Official rule text as published in the U.S. Coast Guard Navigation Rules
(a) Any action taken to avoid collision shall be taken in accordance with Rules 4-19, and shall if the circumstances of the case admit, be positive, made in ample time and with due regard to the observance of good seamanship.
(b) Any alteration of course and/or speed to avoid collision shall, if the circumstances of the case admit, be large enough to be readily apparent to another vessel observing visually or by radar; a succession of small alterations of course and/or speed should be avoided.
(c) If there is sufficient sea room, alteration of course alone may be the most effective action to avoid a close-quarters situation provided that it is made in good time, is substantial and does not result in another close-quarters situation.
(d) Action taken to avoid collision with another vessel shall be such as to result in passing at a safe distance. The effectiveness of the action shall be carefully checked until the other vessel is finally past and clear.
(e) If necessary to avoid collision or allow more time to assess the situation, a vessel shall slacken her speed or take all way off by stopping or reversing her means of propulsion.
(f)(i) A vessel which, by any of these Rules, is required not to impede the passage or safe passage of another vessel shall, when required by the circumstances of the case, take early action to allow sufficient sea room for the safe passage of the other vessel.
(ii) A vessel required not to impede the passage or safe passage of another vessel is not relieved of this obligation if approaching the other vessel so as to involve risk of collision and shall, when taking action, have full regard to the action which may be required by Rules 4-19.
(iii) A vessel, the passage of which is not to be impeded remains fully obliged to comply with Rules 4-19 when the two vessels are approaching one another so as to involve risk of collision.
MyCOLREGS explanation
Summary
Rule 8 sets the standard for how avoidance action should look: positive, taken in ample time, and - if an alteration of course or speed is used - large enough to be readily apparent to another vessel watching visually or by radar. Rule 8(b) specifically warns against a succession of small alterations, which can leave the other vessel unsure whether you are actually doing anything at all. Rule 8(c) and (d) go further: the maneuver has to be checked against the whole situation, not just the one vessel prompting it - an alteration of course must not result in another close-quarters situation, and its effectiveness has to be carefully monitored until the other vessel is finally past and clear. Solving one collision risk by creating a second one fails the rule just as surely as doing nothing.
Real World Example
Acting early isn't caution for its own sake - the sooner you alter course, the more room you have to make that change large and gentle; wait too long and the same maneuver becomes a full rudder order or a hard reverse instead of an easy, obvious turn. Tunnel vision on the immediate threat is one of the most common ways a close call becomes an actual collision - a maneuver that clears one vessel but ignores the rest of the water isn't finished just because you've stopped looking at the first threat.
Practice Questions
Q1. You determine you must give way to another vessel. Under Rule 8(a) and (b), what should your course alteration look like?
A. Positive, made in ample time, and large enough to be readily apparent to the other vessel observing visually or by radar.
B. Delayed until the other vessel signals its own intentions first.
C. As small as possible, to minimize the distance off your original course.
D. A series of small, cautious course nudges so the other vessel barely notices you are maneuvering.
Q2. You turn hard to starboard to avoid Vessel A, but that turn puts you on a collision course with Vessel B, which you had not been tracking closely. What does Rule 8 say about this outcome?
A. It is acceptable, because Rule 8 only concerns the vessel you were originally avoiding.
B. Rule 8 does not address multi-vessel situations at all.
C. It fails Rule 8(c), which requires that an alteration of course not result in another close-quarters situation.
D. It is acceptable as long as the turn was made in good time.