Module 5 - COLREGs

COLREG Rule 7: assessing risk of collision

What you will learn

Determine collision risk from repeated visual and radar evidence, express uncertainty, and resist false reassurance from one bearing, CPA or AIS target.

Collision risk is a developing relationship. Build a trend with all appropriate means, and whenever doubt remains, work on the basis that risk exists.

Rule 7 requires all available means appropriate to the circumstances 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.

A compass bearing that does not appreciably change while range decreases is a strong warning. Risk can still exist despite an appreciable bearing change when a vessel is very large, towing or already close. Record timed bearing and range observations so the team can see the trend rather than remember a reassuring snapshot.

Radar CPA and TCPA help quantify the plot, but the Rules set no universal safe minimum. Passing distance depends on speed, closure, sea room, manoeuvrability, visibility, traffic, size and uncertainty. MCA guidance treats AIS as situational awareness and bases collision decisions primarily on systematic visual and/or radar observation; VHF does not replace a lawful, observable manoeuvre.

Worked example

A ship bears 038°, 039° and 038° at two-minute intervals while range falls from 4.8 to 3.6 to 2.5 miles. AIS predicts 0.8 mile CPA.

  1. 1Treat the near-steady bearing and reducing range as collision risk.
  2. 2Corroborate with systematic radar observation, check own inputs and identify the applicable rules.
  3. 3Take early rule-compliant action and continue the observations until safe passing distance is established.

Sense check: The 0.8-mile AIS CPA is not a clearance certificate: no universal CPA minimum exists and the visual evidence still demands action.

Turn uncertainty into a command decision

The radar plot and visual bearings disagree while the target grows quickly on the starboard bow.

  1. 1. Name the conflict

    Check target association, radar set-up, own-ship inputs and the visual-bearing method.

  2. 2. Deem the risk

    Because doubt and closing evidence remain, plan on the basis that collision risk exists.

  3. 3. Set a review point

    State the intended action and the next observation that must prove it is working.

Sense check: Choosing whichever display gives the largest CPA is selecting reassurance, not resolving uncertainty.

Common mistake or limitation

  • One bearing or one electronic CPA is scanty information; the decision needs a time-sequenced trend.
  • An appreciable bearing change does not always prove clearance, especially with a large vessel, tow or close contact.

Recap

  • Use all appropriate means and deem risk to exist whenever doubt remains.
  • Repeated bearing and range observations reveal a trend; radar must be used systematically.
  • CPA and TCPA have no universal safe threshold, and AIS remains supplementary.

Optional quick check

Section 4 of 12

A target's bearing changes appreciably, but it is a very large vessel already at close range. What is the sound conclusion?

Choose one answer
Sources and factual review

Reviewed 2 September 2026 - Coastal Skipper Revision.

Apply Rule 7 to complete encounters

The COLREGs module links lookout, safe speed, risk assessment, avoiding action and vessel responsibilities.

Practice COLREGs
PreviousRule 5 — Look-outNextRule 8 — Action to Avoid Collision