Official rule text as published in the U.S. Coast Guard Navigation Rules
(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 |
|---|
| (b) Notwithstanding Rule 15(a), on the Great Lakes, Western Rivers, or waters 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. |
MyCOLREGS explanation
Summary
When two power-driven vessels are crossing with risk of collision, the one that has the other vessel on her own starboard side is the one who keeps clear — equivalent to seeing the other vessel's red (port) sidelight off your own starboard bow. The default maneuver is to cross astern of the other vessel — only cross ahead if you're confident you can complete the crossing well clear, without forcing the other vessel to alter her own course or speed to stay safe. On the Great Lakes, Western Rivers, and other Secretary-specified waters, there's an additional inland-only rule: a power-driven vessel crossing a river keeps clear of a power-driven vessel that's heading up or down the river along its length.
Real World Example
The starboard/port split lets both boats work out who keeps clear just by which of the other vessel's sidelights they can see, red or green, without needing to radio each other or guess at courses and speeds. Turning to pass astern gives a forgiving margin if you misjudge the other vessel's speed; crossing ahead turns any miscalculation into the exact moment your paths intersect.
Practice Questions
Q1. Two power-driven vessels are crossing with risk of collision. Vessel A has Vessel B on her own starboard side. Under Rule 15(a), who keeps clear?
A. Whichever vessel is moving faster.
B. Vessel B, because she is to port of Vessel A.
C. Vessel A, because she has the other vessel on her own starboard side.
D. Neither - both hold course and speed until one signals.
Q2. The other vessel is on your port side in a crossing situation, making her the give-way vessel. Under Rule 15(a), when would it be acceptable for her to cross ahead of you instead of passing astern?
A. If there is no possibility of a collision and it does not require your boat to slow down or change direction.
B. If reducing her speed would be inconvenient for her voyage.
C. If she sounds a signal to announce her intentions before crossing.
D. If she is a larger or faster vessel with the right of way.
Practice Question Answers
Q1.
C. Vessel A, because she has the other vessel on her own starboard side. Rule 15(a) assigns the give-way duty to the vessel that has the other vessel on her own starboard side. A substitutes speed for position. B identifies the wrong vessel as give-way - it's the vessel with the other on her own starboard side that must keep clear, which is Vessel A here, not B. D has both vessels waiting on a signal that isn't required.
Q2.
A. Crossing ahead of the other vessel, if the circumstances of the case admit avoiding it. Rule 15(a) only permits the give-way vessel to cross ahead when doing so creates no collision risk and doesn't force the stand-on vessel to alter her own course or speed — not because of her size, speed, signaling, or convenience. B substitutes convenience for the actual safety test — Rule 15(a) doesn't care whether slowing down is inconvenient. C substitutes signaling for the real criterion — announcing intentions doesn't make crossing ahead safe. D invents a right-of-way based on size or speed that Rule 15(a) doesn't grant.