The main situations of collision risk are overtaking, meeting head-on,
and crossing. When one of two vessels is to keep out of the way
(give-way vessel), the other, the stand-on vessel, must maintain
course and speed. The stand-on vessel must take avoiding action
when it becomes apparent that the vessel required to give way is not
taking appropriate action.
The Crossing Rule
Both International and Inland Rules state that when two power-driven vessels are crossing so as to involve risk of collision, the vessel which has the other on her starboard side (the give-way vessel) must keep
out of the way.
As the give-way vessel it is your duty to avoid a collision. Typically, this means you must alter speed or direction to cross behind the other
vessel (the stand-on vessel).
At night, if you see a red light crossing right-to-left in front of you,
you need to change your course. If you see a green light crossing
from left-to-right, you are the stand-on vessel, and should maintain
course and speed.
The Meeting Situation
At times there may be some doubt whether the situation is a crossing
or a head-on meeting.
In case of doubt, you should assume that it is a meeting situation, in which neither vessel has a clear-cut "right-of-way,"
and each must act to avoid the other.
Each vessel in a meeting situation
must alter course to starboard so that each will pass on the port side of
the other. At night, you will recognize a head-on meeting situation if you
see both red and green side lights at the same time.