Module 12 - Visual Aids to Navigation
Fog Signals
What you will learn
Recognise a charted aid or shore sound character while refusing to infer precise bearing, distance or collision status from apparent loudness and direction.
Audible aids can warn of a hazard or support identification in reduced visibility. Sound propagation and masking make them unsuitable for precise positioning, and they never replace the vessel's restricted-visibility duties.
Charted audible aids include horns, sirens, bells, gongs and whistles. Horn(2) 30s describes two blasts in a complete 30-second period. Time the full character and compare it with current charted information; equipment can be altered, damaged or out of service.
Wind and temperature gradients refract sound, land and structures reflect it, and machinery, wind farms, waves and the yacht itself can mask it. MCA restricted-visibility guidance warns that the direction of a heard fog signal may be difficult to determine. Loudness and apparent bearing therefore do not provide a measured range or line of position.
Continue the restricted-visibility plan: effective lookout, safe speed, engines ready, radar and other available aids used competently, depth and position monitored, COLREG sound signals made, and the pilotage limits observed. An aid's sound is supporting evidence, not permission to steer towards it.
Use a heard signal without chasing it
Two horn blasts repeat near the expected time, but their apparent direction swings as the yacht passes a headland in fog.
1. Record the full character
Time group and period and compare them with the current charted aid.
2. Reject apparent bearing
Do not turn towards the latest or loudest apparent direction.
3. Navigate independently
Use radar, depth, electronic position, lookout, safe speed and the preplanned limits.
Sense check: If the signal stopped, the safe-speed, collision-avoidance and pilotage decisions should still remain valid.
Common mistake or limitation
- Louder does not reliably mean closer, and the apparent source direction can be displaced by atmospheric or topographic effects.
- Hearing the expected aid does not remove uncertainty about another vessel's signal or the requirement to comply with Rules 19 and 35.
Recap
- Use sound character as supporting identification only.
- Do not derive precise distance or bearing from loudness or direction.
- Maintain every independent restricted-visibility control.
Optional quick check
Section 6 of 10
A matching two-blast shore signal seems to move from ahead to the beam. What should control the yacht?
Sources and factual review
Reviewed 2 September 2026 - Compass Revision editorial review.
Continue studying Visual Aids to Navigation
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