2.4 Light Characteristics & Buoy Numbering
Key Takeaways
- Fl is flashing (light shorter than dark), Oc is occulting (light longer than dark), Iso is isophase (equal light and dark), and Q is quick (about 60 flashes per minute).
- The period, written as a number of seconds, is the time for one complete light cycle — 'Fl G 4s' flashes green once every four seconds.
- Light color matches the aid: red lights on red marks, green on green, white on safe-water and many non-lateral marks, and yellow on special marks.
- The Light List gives the full details (characteristic, height, range, structure) that the chart shows only in abbreviated form.
- Aid numbers increase from seaward — red even, green odd — while safe-water, isolated-danger, and special marks are usually lettered rather than numbered.
Light Characteristics & Buoy Numbering
Quick Answer: A light's characteristic is its on/off rhythm and color, written in shorthand like "Fl R 4s" (flashing red, one flash every four seconds). Flashing means the light is on less than it is off; occulting means on more than off; isophase means equal; and quick means a rapid roughly-once-per-second flash. The color matches the aid, and the number in seconds is the period of one full cycle.
At night an aid is only as useful as your ability to read its light. These abbreviations appear on the chart, in the Light List, and on the exam.
The Core Rhythms
| Abbrev. | Name | Rhythm |
|---|---|---|
| F | Fixed | Continuous, steady light |
| Fl | Flashing | Single flashes; light shorter than the darkness between |
| Q | Quick | Rapid flashing, about 60 per minute |
| Oc | Occulting | Steady light eclipsed briefly; light longer than darkness |
| Iso | Isophase | Equal periods of light and darkness |
| Mo(A) | Morse "A" | Short flash then long flash (dot-dash), repeated |
| LFl | Long-flashing | A single long flash (two seconds or more) each period |
| Fl(2) | Group flashing | Flashes come in groups, here groups of two |
| Fl(2+1) | Composite group flashing | Two flashes then one — the preferred-channel rhythm |
A simple way to keep the two most-confused rhythms straight: flashing = mostly dark (short bright flashes), while occulting = mostly lit (brief blackouts). Isophase sits exactly between them with equal light and dark. A few charts also show Al (alternating, changing color) and F Fl (a fixed light with a brighter flash).
Reading the Period and Color
The period is the time for one complete cycle, printed as a number of seconds. "Fl G 4s" means a green light that flashes once every four seconds; "Oc R 6s" means a red light that is on, briefly eclipsed, and back on every six seconds. To identify an aid at night you time the cycle with a watch — start counting at one flash and stop at the start of the next identical flash — then match the color, rhythm, and period to the chart.
Color follows the aid. Red lateral marks show red lights; green marks show green; safe-water marks show white (Morse "A"); isolated-danger marks show white Fl(2); and special marks show yellow. So "Fl (2+1) R 6s" on a banded buoy is a preferred-channel mark with a red top band, while "Q G" on a square mark is a quick-flashing green port-hand mark, often placed where the channel turns sharply and demands attention.
Sectored Lights
A major light such as a lighthouse may show different colors over different arcs of visibility, called sectors. A red sector warns that you are looking at the light from a direction where a danger (a shoal or reef) lies; a white sector shows the safe approach. Staying in the white sector keeps you clear. The chart and Light List give the bearings that bound each sector.
Worked Reading
You see a lighted mark and time it: a green flash, then darkness, repeating every four seconds. On the chart the nearby aid reads "Fl G 4s '5'." The match confirms it: a green, odd-numbered ("5") port-hand mark flashing green every four seconds. Because it is green and odd, you keep it to port when returning. Everything reinforces: color gives the side, number confirms direction of travel, and the rhythm/period lets you pick this aid out from others nearby.
The Light List and Sound Signals
The chart shows a compressed version of a light's data. The Light List — a Coast Guard publication — gives the full description: the exact characteristic, the light's height above water, its nominal range (how far it can be seen in clear weather), the structure it sits on, and any sound signal. When precision matters, the Light List is the authority.
Many aids also carry sound signals for fog and darkness: a bell, gong, whistle, or electronic horn. On buoys, wave action often rings the bell or gong; shore stations use powered horns with a timed blast pattern. These help you locate an aid you cannot yet see, though a buoy in calm water may stay silent.
Buoy Numbering
Numbers and letters on aids are systematic:
- Numbers increase from seaward toward the head of navigation, so rising numbers confirm you are returning.
- Red marks carry even numbers; green marks carry odd numbers, matching the lateral color logic.
- Safe-water, isolated-danger, and special marks are usually lettered, not numbered, because they do not mark a channel side. A landfall safe-water buoy might read "RW 'A'."
- Letters may be added to a numbered aid to insert a new one without renumbering the whole channel (e.g., "2A" between "2" and "4").
Put together, a fully identified aid such as "Fl R 4s '8'" tells you at a glance: a red, even-numbered, starboard-hand mark you keep to starboard when returning, flashing red once every four seconds.
A chart shows a light labeled 'Oc G 6s.' How does this light behave?
What does the '4s' mean in the light characteristic 'Fl R 4s'?
Which publication gives the complete details of a light — its characteristic, height, nominal range, and structure — that the chart shows only in abbreviated form?