Official rule text as published in the U.S. Coast Guard Navigation Rules
Every vessel shall at all times proceed at a safe speed so that she can take proper and effective action to avoid collision and be stopped within a distance appropriate to the prevailing circumstances and conditions. In determining a safe speed the following factors shall be among those taken into account:
(a) By all vessels:
(i) The state of visibility;
(ii) The traffic density including concentrations of fishing vessels or any other vessels;
(iii) The maneuverability of the vessel with special reference to stopping distance and turning ability in the prevailing conditions;
(iv) At night, the presence of background light such as from shore lights or from back scatter from her own lights;
(v) The state of wind, sea and current, and the proximity of navigational hazards;
(vi) The draft in relation to the available depth of water.
(b) Additionally, by vessels with operational radar:
(i) The characteristics, efficiency and limitations of the radar equipment;
(ii) Any constraints imposed by the radar range scale in use;
(iii) The effect on radar detection of the sea state, weather and other sources of interference;
(iv) The possibility that small vessels, ice and other floating objects may not be detected by radar at an adequate range;
(v) The number, location and movement of vessels detected by radar;
(vi) The more exact assessment of the visibility that may be possible when radar is used to determine the range of vessels or other objects in the vicinity.
MyCOLREGS explanation
Summary
Rule 6 defines safe speed by what it lets you do, not by a number: a speed at which you can take effective action to avoid a collision and stop within a distance appropriate to the conditions. Rule 6(a) lists factors every vessel must weigh - visibility, traffic density, your own maneuverability and stopping distance, background lighting at night, wind/sea/current, and draft relative to available depth. Rule 6(b) adds a second layer for vessels running with operational radar: the equipment's own characteristics and limitations, the range scale in use, how sea state and weather degrade detection, the chance small vessels or floating objects go undetected, and the number, location, and movement of radar targets. Radar gives you more information, but Rule 6 treats it as one more factor to weigh - not a justification for running faster than conditions otherwise allow.
Real World Example
Safe speed is the single factor most often cited when a court assigns blame after a collision, because it isn't measured against a posted limit but against whether you could actually stop or maneuver in time for what was happening around you. Two of history's most notorious ocean collisions happened between ships that both had operational radar and both were using it - the equipment told each captain the other vessel was out there, but neither one slowed down, which is exactly the failure Rule 6 is written to prevent.
Practice Questions
Q1. A powerboat runs at 22 knots in open water with good visibility and no other traffic in sight. The same boat later enters a crowded, fog-bound harbor entrance still doing 22 knots. What does Rule 6 say about this?
A. 22 knots may be a safe speed in the first case and an unsafe speed in the second, because Rule 6(a) requires weighing visibility, traffic density, and stopping distance among other factors.
B. Speed compliance is fixed once and does not need to be reassessed, since Rule 6 only requires an initial safe-speed determination when getting underway for the day.
C. Rule 6 only applies to vessels over 20 meters, since smaller recreational vessels are assumed to be maneuverable enough to avoid collision at any speed.
D. Speed is only regulated by posted speed-limit signs, not by Rule 6, which merely offers general guidance rather than an enforceable requirement.
Q2. You are running in thick fog with radar operating. Under Rule 6(b), does having radar justify maintaining a higher speed than you would without it?
A. Yes, as long as the radar is a recent model, since modern radar sets are accurate enough to fully offset the reduced visibility from fog.
B. Yes — radar sees through fog, so normal speed restrictions do not apply once a vessel is equipped and operating radar in restricted visibility.
C. No — Rule 6(b) requires weighing the radar's own limitations, range-scale constraints, sea-state effects on detection, and the chance small targets go undetected, in addition to the Rule 6(a) factors.
D. No — Rule 6(b) prohibits using radar to determine safe speed at all, requiring speed to be based solely on visual range instead.
Practice Question Answers
Q1.
A. 22 knots may be a safe speed in the first case and an unsafe speed in the second, because Rule 6(a) requires weighing visibility, traffic density, and stopping distance among other factors. Rule 6 defines safe speed functionally — by whether you can take effective action and stop in time for the prevailing circumstances — so the same numeric speed can be safe or unsafe depending on visibility, traffic, and the other Rule 6(a) factors. B treats safe speed as a one-time determination, which defeats the whole point of Rule 6(a)'s multi-factor test. C invents a size cutoff the rule doesn't have. D confuses posted limits with Rule 6, which applies regardless of local signage.
Q2.
C. No — Rule 6(b) requires weighing the radar's own limitations, range-scale constraints, sea-state effects on detection, and the chance small targets go undetected, in addition to the Rule 6(a) factors. Rule 6(b) adds radar-specific factors on top of Rule 6(a), including the equipment's limitations and the risk that small vessels or floating objects go undetected — radar informs the safe-speed judgment, it does not override it. A treats this as an equipment-quality question when it isn't. B is the mistaken assumption Rule 6(b) exists to correct. D swings too far the other way - radar isn't prohibited, it just has to be weighed against its own real limitations.