Visibility closes in
Choose a safe speed. Developing level, about 3 min. International COLREGs / IRPCS. COLREGs Rules 6 & 19(b).
The situation
Visibility has fallen to about 90 m. At 12 knots your boat cannot stop within the water you can assess. In these conditions four to five knots permits effective action and stopping within an appropriate distance. A charted obstruction lies ahead.
What should you do with your speed?
- Reduce to a speed suited to the visibility: Create time and room to assess and avoid hazards.
- Keep 12 knots because the chart is accurate: Rely on the plotted route alone.
- Increase speed to leave the fog sooner: Spend less time in restricted visibility.
Answers and explanations
Lookout: Safe speed depends on visibility, traffic, stopping and turning ability, weather, current and hazards. There is no universal fog speed limit.
The sound answer
Reduce to a speed suited to the visibility
Rule 6 requires a speed that permits effective avoiding action and stopping within an appropriate distance. This authored reduction to 4.8 knots addresses the stated stopping ability; it is not a general legal speed limit.
Why the other options fall short
Keep 12 knots because the chart is accurate
A chart does not establish what uncharted or moving obstacles are ahead. The stated visibility and stopping ability make the present speed unsafe.
Increase speed to leave the fog sooner
Increasing speed reduces the time available to assess hazards and increases the stopping distance in already unsafe conditions.
Rules 6 & 19(b) · Safe speed in restricted visibility
Proceed at a safe speed adapted to the prevailing conditions. A power-driven vessel keeps her engines ready for immediate manoeuvre. Safe speed must permit effective avoiding action and stopping within an appropriate distance.