A horn in the fog
Restricted visibility. Developing level, about 3 min. COLREGs Rule 19.
The situation
Dense fog, visibility about 250 m. You are making 6 knots and sounding your own fog signal. Out of the grey, a prolonged blast is heard, apparently forward of your starboard beam. You cannot see the vessel.
What should you do?
- Reduce to the minimum speed to keep steerage: Take all way off if necessary, and navigate with extreme caution.
- Alter to port, away from the sound: Open the distance from the signal.
- Keep your speed and listen again: Wait for a better idea of where she is.
Lookout: In fog a sound tells you a vessel is near, but not reliably where or how she is heading. Do not let the readouts tempt you into confident manoeuvres on scanty information.
The sound answer
Reduce to the minimum speed to keep steerage
Rule 19(e): a vessel that hears the fog signal of another apparently forward of her beam reduces speed to the minimum at which she can be kept on course, takes all way off if necessary, and navigates with extreme caution until the danger is over.
Why the other options fall short
Alter to port, away from the sound
The direction of a sound in fog is unreliable, and Rule 19(d) warns against altering to port for a vessel forward of the beam. The required response to a fog signal forward of the beam is to slow right down.
Keep your speed and listen again
Keeping 6 knots toward an unseen vessel forward of the beam is what Rule 19(e) exists to prevent. Reduce speed first, then keep listening.
Rule 19 · Conduct in restricted visibility
Every vessel proceeds at a safe speed adapted to restricted visibility. On hearing a fog signal apparently forward of the beam, or when a close-quarters situation cannot be avoided with a vessel forward of the beam, she reduces speed to the minimum at which she can be kept on course.