Module 12 - Visual Aids to Navigation
Fog Signals
What you will learn
Recognise a charted shore or aid fog-signal character while treating apparent direction and distance as unreliable.
A fog signal can corroborate that a charted aid may be audible nearby, but sound bends, reflects and fades unpredictably over water. It cannot replace restricted-visibility navigation or a measured position.
Some lighthouses and aids have charted sound signals such as horns, sirens, bells or gongs. A notation such as Horn(2) 30s describes two blasts in a complete 30-second period. Check the current chart or light list because equipment and service status can change.
Sound can be refracted by temperature and wind gradients, reflected by land or structures and masked by vessel noise. Apparent loudness and direction are therefore poor measures of range and bearing. Hearing the expected character is supporting evidence, not confirmation of proximity or a precise position.
In restricted visibility maintain the required lookout, safe speed and collision-avoidance actions, use radar and other available aids competently, monitor depth and position, and follow the pilotage plan. A shore fog signal does not replace the vessel's COLREG sound-signal duties.
Use a heard signal without chasing it
In fog, two horn blasts repeat near the expected time, but the apparent direction swings as the yacht passes a headland.
1. Record the character
Time the full period and compare it with the current charted aid.
2. Reject apparent bearing
Do not steer towards the loudest or most recent direction.
3. Navigate independently
Use radar, depth, position, lookout and the preplanned safe limits.
Sense check: If the signal vanished, the safe navigation method should still control the vessel.
Common mistake or limitation
- Louder does not reliably mean closer, and apparent direction can be displaced by refraction or reflection.
- A charted aid signal is different from the manoeuvring and warning signals required of vessels by the COLREGs.
Recap
- Time and identify the complete charted sound character.
- Do not infer precise bearing or distance from sound over water.
- Continue the full restricted-visibility navigation and collision-avoidance plan.
Optional quick check
Section 6 of 6
What is the safest use of a matching shore fog-signal character?
Continue studying Visual Aids to Navigation
This public lesson is part of Revision Module 12. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.