The following methods are to be used while determining if ROC exists:
1) Take compass bearings: This is one of the most important means of determining risk of collision.
2) Use Radar: In restricted visibility, the primary tracking instrument is radar, if fitted and operational. Radar should also be used to track a vessel in good visibility in open areas after the vessel has been sighted visually.
3) Use Navigational Equipments: All available means includes the use of the above techniques plus the use of VHF, visual look-out etc.
4) When determining risk of collision a number of factors are involved:
i) Closest distance of approach
ii) Type of waterway
iii) Vessel size and manoeuvrability
iv) Distance out from closest point of approach
5) COLREG Rule 7(d) states the following for determining of risk of collision exists or not:
(i) such risk shall be deemed to exist it 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.
Note: Collision avoidance must be carried out in strict compliance with the COLREG. There is no provision in the COLREG for use of AIS information, therefore, decisions should be taken based primarily on systematic visual and/or radar observations. The availability and display of AIS data similar to one produced by systematic radar target tracking (e.g. automatic radar plotting or tracking aid (ARPA, ATA)) should not be given priority over the latter.
Ref: The seamanship Center website
