Section I: Conduct of vessels in any condition of visibility
Action to avoid collision: Rule 8
In practice
- The manoeuvre is to be bold, made early and readable for the other vessel, both visually and on radar.
- A series of small alterations of course or speed can be hard to notice and interpret on the other bridge. That is why you avoid it.
- The manoeuvre does not close the case: you check its effect until the other vessel is finally past and clear.
Text of Rule 8: Action to Avoid Collision
(a) Any action taken to avoid collision shall be taken in accordance with the Rules of this Part 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 the rules of this part.
- (iii) A vessel, the passage of which is not to be impeded remains fully obliged to comply with the rules of this part when the two vessels are approaching one another so as to involve risk of collision.
Text of the International Regulations as amended, quoted from the consolidated U.S. Coast Guard Navigation Rules (public domain); spelling follows the IMO original. When in doubt, the Convention decides.
The common mistake
Hesitation and cosmetic 5-degree course changes. Rule 8b says it straight: the alteration is to be large enough to be readily apparent to the other vessel.