IMO COLREG Rule 7

Rule 7 requires every vessel to use all available means appropriate to the prevailing circumstances to determine whether risk of collision exists, and states that if there is any doubt, the risk shall be deemed to exist. It mandates proper use of operational radar, including long-range scanning and radar plotting or equivalent systematic observation. A steady compass bearing with decreasing range is the classic indicator of risk, but Rule 7(d)(ii) warns that risk may exist even with an appreciable bearing change when approaching a very large vessel, a tow, or a close-range target.