When MUST the stand-on vessel act?

When collision cannot be avoided by the give-way vessel alone

As soon as she sees the other vessel

Acting under 17(a)(ii) in a power-driven crossing, what should she avoid if possible?

Altering course to port for a vessel on her own port side

Rule 17: Action by Stand-on Vessel (COLREGS)

Last reviewed October 21, 2026

The short answer

Rule 17 says the stand-on vessel keeps her course and speed. She may act by her own maneuver once it's clear the give-way vessel isn't taking appropriate action, and she must take the best action to avoid collision when the give-way vessel alone can no longer prevent it.

The rule text

Where one of two vessels is to keep out of the way the other shall keep her course and speed.
International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 17

she shall take such action as will best aid to avoid collision.
International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 17

What it means on the water

Stage 1: keep course and speed (17(a)(i)).
Stage 2: she may act alone once it becomes apparent the give-way vessel isn't acting (17(a)(ii)).
Stage 3: she shall act when collision can't be avoided by the give-way vessel alone (17(b)).
In a crossing between power-driven vessels, if acting under 17(a)(ii), avoid altering to port for a vessel on her own port side, if circumstances admit (17(c)).
The give-way vessel is never relieved of her duty (17(d)).

International vs Inland

Same in both International and Inland.

Common exam traps

Mixing up "may" (17(a)(ii)) and "shall" (17(b)).
Turning to port for a crossing vessel on your port side (17(c)).

Try 3 questions on Rule 17

What does the stand-on vessel normally do?
1. Alter to starboard
2. Keep course and speed
3. Stop
4. Sound three short blasts
Show answer
B. Keep course and speed Rule 17(a)(i).
When MUST the stand-on vessel act?
1. As soon as she sees the other vessel
2. When collision cannot be avoided by the give-way vessel alone
3. Never
4. Only in fog
Show answer
B. When collision cannot be avoided by the give-way vessel alone Rule 17(b).
Acting under 17(a)(ii) in a power-driven crossing, what should she avoid if possible?
1. Altering to starboard
2. Altering course to port for a vessel on her own port side
3. Slowing down
4. Sounding signals
Show answer
B. Altering course to port for a vessel on her own port side Rule 17(c).

Frequently asked questions

Can the stand-on vessel change course?

Yes, under Rule 17(a)(ii) once the give-way vessel clearly isn't acting, and she must act under 17(b) when collision can't be avoided by the give-way vessel alone.