The Rule:        

(a) When two power-driven vessels are crossing so as to involve risk of collision, the vessel which has the other on her own starboard side shall keep out of the way and shall, if the circumstances of the case admit, avoid crossing ahead of the other vessel.

Inland Only: (b) Notwithstanding Rule 15(a), on the Great Lakes, Western Rivers, or water specified by the Secretary, a power-driven vessel crossing a river shall keep out of the way of a power-driven vessel ascending or descending the river.

Discussion: This rule describes situations in which vessels are neither overtaking or meeting, but that risk of collision exists. Another way to look at this is if two vessels are closing together and each is less than 22.5 degrees abaft the beam to almost all the way dead ahead, a crossing situation applies. This rule is third in the listing of the three main maneuvering rules because it requires the most interpretation of the rules. 

Often, students think of two power driven vessels crossing at night, and they imagine themselves on each vessel – which sidelights would you see at night? If you see the red light, you are the give way vessel, and if you see the green sidelight you are the stand on vessel.  This memory aid is helpful but incomplete and requires an understanding of Part C of the Rules.

Another key point is that the vessels must both be power-driven vessels. If they are not, other rules (namely rule 18) will govern the situation.  Also remember that risk of collision must exist for this rule to be in effect. Finally, note the international only provision regarding power-driven vessels on rivers – similar to Narrow Channels, but more explicit.  

Test Strategy: There are about 20 questions in the database for this Rule specifically, but many other questions use crossing situations as an element of the question. For example, shapes, lights, sounds and other signals are often included in crossing situation questions.