1The rule in plain words
In one sentence…
Proceed at a safe speed so you can take proper, effective action to avoid collision and stop within an appropriate distance.
What must it let you do?
Choose a speed that lets you take proper and effective action, avoid collision and stop within an appropriate distance.
What counts for all vessels?
Check visibility and traffic: all these factors count alongside your maneuverability, stopping distance and turning ability; background light, such as shore lights that dazzle you at night; wind, sea, current, hazards and draft against available depth.
What does operational radar add?
With operational radar, radar still has limits: add its characteristics, efficiency and limitations; the range scale; sea, weather and interference; and the possibility of missing small vessels, ice or other floating objects, among other factors.
Is there a fixed number?
No. Rule 6 lists factors among those taken into account and requires the prevailing circumstances and conditions to be assessed; so speed changes with the situation.
2Key points
- 1.
Goal: act effectively and stop within an appropriate distance.
- 2.
6(a): six factor groups apply to all vessels.
- 3.
6(b): operational radar adds six checks.
- 4.
Prevailing circumstances and conditions decide the speed.
3Frequent mistakes
Wrong: "If radar works, you can go fast."
Right: Radar adds factors; its limits and the situation still matter.
Wrong: "Safe speed is always the same."
Right: It depends on the prevailing circumstances and conditions.
Wrong: "Only stopping distance matters."
Right: You must also take proper, effective action to avoid collision.
4Exam keys & interpretation
Exam keys
"All vessels" = paragraph 6(a); do not confuse its factors with the radar additions.
"Additionally" + "operational radar" = 6(b): look for limitations, range scale and undetected targets.
"Among those taken into account" + "prevailing circumstances" rules out a fixed or universal speed.
How to read it
If conditions worsen, reassess speed: respond to the real situation rather than memorising a number.
With radar, use it to refine ranges, but remember that some objects may not be detected.
5Official text
IMO COLREG 1972U.S. Coast Guard — International Navigation Rules (2024)
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.6Test your knowledge
What must safe speed allow a vessel to do?
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.
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