Module 12 - Visual Aids to Navigation
Yachtmaster Offshore Standard: Visual Aids
What you will learn
Command a visual-aid identification gate by predicting what should be seen, classifying the observation honestly and preserving a safe option until independent evidence agrees.
Yachtmaster visual-aid work is not faster recognition. It is a disciplined decision about whether an observed light or mark is identified, provisional or not identified, and whether that evidence is strong enough to support the next commitment.
Before a night or reduced-visibility stage, record the expected aid, approximate bearing window, structure or topmark, colour, rhythmic character, complete period, sector, usable range and order in the sequence. Add the current publication or warning source and a decision limit: the last point at which the vessel can hold, turn away or use another route if the cue is absent or inconsistent.
Classify the observation rather than forcing a name onto it. Identified means the measured character and period, bearing, charted context and independent position evidence agree. Provisional means some useful attributes match but a material check is missing. Not identified means the observation conflicts, is too incomplete or could plausibly be another aid. Brightness, colour, one flash group or an approximate bearing cannot carry a close-water decision alone.
An expected light that is not seen does not provide a line of position. The vessel may be outside geographical or luminous range; visibility, background lighting, obstruction, sector geometry or serviceability may intervene; a floating aid may be off station; or the assumed vessel position may be wrong. Test these alternatives with current publications, depth, radar where fitted and operational, bearings, time and distance, and position evidence that does not merely repeat one sensor or chart source.
When visibility is restricted, loss of a visual cue is also a watchkeeping and collision-avoidance condition. Maintain a proper lookout by sight, hearing and all available appropriate means, keep safe speed under continuous review and use radar properly if fitted and operational. The correct Rule 19 action depends on the traffic situation; a light-identification exercise cannot prescribe a live course alteration.
Worked example
A charted light has elevation 30 m and nominal range 18 M. The observer's height of eye is 4 m. What does the geographical-range estimate prove before a hazy night approach?
- 1Use the IALA training approximation with the applicable elevations: 2.03 × (√30 + √4).
- 2√30 is about 5.477 and √4 is 2, so the sum is 7.477.
- 32.03 × 7.477 = 15.178 M, rounded to 15.2 M geographical range under the stated refraction assumption.
- 4Compare 15.2 M with the 18 M nominal range and the luminous range for the prevailing visibility. The most restrictive credible limit governs first; background, obstruction and serviceability may shorten detection further.
Sense check: At 16 M the 18 M nominal figure does not prove that the light should be visible because the estimated geographical horizon is only 15.2 M. At 14 M, non-detection still would not prove position or an outage because atmospheric and operational causes remain.
| Observation | Evidence status | Command implication |
|---|---|---|
| Colour and group agree; period not timed | Provisional | Continue measuring without committing beyond the safe limit |
| Period, bearing and sequence agree with independent position | Identified for the planned use | Open only the gate that this evidence actually supports |
| Expected light absent inside a predicted detection envelope | Unresolved discrepancy | Test range, visibility, obstruction, serviceability and vessel position |
| Floating mark conflicts with depth and radar | Not identified or possibly off station | Do not use it as an exact fix or a side-of-channel instruction |
Open the visual-identification gate
The approach plan expects a white group-flashing light before a charted sector. A white group appears near the expected bearing, but one preceding buoy was missed and the period has not settled.
1. Predict
State the expected bearing window, complete character, period, sequence and the last safe decision point.
2. Classify
Call the sighting provisional because group and colour agree but timing and sequence evidence are incomplete.
3. Corroborate
Time several complete periods and compare depth, radar, bearings and independent position with the charted picture.
4. Decide
Keep the gate closed, protect sea room and use the briefed hold or alternative if material evidence remains unresolved at the decision limit.
Sense check: The safest label can change from provisional to identified only when the missing evidence is obtained. Repeating the same incomplete sighting does not make it independent.
Common mistake or limitation
- A charted nominal range is a standard luminous-performance reference, not a guarantee of detection at that distance.
- A light on the expected bearing can still be a different aid, shore light or floating mark displaced from its charted position.
- Two plotters sharing one GNSS receiver and chart database do not provide two independent position checks.
- Losing a light in restricted visibility does not suspend the lookout, safe-speed or collision-avoidance duties that govern the whole situation.
Recap
- Predict the full observation and the last safe decision point.
- Use identified, provisional and not-identified labels honestly.
- Compare nominal, luminous and geographical limits before interpreting absence.
- Open a commitment gate only when independent evidence agrees.
Optional quick check
Section 9 of 10
A white group-flashing light is close to the predicted bearing, but its period and the preceding buoy are unconfirmed. What is the strongest Yachtmaster call?
Sources and factual review
Reviewed 2 September 2026 - Compass Revision editorial review.
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.