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
Pretty straight forward here, you as captain of your vessel shall use all means available to sort out whether you are at risk of collision with another object (notice I do not say vessel, this rule goes for anything you may run across, lighthouse, daymarker, bouy etc.). Notice above section (d) (i) making mention of a bearing to an object not appreciably changing. CBDR or Constant Bearing Decreasing Range was something they taught in the Navy. Basically observe the relative position of the object in relationship with something stationary on your vessel, say the bow light, or your VHF antenna etc (in the Navy we had rangefinder showing relative bearing available to us). Take note of the position of the object, wait some period of time say a minute or so (depending on the proximity of the object) and notice if the object has moved one way or the other compared to your reference. If the object is right of where you first took notice in Navy speak the object is drawing right and you should pass ahead it if everything stays constant (speed course etc), if it is to the left of your reference we termed that drawing left and you should pass in behind it, if the object stays stationary in relationship to your reference you are quite possibly on a collision course and must take action. In Rule #8 we will talk about Action to Avoid Collision. In all situations you should be diligent in your situational awareness (what is going on all around you), if the object is a moving vessel and is drawing right or left pay attention, any speed or course change by either vessel will alter this relationship and would need to be re evaluated.