Official rule text as published in the U.S. Coast Guard Navigation Rules
(a)(i) Where one of two vessels is to keep out of the way, the other shall keep her course and speed.
(ii) The latter vessel may, however, take action to avoid collision by her maneuver alone, as soon as it becomes apparent to her that the vessel required to keep out of the way is not taking appropriate action in compliance with these Rules.
(b) When, from any cause, the vessel required to keep her course and speed finds herself so close that collision cannot be avoided by the action of the give-way vessel alone, she shall take such action as will best aid to avoid collision.
(c) A power-driven vessel which takes action in a crossing situation in accordance with Rule 17(a)(ii) to avoid collision with another power-driven vessel shall, if the circumstances of the case admit, not alter course to port for a vessel on her own port side.
(d) This Rule does not relieve the give-way vessel of her obligation to keep out of the way.
MyCOLREGS explanation
Summary
If you're the stand-on vessel, your first move is to hold your course and speed. But you're allowed to maneuver on your own once it's clear the give-way vessel isn't doing what she should, and you must take whatever action helps most to avoid collision if things get so close that the give-way vessel alone can't prevent it anymore. In a crossing situation, if you do have to maneuver, avoid turning to port for a vessel on your own port side when you can help it - that can make things worse rather than better. None of this lets the give-way vessel off the hook; she still has to keep clear.
Real World Example
Holding course and speed isn't passivity - it's what keeps the give-way vessel's evasive maneuver from becoming a guess, since a stand-on vessel that starts jinking early can turn straight into the very turn the other boat was making to avoid her. If she does have to act, a port turn in this geometry swings her bow toward the give-way vessel's path instead of away from it - the one maneuver a panicking stand-on skipper might instinctively reach for is the one most likely to turn a close call into a collision.
Practice Questions
Q1. You are the stand-on vessel. It becomes apparent the give-way vessel is not taking appropriate action to comply with the Rules. Under Rule 17(a)(ii), what may you do?
A. Take action to avoid collision by your own maneuver alone.
B. Nothing - you must hold course and speed no matter what.
C. Wait until contact is imminent before doing anything.
D. Sound five short blasts and immediately stop your engines.
Q2. You are the stand-on power-driven vessel in a crossing situation and must finally act because the give-way vessel is not responding. The give-way vessel is on your port side. Under Rule 17(c), which turn should you be especially cautious about making?
A. Sounding five short blasts instead of maneuvering.
B. Stopping engines entirely.
C. Altering course to starboard.
D. Altering course to port, toward the give-way vessel.
Practice Question Answers
Q1.
A. Take action to avoid collision by your own maneuver alone. Rule 17(a)(ii) allows the stand-on vessel to maneuver on her own once it becomes apparent the give-way vessel is not taking appropriate action. B treats the stand-on duty as unconditional, when Rule 17(a)(ii) specifically allows action once it's apparent the give-way vessel isn't responding. C describes waiting until the last possible moment, which defeats the purpose of acting early. D jumps to a specific maneuver the rule doesn't prescribe.
Q2.
D. Altering course to port, toward the give-way vessel. Rule 17(c) says a power-driven stand-on vessel acting under 17(a)(ii) should not, if circumstances admit, alter course to port for a vessel on her own port side, since that turn tends to close the distance rather than open it. A and B each substitute a different action for the question asked. C names the turn away from the give-way vessel, which is generally the safer option - the caution in Rule 17(c) is specifically about turning toward a vessel on your own port side.