. These are the same words as Rule 5 (look-out): eyes, ears, radar, AIS and the rest. The look-out of Rule 5 exists to make a full appraisal of the situation and of the risk of collision, so Rules 5 and 7 work as a pair.
• If there is any doubt, you treat the risk as real (Rule 7(a)).
• Rule 7(b) — if radar is fitted and working, use it properly: long-range scanning for early warning, and radar plotting or an equivalent systematic observation of detected targets.
• Rule 7(c) — no assumptions on scanty information, especially scanty radar information. One echo position tells you nothing about the target’s course or speed.
• Rule 7(d)(i) — risk of collision is deemed to exist if the compass bearing of an approaching vessel does not appreciably change.
• Rule 7(d)(ii) — risk may still exist when the bearing is changing, particularly when approaching a very large vessel, a tow, or any vessel at close range.

Compass bearings in practice

• Take a series of compass bearings, not one. Use a bearing repeater, an azimuth ring or the radar EBL, always against a compass (not a relative bearing to the bow, which changes every time you yaw).
• Steady bearing + range closing = risk of collision.
• A small change is not enough: with a big ship, a tow or a target at close range, the bow or the tow can still hit you while the bearing of the bridge drifts.
• In the oral, when you describe a vessel from her lights or shapes, the order is: type → length → aspect → special circumstances → underway → making way, and then say: “I take a series of compass bearings to determine if risk of collision exists.”

Exam trap: the examiner shows one plotted position or one bearing and asks what you will do. Answer: scanty information (Rule 7(c)) — more bearings or plots first. If doubt remains, risk of collision exists.

Risk of collision with radar: CPA and TCPA limits

In short: On radar, risk of collision is judged with two numbers: the CPA (closest point of approach) and the TCPA (time to CPA), compared with the limits you set. Small CPA and short TCPA = dangerous.

The decision chain

• From the radar and AIS data, work through: identify the situation → is there a risk? → decide → act → check the action works → decide when to resume.
• Where the data come from:
  • Target tracking (TT) → range, bearing, course and speed. This is the primary method: independent detection, automatic, many targets at once, reliable.
  • AIS → type, position, heading, course, speed, navigational status, destination. It depends on GNSS on the other ship, so it can be wrong.
  • Association of TT and AIS → the best picture, with less duplicate information on screen.
  • Manual plot / systematic observation → slower, fewer targets, less accurate.
• Situations change all the time. Keep identifying: an uninterrupted look-out, information exchange with other ships, and cooperation inside the bridge team.

CPA/TCPA logic

• A plot or the radar’s tracking gives 7 items: CPA, TCPA, BCR (bow crossing range), BCT (bow crossing time), target course, target speed and aspect. AIS adds type, position, heading, course, speed and navigational status.
• The CPA limit is your guide: if you want 1 NM, act on anything that will pass closer than 1 NM.
• Examples: limit 1.5 NM, target CPA 0.9 NM → a small alteration solves it. Limit 1 NM, target CPA 0.1 NM → a serious situation.
• Set a minimum BCR as well, and increase it for large ships, tows and targets at close range (the Rule 7(d)(ii) cases).
• Modern radars flag dangerous targets in red automatically, even targets you have not acquired.
• The numbers are only as good as the person reading them. A careless plot is worse than none, because it gives false confidence: know what you do not know, and ask.

Rule 8 — action to avoid collision

In short: When you must act, the action follows the rules of Part B and is positive, made in ample time and with good seamanship. Make it large, so the other ship sees it, and make sure it ends in a pass at a safe distance.

• Rule 8(a) — any action follows the Rules of Part B and, if the circumstances allow, is positive, made in ample time and with due regard to good seamanship.
• Rule 8(b) — any change of course or speed is large enough to be readily apparent to the other vessel, whether she watches by eye or by radar. Avoid a succession of small alterations: on a radar screen they look like no alteration at all.
• Rule 8(c) — with enough sea room, an alteration of course alone may be the most effective way to avoid a close-quarters situation, provided it is made in good time, is substantial and does not create another close-quarters situation.
• Rule 8(d) — the action must give a pass at a safe distance. Check its effectiveness until the other vessel is finally past and clear.
• Rule 8(e) — if needed to avoid collision or to gain time to assess the situation, slow down or take all way off by stopping or going astern.
• Rule 8 sits in Part B Section I (Rules 4–10), so it applies in any visibility. The steering and sailing rules that tell you who acts are Rules 11–19; every action must comply with them.

Several targets at once

• Real traffic is rarely one ship against one: crossing, overtaking and head-on situations come together, often at night or in restricted visibility.
• In a complex situation you may need a series of actions. Deal with the targets in order of danger, using each one’s CPA and TCPA.
• Coordinated action: when your change and the other ship’s change move the relative motion line the same way, the CPA grows. When they work against each other, the CPA can shrink.
• Before you act, test the idea with the radar’s trial manoeuvre (and a manual plot): it shows the CPA each target would have after your planned course or speed change.

Rule 8(f) — “not to impede”

In short: Some vessels must not impede others (for example in a narrow channel or a TSS). That duty starts early, and it does not end when risk of collision appears. The protected vessel still keeps all her own duties.

• Rule 8(f)(i) — a vessel that must not impede the passage of another takes early action, when the circumstances require it, to leave enough sea room for the other vessel’s safe passage.
• Rule 8(f)(ii) — she is not relieved of that duty if she gets so close that risk of collision develops. When she acts, she must also take full account of the steering and sailing rules. In practice she stays the give-way vessel.
• Rule 8(f)(iii) — the vessel whose passage must not be impeded keeps all her obligations under Part B once risk of collision exists. She is not automatically “stand-on with no duties”.

Checking the action and resuming course

In short: After you act, keep checking until the other ship is finally past and clear. Resume course and speed only when the CPA will stay safe after you do.

• Is the action working? (Rule 8(d)) Three checks:
  1. it complies with the COLREG;
  2. it is visible to the other ship, by eye or on her radar;
  3. the ships will pass at a safe distance.
• Keep plotting: if the bearing is still steady or the CPA is not opening, do more (larger alteration, slow down, stop).
• When to resume: “past and clear” means the CPA stays safe even after you return to your course and speed. Use the trial manoeuvre to see the CPA you would get by resuming now.
• Resuming must not create another close-quarters situation with this ship or any other (Rule 8(c)).
• A safe distance leaves margin for ship interaction, yaw, a sudden sheer, or an engine or rudder failure on either ship.
• The situation can change after you act: the other ship may lose steering or power, or have a fire, and become NUC — top of the Rule 18 order. Keep monitoring. A vessel that runs aground changes her lights to the aground signal.

VHF and collision avoidance — MGN 324

In short: Do not agree manoeuvres by VHF (“let’s pass green to green”). Apply the COLREG. VHF can help in rare cases, but the rules always come first.

• MGN 324 (M+F) Amendment 1 (paras 3.4–3.6): using VHF to agree a passing arrangement carries a real danger of misunderstanding — wrong ship identified, language problems, an agreement that breaks the rules.
• MGN 324 (M+F) Amendment 2 (22 March 2022, current): VHF may sometimes help in collision avoidance, but the COLREG remain uppermost and VHF use must follow the ITU Radio Regulations.
• Study guides often say “never” use VHF to arrange a pass. The MCA notice is less absolute (“fraught with danger”, “sometimes justified”).
• Safe oral answer: “I do not agree manoeuvres on VHF. I apply the COLREG.” Then add that VHF may be used, if at all, only to support a rule-compliant action.

Real situations on the bridge

In short: Exam and real scenarios mix several rules. Identify each vessel correctly, keep well clear of tows and fishing gear, and slow down early when visibility drops.

• Tows: never pass between a tug and her tow. A tow wire can be hundreds of metres long (around 800 m for a large rig tow) and may be hard to see; tows of fish cages are also low and hard to spot. Increase your BCR and CPA limits for tows (Rule 7(d)(ii)).
• Fishing vessel (trawler: green over white) crossing and showing you her starboard side, nets astern:
  • “Engaged in fishing” (Rule 3(d)) = fishing with nets, lines, trawls or other gear that restrict manoeuvrability. It excludes trolling lines only — do not confuse trawling with trolling.
  • A trawler engaged in fishing ranks above a power-driven vessel (Rule 18): you keep out of her way.
  • Options: take a series of compass bearings; do not cross close astern (nets of unknown length); make a round turn; alter to starboard, run parallel and pass astern at a good distance (or ahead only if you are much faster and well clear); or take all way off and let her pass.
• Overtaking (Rule 13): you are overtaking when coming up from more than 22.5° abaft the beam of the other vessel. Any vessel can be the overtaking one, even a sailing vessel overtaking a power-driven vessel. In doubt, assume you are overtaking (Rule 13(c)).
• Restricted visibility: first reduce speed, inform the Master, post an extra look-out. Example: container ships that run at 22 kn slow to 6–7 kn in northern European fog banks. Arriving late is better than not arriving: stand your ground with owners and charterers.
• Busy yachting areas in summer (Monaco, St-Tropez, Capri, Sardinia) mean many inexperienced skippers: expect actions that break the rules.
• Oral exam: scenarios start at the critical moment, with no build-up. Many answers are grey: explain your reasoning with the rule numbers. A well-argued answer is hard for the examiner to dismiss.