8.3 Isolated Danger, Safe Water, Special & Emergency Wreck Marks
Key Takeaways
Isolated Danger Marks are moored directly on or over an isolated hazard with navigable water all around; they feature black hulls with red horizontal bands, two black ball topmarks, and a white Fl(2) light.
Safe Water Marks designate navigable water all around the mark (used for fairways, mid-channels, and landfalls); they carry red and white vertical stripes, a single red ball topmark, and Iso, Occ, LFl 10s, or Morse 'A' white lights.
Special Marks indicate specific administrative, recreational, or industrial areas (spoil grounds, pipelines, military areas, ODAS buoys); they are solid yellow with an optional yellow 'X' topmark and a yellow light.
Emergency Wreck Marking Buoys provide immediate, high-visibility temporary marking for new hazards; they feature blue and yellow vertical stripes, an upright yellow cross topmark, and an alternating blue and yellow light (Al.Fl.BuY).
All four non-lateral mark types are universally standardized and identical across both IALA Region A and Region B worldwide.
8.3 Isolated Danger, Safe Water, Special & Emergency Wreck Marks
Note
Beyond lateral and cardinal marks, the IALA Maritime Buoyage System establishes four specialized aid-to-navigation categories: Isolated Danger Marks, Safe Water Marks, Special Marks, and Emergency Wreck Marking Buoys. Like cardinal marks, these four types are universally identical in both Region A and Region B.
Overview of Non-Lateral Aids to Navigation
While lateral marks outline channels and cardinal marks designate compass quadrants of open sea, mariners frequently encounter isolated rocks, offshore fairway approaches, and specialized maritime zones. The IALA framework resolves these with four distinct mark categories.
Isolated Danger Marks: Hazards in Navigable Water
An Isolated Danger Mark is erected on, or moored directly over, an isolated hazard of limited extent with navigable water all around. Check the chart for its extent and a suitable passing distance.
Common Hazards & Placement
Isolated danger marks indicate submerged rock pinnacles, isolated drying boulders, sunken wrecks, or isolated shoals in open water.
Visual Identification & Topmark
- Body Shape: Pillar or spar buoy.
- Color Pattern: Black with one or more broad red horizontal bands.
- Topmark: Two black spheres (balls) mounted vertically, separated by clear daylight.
- Topmark Mnemonic: "Two balls in the air = Danger down there."
Light Signature: The Fl(2) W Characteristic
- Color: White.
- Rhythm: Group flashing in pairs: Fl(2) W.
- Mnemonic: "Two balls, two flashes"—the two spherical topmarks correspond directly to the two white flashes.
Navigation Practice
Vessels may pass on any side of an isolated danger mark. However, skippers must consult the chart to evaluate the hazard's underwater footprint and give the mark a generous berth.
Safe Water Marks: Landfall & Fairway Guides
A Safe Water Mark indicates navigable water all around, commonly marking a fairway, mid-channel or landfall position. It does not mark a danger; check charted depths against the vessel’s draught.
Primary Applications
- Landfall Marks: Positioned offshore as a visual waypoint terminating an open-water crossing.
- Fairway / Mid-Channel Marks: Used to mark a fairway or channel centreline; consult charted depths and local traffic rules.
- TSS Termination Marks: Positioned at seaward entry points of Traffic Separation Schemes.
Physical Characteristics & The Spherical Hull
- Body Shape: Spherical, pillar, or spar. Identify it by red-and-white vertical stripes and its red spherical topmark; special marks may also have spherical hulls.
- Color Pattern: Distinctive Red and White vertical stripes (never horizontal).
- Topmark: A single solid red sphere (ball).
Light Sequences
Safe water marks exhibit a white light using one of four specific rhythms:
- Isophase (Iso W): Equal durations of light and dark.
- Occulting (Occ W): Light duration strictly exceeds darkness.
- Long Flash every 10 seconds (LFl 10s W): A single flash lasting at least 2.0 seconds every 10 seconds.
- Morse Code 'A' (Mo(A) W): One short flash followed by one long flash (, "Alpha").
Seamanship Practice
Pass on the side required by local regulations and the channel layout. Safe-water marking alone does not specify a compulsory passing side or guarantee clearance for the vessel.
Special Marks: Marking Channels, Features & Hazards
Special Marks do not primarily guide navigation. Instead, they indicate special areas or features recorded on charts or nautical notices.
Common Applications
- Spoil grounds (dredging disposal zones).
- Military exercise and firing ranges.
- Submarine pipelines, cables, and outfalls.
- Recreation and swimming reserves.
- Offshore wind farms and wave energy zones.
- ODAS Buoys: Ocean Data Acquisition Systems gathering marine meteorological data.
Visual & Light Characteristics
- Body Shape: Optional (can, conical, spherical, pillar, or spar), provided the shape does not conflict with lateral marks in the same fairway.
- Color: Solid Yellow.
- Topmark (if fitted): A single yellow diagonal cross (St. Andrew's cross, 'X').
- Light Color: Yellow.
- Light Rhythms: Any rhythm not used for white cardinal, isolated danger, or safe water marks. Examples include
Fl Y,Fl(4) YorFl(5) Y; avoid the rhythms reserved for other mark types.
Emergency Wreck Marking Buoys: Rapid Maritime Response
The Emergency Wreck Marking Buoy (EWMB) provides conspicuous temporary marking for fresh wrecks and other new dangers before permanent aids are established.
Visual & Structural Design
- Body Shape: Pillar or spar.
- Color Pattern: Equal number of vertical Blue and Yellow stripes (typically 4 or 8 stripes).
- Topmark: Standing upright Yellow cross ('+'), perpendicular to the horizon (distinct from the diagonal 'X' of a special mark).
Light Sequence & Electronic Aids
- Light Rhythm: Alternating flashing Blue and Yellow light:
- 1.0 second Blue light + 0.5s dark + 1.0 second Yellow light + 0.5s dark = 3.0s total period (
Al.Fl.BuY 3s).
- 1.0 second Blue light + 0.5s dark + 1.0 second Yellow light + 0.5s dark = 3.0s total period (
- Racon Transmission: May carry a radar transponder transmitting Morse Code 'D' () on 3cm and 10cm radar.
- AIS AtoN: May broadcast an electronic AIS Aid to Navigation signal displaying its position on compatible equipment.
Comprehensive Matrix of Non-Lateral Marks
| Mark Type | Purpose | Hull Shape | Color Pattern | Topmark | Light Color & Rhythms |
|---|---|---|---|---|---|
| Isolated Danger | Isolated hazard with safe water all round | Pillar or Spar | Black with Red horizontal bands | Two black spheres | White: Fl(2) W |
| Safe Water | Navigable water all round; fairway / landfall | Spherical, Pillar, or Spar | Red & White vertical stripes | Single red sphere | White: Iso, Occ, LFl 10s, or Mo(A) |
| Special Mark | Special feature, cable, spoil, or ODAS buoy | Optional (can, cone, pillar) | Solid Yellow | Single yellow diagonal cross ('X') | Yellow: Any non-conflicting rhythm (Fl Y) |
| Emergency Wreck | Immediate temporary marking of new hazards | Pillar or Spar | Blue & Yellow vertical stripes | Upright yellow cross ('+') | Alternating Blue & Yellow: Al.Fl.BuY 3s |
Passage Planning, Chartwork & Examination Traps
- Horizontal vs. Vertical Striping: Remember: Safe Water has vertical stripes (Red & White), whereas Isolated Danger has horizontal bands (Black & Red). Safe water marks may be spherical. Isolated danger marks normally use pillar or spar buoys; their black/red bands and double black-ball topmark distinguish them.
- Cross Topmarks: Do not confuse the diagonal 'X' topmark of a yellow Special Mark with the upright '+' cross of a blue-and-yellow Emergency Wreck Marking Buoy.
- Yellow Lights at Night: In industrial ports, yellow lights from special marks can blend into shore sodium lighting. Check the chart characteristic (e.g.,
Fl(4) Y 12s) and use a hand-bearing compass to verify the bearing.
In the event of a newly discovered, uncharted navigation hazard or a freshly sunken vessel, which temporary aid to navigation may maritime authorities deploy, and what light characteristic does it exhibit?
A Safe Water mark exhibiting an isophase white light with a 10-second period.
An Emergency Wreck Marking Buoy with blue and yellow vertical stripes, exhibiting an alternating blue and yellow flashing light.
A Special Mark painted yellow with an 'X' topmark, exhibiting a continuous quick flashing white light.
An Isolated Danger mark with red and white horizontal bands, exhibiting Morse code 'A' in red.
On an Admiralty chart, a symbol depicts a pillar buoy painted black with a single broad red horizontal band, surmounted by two black spherical topmarks. What does this mark signify, and what is its standard light rhythm?
A mid-channel safe water mark; light is Morse code 'A' (Mo(A) W).
A bifurcation mark for a preferred channel to port; light is Fl(2+1)G.
A military exercise boundary; light is flashing yellow (Fl Y).
An isolated danger mark with navigable water all around it; light is white group flashing two (Fl(2) W).
Which visual and operational characteristics uniquely define an IALA Safe Water Mark?
Spherical shape, red and yellow horizontal bands, single cone topmark, and quick flashing green light.
Can or conical shape, yellow body, yellow 'X' topmark, and group flashing yellow light.
Spherical, pillar, or spar shape with red and white vertical stripes, single red spherical topmark, and white Iso, Occ, LFl 10s, or Morse code 'A' light.
Pillar shape with black and yellow vertical stripes, two black cones pointing downwards, and continuous white light.
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