Seaman's Handbook

PART B. Steering and Sailing Rules : 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 as 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 observations 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.
  1. Such risk shall be deemed to exist it the compass bearing of an approaching vessel
    does not appreciably change;
  2. 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.