Rule 6 requires every vessel to 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.
Factors for all vessels include:
- 1the state of visibility
- 2the traffic density, including concentrations of fishing vessels or any other vessels
- 3the manoeuvrability of the vessel, especially stopping distance and turning ability
- 4at night, the presence of background light such as shore lights or back scatter from the ship's own lights
- 5the state of wind, sea and current, and the proximity of navigational hazards
- 6the draught in relation to the available depth of water
Additional factors for vessels with operational radar include:
- 1the characteristics, efficiency and limitations of the radar equipment
- 2constraints imposed by the radar range scale in use
- 3the effect of sea state, weather and interference on radar detection
- 4the possibility that small vessels, ice and other floating objects may not be detected at an adequate range
- 5the number, location and movement of vessels detected by radar
- 6the more exact assessment of visibility that radar can provide when vessels or objects are detected nearby
IMO COLREG 1972Official text
Full wording of the rule, reproduced from the official source below.
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 of 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.STCW Bridge Watch Lens
Rules 4–10 apply in any visibility.
Rules 11–18 apply to vessels in sight of one another; Rule 19 applies to vessels not in sight when navigating in or near an area of restricted visibility.
Build the traffic picture with sight, hearing, radar/ARPA and chart context.
Do not let AIS or one isolated bearing replace systematic observation.
Safe speed is judged by what the ship can actually do in the moment: stop, turn, reduce exposure and avoid collision within the available sea room.
Exam Focus
Start every scenario by classifying the encounter: overtaking, head-on, crossing, narrow channel, traffic separation, or restricted visibility.
For vessels in sight, every overtaking vessel keeps clear until finally past and clear.
Rule 13 takes precedence over Part B Sections I and II; it does not override the other Parts of COLREG.
Schedule, fuel economy and comfort are not among the factors expressly listed in Rule 6.
They do not justify an unsafe speed.
Apply Rule 6: two safe-speed decisions
COLREG · USCG NAVCEN · 2026-09-10A power-driven vessel approaches a busy harbour at night. Shore lights obscure small craft, a following current increases speed over ground, and shoals reduce the room available to turn or stop. The vessel still has its open-water speed.
Is the previous speed still justified, or is there one safe number to choose?
Reassess now and reduce speed as necessary so effective avoiding action and stopping remain possible within the available space. Check traffic, visibility and background lights, actual stopping and turning ability, current, nearby hazards, and draught against water depth. Keep reassessing as conditions change.
Rule 6 sets a performance requirement, not a universal speed in knots. The old speed, engine setting or harbour limit alone cannot establish safety. If radar is operational, its limitations and the detected traffic add further factors; they do not remove the others.
In fog, your power-driven vessel hears another vessel's fog signal apparently forward of the beam. The vessels are not in sight, the radar picture is inconclusive, and you have not determined that collision risk is absent.
Keep speed until radar confirms a target?
No. In this Rule 19(e) situation, reduce to the minimum speed at which the vessel can keep her course; if necessary, take all way off. Navigate with extreme caution until the collision danger is over. Have engines ready for immediate manoeuvre and maintain the lookout.
The trigger includes a fog signal apparently forward of the beam unless absence of collision risk has been determined. Waiting for a clear echo does not establish that absence. The sound does not give an exact bearing or justify an automatic turn; no fixed speed in knots can replace this assessment.
Key Takeaways
Safe speed is whatever allows timely, effective collision-avoidance action in the actual conditions
Stopping distance, turning ability and available sea room are part of the decision
Radar does not justify higher speed; it adds more factors that must be considered
Commercial pressure, schedule and convenience are never Rule 6 factors
Common Mistakes
Keeping too much speed in restricted visibility or dense traffic
Looking only at engine speed instead of the ship's actual stopping and turning performance
Assuming radar removes the need to slow down
Test Your Knowledge
Practise this topic and review the explanation for each answer.
More rules in this Part
- ApplicationRules in Section I apply in any condition of visibility.
- Action to Avoid CollisionAny action to avoid collision shall be positive, made in ample time, and with due regard to good seamanship.
- Narrow ChannelsA vessel proceeding along the course of a narrow channel shall keep as near to the outer limit of the channel which lies on her starboard side.
- Traffic Separation SchemesVessels using traffic separation schemes shall proceed in the appropriate traffic lane in the general direction of traffic flow.
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