Section I: Conduct of vessels in any condition of visibility
Risk of collision: how to tell. Rule 7
In practice
- No appreciable change in the compass bearing of an approaching vessel means you assume risk of collision exists. Confirm the assessment with successive bearings, the change of range and systematic radar observation.
- Doubt always resolves in favour of the risk: you doubt, so it exists.
- One glance at the radar screen is not enough (paragraph c): you need plotting or equivalent systematic observation.
Text of Rule 7: Risk of Collision
(a) Every vessel shall use all available means appropriate to the prevailing circumstances and conditions to determine if risk of collision exists. If there is any doubt such risk shall be deemed to exist.
(b) Proper use shall be made of radar equipment if fitted and operational, including long-range scanning to obtain early warning of risk of collision and radar plotting or equivalent systematic observation of detected objects.
(c) Assumptions shall not be made on the basis of scanty information, especially scanty radar information.
(d) In determining if risk of collision exists the following considerations shall be among those taken into account:
- (i) such risk shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change;
- (ii) such risk may sometimes exist even when an appreciable bearing change is evident, particularly when approaching a very large vessel or a tow or when approaching a vessel at close range.
Text of the International Regulations as amended, quoted from the consolidated U.S. Coast Guard Navigation Rules (public domain); spelling follows the IMO original. When in doubt, the Convention decides.
The common mistake
Treating a changing bearing as a guarantee of safety. With a very large ship or a tow, risk may exist despite an appreciable bearing change (paragraph d.ii).