Module 12 — Buoyage & Compass | OnDeck Academy OOW Oral Prep

Module 12 — OOW 3000 GT Oral Preparation

IALA Buoyage & Compass

IALA Region A & B · Lateral marks · Cardinal marks · Special marks · Gyro compass · Magnetic compass · Errors & checks

IALA Maritime Buoyage System

Two regions — one critical difference: The colour of lateral marks is reversed between Region A and Region B. Cardinal marks are identical worldwide. Superyachts cross between regions regularly — update your mental model every time.

Region	Coverage	Port lateral (entering harbour)	Starboard lateral
IALA Region A	Europe, Africa, India, Asia (except Japan/Philippines/Korea), Australia, New Zealand	RED — can shape, red light	GREEN — conical shape, green light
IALA Region B	Americas (North, Central, South), Japan, Philippines, South Korea	GREEN — can shape, green light	RED — conical shape, red light

TRANSATLANTIC PASSAGES: Med to Caribbean — you leave Region A (red to port) and enter Region B (green to port). The lateral marks mean the opposite. This is a known hazard. Brief all watchkeepers before crossing the boundary.

Cardinal Marks — Identical in Both Regions

Cardinal marks show the direction of safe water from the hazard. The mark is named for the direction of safe water — a North cardinal mark means safe water is to the North of the mark.

Mark	Topmark	Band colours	Safe water	Light
North	Both cones pointing UP ▲▲	Black over yellow	To the NORTH	VQ or Q — continuous very quick or quick flashing
South	Both cones pointing DOWN ▼▼	Yellow over black	To the SOUTH	VQ(6)+LFl or Q(6)+LFl — 6 flashes then long flash
East	Cones points away from each other ▲▼	Black, yellow band, black	To the EAST	VQ(3) or Q(3) — 3 flashes every 5 or 10 seconds
West	Cones points toward each other ▼▲	Yellow, black band, yellow	To the WEST	VQ(9) or Q(9) — 9 flashes every 10 or 15 seconds

Memory aid for lights: Start at North and go clockwise — 1 (continuous), 3 (East), 6+LFl (South), 9 (West). Or: Never Eat Shredded Wheat — North, East, South, West.

Other Buoy Types

Mark type	Colour	Shape	Meaning
Isolated Danger	Black and red horizontal bands	Any — topmark: 2 black spheres	Marks an isolated danger with navigable water all around. Pass at a safe distance on any side
Safe Water	Red and white vertical stripes	Spherical or spar — topmark: single red sphere	Safe water all around — used for mid-channel and landfall marks
Special Mark	Yellow	Any — topmark: yellow X	Special purpose — separation zones, military exercise areas, aquaculture, outfall pipes
Emergency Wreck Marking Buoy	Blue and yellow vertical stripes	Pillar or spar — alternating blue and yellow light	Marks a new wreck — deployed before permanent chart correction

Gyro Compass

How it works

A rapidly spinning gyroscope (rotor) maintains its orientation in space due to gyroscopic inertia
The Earth's rotation and gravity cause the gyro to precess and align with True North over time
Once settled it points to True North — not magnetic
Typically requires 1–6 hours to settle after starting from rest — always start before departure

Gyro errors

Latitude error: Increases at higher latitudes — gyro tends to deviate from True North. Most accurate between 30°N and 30°S
Speed error: Vessel's speed introduces a small error — corrected by speed/latitude correction
Rolling error: Heavy rolling in a seaway can cause gyro instability
Settling error: Gyro not fully settled after start-up — allow full settling time
Check gyro error at least once per watch by taking an azimuth of the sun or a celestial body

Magnetic Compass

Advantages

Completely independent — works without power
Essential backup to gyro compass
SOLAS requires a magnetic compass on all vessels — it is the last-resort navigation reference
Checked against gyro at least once per watch

Compass checks

Azimuth check: Compare compass bearing of the sun (or star) at a known time with the calculated true bearing — difference gives total compass error (variation + deviation)
Transit check: When two charted objects are in line, compare the compass bearing with the charted true bearing
Record compass error in the deck log at least once per watch when at sea
Deviation card should be checked and updated when the vessel's magnetic signature changes significantly

Night Identification — The Complete Rhythm Table

At night the shapes vanish and the rhythms carry everything. This table is worth owning cold:

Mark	Light	Memory hook
Lateral (A or B)	Red or green, any rhythm not used for white lights	Colour = side, per the region
Preferred channel	Composite group flashing (2+1), red or green	The 2+1 rhythm IS the junction signature, both regions
North cardinal	VQ or Q, continuous white	12 o'clock — continuous
East cardinal	VQ(3) 5s or Q(3) 10s	3 o'clock — three flashes
South cardinal	VQ(6)+LFl 10s or Q(6)+LFl 15s	6 o'clock — six, PLUS a long flash so a partial count can't be mistaken for east or west
West cardinal	VQ(9) 10s or Q(9) 15s	9 o'clock — nine flashes
Isolated danger	Fl(2) white	Two flashes, two spheres, pass either side — well clear
Safe water	Iso, Oc, LFl 10s, or Morse "A"	The gentle whites — reassurance rhythms
Special	Yellow, any rhythm not usable for white	Yellow = consult the chart
Emergency wreck	Alternating blue/yellow, 1s each	Unlike anything else — deliberately

The design logic is examinable in itself: colour = side (laterals), rhythm = message (cardinals, danger, safe water) — and the rhythm-based marks are white because white carries furthest and can never be confused with a channel edge. Brief a night approach as "coloured lights are channel edges; white lights, count the flashes" and the picture assembles itself.

The Emergency Wreck Marking Buoy

Deployed FAST over new wrecks — precisely the dangers your chart doesn't yet carry: blue and yellow vertical stripes, upright yellow cross topmark, and an alternating blue/yellow light, one second of each. It is deliberately unlike anything else in the system. Response: widest practicable berth (a new wreck has an unknown least depth and a debris field), slow down, and check Navtex/NAVWARNs for the wreck's details — a mark you cannot explain is a prompt to check the warnings, not to sail on. Increasingly the FIRST marking is a virtual AIS aid to navigation, ahead of the physical buoy.

Direction of Buoyage — The Convention Everything Hangs On

Every lateral mark's meaning is relative to the direction of buoyage: generally FROM seaward into rivers, harbours and estuaries, and around coastlines a general direction set by the charting authority — shown by the broad magenta arrow on the chart. Travel with it, Region A puts red cans to port; travel against it — every departure — and the same marks swap sides relative to your bow. Be most alert at junctions, around headlands, and at the places where two buoyage directions meet head-to-head and the laterals flip between one buoy and the next: those flip points are charted, briefable, and exactly where unprepared navigators ground.

The Magnetic Compass — Errors You Manage, Not Suffer

Error	Source	Management
Variation	The Earth — difference between true and magnetic north, changing with place and time	From the chart's compass rose, updated by its annual change note
Deviation	The SHIP — her own iron and electrics deflecting the card, different on every heading	The deviation card, produced by swinging the compass; re-swing after refit, structural work, or magnetic cargo

Working rules: Compass → Deviation → Magnetic → Variation → True, adding east and subtracting west toward true ("Compass to True, add East"), reversed coming back. Deviation is entered with the heading you will actually steer. And the housekeeping that examiners ask because it is real: nothing magnetic near the standard compass — the phone, the speaker, the multitool left on the binnacle each write their own deviation card entry that no one has calibrated.

Checking the error at sea — every watch: a TRANSIT of two charted objects is the free, compass-independent method — read the compass as they touch, compare with the charted bearing, and the difference is total error; deviation falls out once variation is applied. Offshore, an azimuth of the sun or Polaris against the computed true bearing does the same job. Logged, every watch, and compared gyro-to-magnetic — a wandering error caught at the log-entry stage is a grounding that never happened.

Gyro Compass — Different Animal, Different Errors

The gyro seeks true north — no variation, no deviation — but it has its own account: speed error (a function of speed and course, worst steaming fast on north/south headings at high latitude — corrected automatically on modern sets fed with speed and latitude, but only if the feeds are right), settling time after start-up (hours, which is why the gyro is never switched off in port lightly), and the brutal simplicity that when it wanders or topples it does so confidently — the repeaters follow it into the error. Hence the discipline: gyro checked against magnetic every watch, both checked against a transit or azimuth, and the three-way comparison logged. Three sources that agree are navigation; one source trusted alone is a story you tell the MAIB.

Additional Question

You are steering 090°C. Deviation card says 3°E, variation from the chart 2°W. A transit of two charted beacons bears 092° by compass as they touch; the chart says the transit is 094°T. Reconcile all of it.

Expected total error is deviation plus variation: 3°E + 2°W = 1°E — so compass readings should be 1° less than true. The transit proves it: charted 094°T, observed 092°C — an actual total error of 2°E. The difference of 1° means the card or the variation is out — most likely deviation has shifted since the last swing. Action: log the observed error, apply the OBSERVED 2°E to my courses and bearings for now, check on another heading when a transit offers, and report it — the deviation card may need re-swinging. The measured error always beats the documented one.

Oral Exam Practice Questions

Question 1

You are arriving in the Caribbean after an Atlantic crossing. What do you need to brief the bridge team about with regard to buoyage?

The Caribbean is IALA Region B — the lateral marks are the opposite of Europe. In Region A, red marks are left to port when entering harbour. In Region B, green marks are left to port entering harbour and red marks are left to starboard. Every bridge team member must have this updated before we arrive. I would brief this at the start of the watch before any coastal approach, and confirm it is understood. The charted marks will be the same colours — but the safe side to pass them changes completely.

Question 2

You see a buoy with black and yellow bands and two cone topmarks pointing upward. What is it and what does it tell you?

That is a North cardinal mark. The topmarks — both cones pointing up — confirm North cardinal. Black over yellow colour bands confirm it. It tells me that safe water lies to the North of the mark. I should pass the mark on its north side. It will display a continuous quick or very quick white flashing light at night.

Question 3

How do you check the gyro compass error at sea?

By taking an azimuth — a compass bearing of the sun, moon, or a star at a known time and position. I calculate the true bearing of that body using azimuth tables or an azimuth calculator. For the gyro, the difference between the calculated true bearing and the gyro bearing IS the gyro error — no variation is involved, because a gyro seeks true north. (The same azimuth taken with the magnetic compass gives total compass error; remove variation and what remains is deviation.) This should be done at least once per watch and recorded in the deck log. I can also use a transit — when two charted objects are in line, compare the compass bearing with the charted true bearing.