4.3 Non-Lateral Aids, Light Characteristics & Numbering
Key Takeaways
- Safe-water marks are red-and-white vertical stripes (often with a red ball topmark) and may show a white Morse Code “A” (Mo(A)) light — pass either side in safe water.
- Isolated-danger marks are black with one or more broad red horizontal bands and two black spheres as topmark; they sit on or over a danger with navigable water around.
- Special marks are yellow (often with an “X” topmark) and mark special areas such as traffic separation, spoil, recreation, or cables — not channel sides.
- Information and regulatory marks (orange-and-white) convey rules, warnings, or information; they are not lateral channel edges.
- Light characteristics (Fl, Oc, Iso, Q, Mo(A)), period, and nominal range are taken from the Light List (and chart abbreviations); do not invent rhythms from memory alone.
Beyond Red and Green
Not every aid tells you port or starboard. Non-lateral aids mark safe water, dangers, special features, and regulations. Q162 mixes these with light-characteristic questions because night navigation depends on recognizing a rhythm and color, not only a paint job you saw at noon.
Safe-Water Marks
Safe-water marks (also called mid-channel or fairway marks in many contexts) indicate navigable water all around — typically the middle of a fairway or landfall approach.
| Feature | U.S. / IALA-B pattern |
|---|---|
| Color | Red and white vertical stripes |
| Topmark (if fitted) | Single red sphere |
| Light (if lighted) | White, often Morse Code “A” — short flash, long flash: Mo(A) |
| Action | May be passed on either side; marks safe water, not a shoal |
Do not treat a safe-water buoy as a lateral nun. Cutting close is usually fine laterally, but still watch traffic and depth — “safe water” means the mark’s system says water is around it, not that your draft fits every nearby ledge.
Isolated Danger Marks
Isolated danger marks sit on, or immediately over, a danger with navigable water around it (rock, wreck, isolated shoal).
| Feature | Pattern |
|---|---|
| Color | Black with one or more broad red horizontal bands (black-red-black appearance) |
| Topmark | Two black spheres, one above the other |
| Light (if lighted) | White, group flashing in a pattern associated with danger (often a group of two flashes — know the Light List entry) |
| Action | Keep clear of the mark; do not “hug” it like a mid-channel buoy |
Exam contrast: safe water invites either-side passage; isolated danger says there is something bad under or at the mark.
Special Marks
Special marks are yellow. They mark a special area or feature — examples include traffic separation scheme features, spoil grounds, military exercise areas, recreation zones, pipelines/cables, or other features defined locally — not the port/starboard edge of the main channel.
- Color: yellow
- Topmark (if fitted): yellow “X” (saltire)
- Light (if lighted): yellow, rhythm as charted/listed
Special marks do not replace lateral reasoning. Seeing yellow does not mean “leave it to starboard returning.” Read the chart note and Coast Pilot for what the special mark protects or indicates.
Information and Regulatory Marks
Information and regulatory marks (common on U.S. waters, including inland) use orange and white geometric shapes on white backgrounds with orange borders:
- Diamond — danger / hazard warning
- Circle — controlled / restricted operation (speed, wake, etc.)
- Diamond with cross — exclusion / keep-out area as depicted
- Rectangle / square — information or directions
These marks communicate rules and warnings (no-wake, swim area, dam danger) rather than IALA lateral hand. On a Master 100 passenger run through a no-wake zone, the orange-and-white board is as operationally important as a green can.
Light Characteristics — The Alphabet of Night ATON
Charts and the Light List use standard abbreviations for light rhythms. Master candidates should read these fluently:
| Abbreviation | Meaning (concept) |
|---|---|
| F | Fixed — continuous light |
| Fl | Flashing — light shorter than dark (single flash at regular intervals) |
| Fl(2) / Fl(3) | Group flashing — groups of 2 or 3 flashes |
| Q | Quick flashing — rapid flashes (often ~50–60/min class; know as “quick”) |
| VQ | Very quick flashing |
| Oc | Occulting — light longer than dark; dark intervals are the “eclipses” |
| Iso | Isophase — equal light and dark intervals |
| Mo(A) | Morse Code A — short-long white typical of many safe-water lights |
| Al | Alternating colors (less common on simple lateral sets) |
Period is the time in seconds from the start of one cycle to the start of the next (e.g., Fl 4s means a flashing light whose cycle is 4 seconds). Period is how you tell “your” light from another similar flash down the beach.
Color abbreviations appear with the characteristic: R red, G green, W white, Y yellow. Example chart style: Fl R 4s = flashing red, 4-second period.
Nominal Range
Nominal range is the maximum distance at which a light can be seen in meteorological visibility of 10 nautical miles, based on the light’s intensity (and stated in the Light List). It is not always how far you will see it tonight:
- Fog, rain, and haze cut visibility below nominal range
- Your height of eye and the light’s geographic range (curvature of the Earth) may limit detection before intensity does
- Background shore glare can wash out a light inside its nominal range
Exam point: when asked where to find characteristic, period, nominal range, fog signal, and position, the primary publication answer is the Light List (see Section 4.4).
Sector Lights and Color Changes
Some major lights show sectors — different colors (or obscured arcs) over different bearings. A white sector may mark the safe approach; red or green sectors may warn of danger on that side of the fairway. When the exam or chart notes a sector light, your job is to stay in the safe sector as described, not merely to “see a light somewhere ahead.” Sector boundaries are bearings; plot or mentally hold them the way you would a range line.
Numbering and Identification Discipline
Lateral numbers were covered in 4.2. Non-lateral aids may use letters, names, or special designations (e.g., safe-water lettered marks, named light structures). Always cross-check three sources:
- What the chart plots and labels (abbreviated characteristic)
- What the Light List describes (full characteristic, period, nominal range, fog signal)
- What your eye actually sees (color, rhythm, period timed with a watch if needed)
If chart, list, and eye disagree, assume something changed — off-station buoy, extinguished or reduced light, wrong identification, or outdated chart — and navigate with extra caution while you update from Local Notice to Mariners. Timing a light’s period with a stopwatch is an old skill that still separates professionals from guessers when two white flashers compete on the same shoreline.
Non-lateral & lights pocket card
- Safe water = red/white vertical; Mo(A) white common; either side.
- Isolated danger = black-red-black; two black spheres; keep off.
- Special = yellow; not a channel side.
- Orange/white = info/regulatory.
- Fl / Oc / Iso / Q / Mo(A) + period + color identify the light; Light List holds full data and nominal range.
Learn the shapes by daylight and the rhythms by night. That pairing is the core of non-lateral ATON on the Master 100 Navigation General module — and it is the same pairing you use when the chartplotter screen freezes and the inlet is only a Morse A and two range lights away.
Which description correctly identifies a safe-water mark in the U.S. system?
What does the chart abbreviation “Oc” indicate about a light?
Where should a mariner look for a lighted aid’s detailed characteristic, period, nominal range, and fog signal data?