Official rule text as published in the U.S. Coast Guard Navigation Rules
(a) This Rule applies to vessels not in sight of one another when navigating in or near an area of restricted visibility.
(b) Every vessel shall proceed at a safe speed adapted to the prevailing circumstances and conditions of restricted visibility. A power-driven vessel shall have her engines ready for immediate maneuver.
(c) Every vessel shall have due regard to the prevailing circumstances and conditions of restricted visibility when complying with Rules 4-10.
(d) A vessel which detects by radar alone the presence of another vessel shall determine if a close-quarters situation is developing and/or risk of collision exists. If so, she shall take avoiding action in ample time, provided that when such action consists of an alteration in course, so far as possible the following shall be avoided:
(i) An alteration of course to port for a vessel forward of the beam, other than for a vessel being overtaken;
(ii) An alteration of course toward a vessel abeam or abaft the beam.
(e) Except where it has been determined that a risk of collision does not exist, every vessel which hears apparently forward of her beam the fog signal of another vessel, or which cannot avoid a close-quarters situation with another vessel forward of her beam, shall reduce her speed to be the minimum at which she can be kept on her course. She shall if necessary take all her way off and in any event navigate with extreme caution until danger of collision is over.
MyCOLREGS explanation
Summary
Rule 19 governs vessels that are not in sight of each other in restricted visibility, and it does not hand anyone a stand-on role the way Rules 12-17 do. Every vessel must run at a safe speed for the conditions with engines ready for immediate maneuver, and stay mindful of restricted visibility when applying the lookout, speed, and risk-assessment duties of Rules 4-10. A vessel that picks up another by radar alone has to judge whether a close-quarters situation or risk of collision is developing and act in ample time - there is no vessel that gets to simply hold course and speed the way an in-sight stand-on vessel would. When avoiding action means altering course, Rule 19(d) says to avoid, so far as possible, two kinds of turns: turning to port for a vessel forward of the beam (unless overtaking her), and turning toward a vessel abeam or abaft the beam - turning toward a target you cannot see tends to close the range instead of opening it. Rule 19(e) backs this up: if you hear another vessel's fog signal apparently forward of your beam, or cannot avoid a close-quarters situation with a vessel forward of your beam, slow to the minimum steerageway speed, take all way off if needed, and navigate with extreme caution until the danger has passed.
Real World Example
Rule 19 drops the stand-on role because radar alone shows you a target's position, not her intentions - vessels have collided in fog precisely because one side held course expecting a deference that Rule 19 never actually grants. The danger radar can't fully solve is precision: a bearing that looks wide by a few degrees can still put a fast contact directly in your path a few minutes later, which is why the rule treats the instinctive save-yourself turn toward an unseen vessel as more dangerous than no turn at all.
Practice Questions
Q1. In thick fog, you pick up a target by radar on your starboard bow; you cannot see the vessel visually. Should you treat this the way you would treat an ordinary in-sight crossing situation under Rule 15, with yourself as stand-on because the target has appeared on your starboard side rather than the reverse?
A. No - Rule 19 applies to vessels not in sight of one another, and it assigns no stand-on role.
B. No - in fog, whichever vessel has more powerful radar automatically has the right of way.
C. Yes - Rule 15 controls whenever another vessel is detected on your starboard side.
D. Yes, but only if the target is within 2 miles.
Q2. By radar alone you detect a vessel forward of your beam in fog, and you determine avoiding action is needed. Under Rule 19(d)(i), which alteration of course should you avoid, so far as possible, for this vessel (assuming you are not overtaking her)?
A. Any use of the danger signal.
B. Any reduction in speed.
C. An alteration of course to port.
D. An alteration of course to starboard.
Practice Question Answers
Q1.
A. No - Rule 19 applies to vessels not in sight of one another, and it assigns no stand-on role. Rule 19(a) applies specifically to vessels not in sight of one another in restricted visibility, and unlike Rules 12-17, it does not designate a stand-on vessel entitled to hold course and speed. B invents a rule based on radar strength that doesn't exist anywhere in the Rules. C wrongly imports Rule 15's stand-on/give-way roles, which only apply when vessels are in sight of each other - Rule 19 covers exactly the situation where they aren't. D adds a distance condition that isn't part of the rule.
Q2.
C. An alteration of course to port. Rule 19(d)(i) says that, so far as possible, an alteration of course to port should be avoided for a vessel detected by radar forward of the beam, other than a vessel being overtaken. A and B each name something other than a turn, when the question is specifically about course alteration. D names the turn that's generally acceptable - the caution in Rule 19(d)(i) is specifically about turning to port toward a vessel forward of the beam.