Danger Bearings and Danger Circles

TL;DR — A danger bearing is a limiting compass bearing to a charted hazard; so long as your observed bearing is on the safe side of that limit, your vessel clears the hazard. Risk of collision is deemed to exist when the compass bearing of an approaching vessel does not appreciably change while range is decreasing (CBDR). 33 CFR §83.07 Bowditch Ch. 7 §704

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What the Rule Says

Constant Bearing, Decreasing Range (CBDR) — the regulatory foundation

Inland Rule 7 (and its mirror-image COLREGs Rule 7) requires every vessel to use all available means appropriate to the prevailing circumstances and conditions to determine whether risk of collision exists. If any doubt remains, risk shall be deemed to exist. 33 CFR §83.07

The rule is explicit about the compass bearing test: risk of collision shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change. 33 CFR §83.07 This is the regulatory statement of the CBDR principle that underlies the danger bearing concept.

The rule adds a critical nuance: risk of collision may sometimes exist even when an appreciable bearing change is evident, particularly when approaching a very large vessel or a tow, or when approaching a vessel at close range. 33 CFR §83.07 A bearing change alone does not guarantee safety.

Radar must be used properly when fitted and operational, including long-range scanning for early warning and radar plotting or equivalent systematic observation of detected objects. Assumptions shall not be made on the basis of scanty information, especially scanty radar information. 33 CFR §83.07

Compass Bearings as the Canonical Collision-Detection Tool

Compass bearings of an approaching vessel are the canonical method of detecting risk of collision. If the bearing remains constant and the range is decreasing (CBDR), risk of collision exists. Bowditch Ch. 7 §704

Bearings to a single fixed object can also produce a running fix when the relative bearing changes substantially. The doubling-the-angle technique uses two relative bearings; the distance run between them equals the distance to the object at the second bearing. Bowditch Ch. 7 §703 This technique is directly applicable to establishing your distance off a charted hazard — the geometric basis of a danger circle.

Sectored Lights as Pre-Plotted Danger Bearings

A sectored light shows different colours in defined arcs of bearing. The mariner's bearing through the sectors indicates safe versus danger water. Light List — light-characteristic (sec) When a vessel is in the white (safe) sector and observes the light change to red or green, that colour change is the equivalent of crossing a pre-established danger bearing — the light itself signals the transition from safe to dangerous water.

Danger Markers

A white buoy with orange bands and a diamond symbol indicates a hazard to navigation. The diamond may contain words describing the hazard. Light List — Danger Marker Danger markers identify the hazard in the water; danger bearings and danger circles are the navigator's tool for keeping clear of the area those markers protect.

Bridge Equipment Supporting Danger Bearing Work

Every regulated vessel must carry an illuminated magnetic steering compass mounted in a binnacle, readable at the main steering stand, along with a current deviation table or compass comparison record. 33 CFR §164.35 A gyrocompass and an illuminated repeater at the main steering stand are also required. 33 CFR §164.35 These instruments are what the navigator uses to observe and record the compass bearings that form the basis of danger bearing monitoring.

Equipment on the bridge for plotting relative motion is also required. 33 CFR §164.35 This is the chart table, parallel rulers, dividers, and plotting sheets — the tools used to construct and monitor danger bearings and danger circles graphically.

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Why It Matters on the Exam

The Danger Bearing Concept

A danger bearing is a limiting compass bearing drawn on the chart from a vessel's intended track to a charted hazard (shoal, rock, wreck, reef). The navigator labels it NLT (Not Less Than) or NMT (Not More Than) depending on which side of the hazard is safe.

If the hazard lies to starboard of the track and the danger bearing is labeled NLT 045°T, the vessel is safe as long as the observed compass bearing to the landmark used as the reference is 045° or greater. If the bearing drops below 045°, the vessel has closed inside the danger arc and is standing into danger.

The exam will test whether you can identify which label (NLT or NMT) applies given the geometry, and whether you understand that the bearing must be monitored continuously — not just checked once. Bowditch Ch. 7 §704 33 CFR §83.07

The Danger Circle Concept

A danger circle is a circle of minimum safe distance drawn around a hazard on the chart. The navigator ensures the vessel's track passes outside that circle. The doubling-the-angle technique provides a practical way to determine distance off a fixed object without a fix from two separate objects — the distance run between the first and second bearing equals the distance to the object at the time of the second bearing. Bowditch Ch. 7 §703 This gives the navigator a distance-off value that can be compared against the radius of the danger circle.

CBDR and the Danger Bearing — Two Applications of the Same Principle

The exam frequently tests both applications of the constant-bearing principle in the same module:

1. Collision avoidance: a constant compass bearing to another vessel with decreasing range means risk of collision exists. 33 CFR §83.07 Bowditch Ch. 7 §704 2. Grounding avoidance: a compass bearing to a charted landmark that crosses the danger bearing limit means the vessel is standing into danger.

In both cases the navigator is watching a compass bearing and comparing it against a threshold. The direction of the "safe" side differs, but the monitoring technique is identical.

Restricted Visibility Adds Urgency

In restricted visibility, every vessel must proceed at a safe speed adapted to the prevailing circumstances. A power-driven vessel shall have her engines ready for immediate maneuver. 33 CFR §83.19 A vessel detecting another vessel by radar alone must determine if a close-quarters situation is developing and, if so, take avoiding action in ample time. 33 CFR §83.19 The same radar that is used to plot another vessel's bearing for collision avoidance is used to take bearings on charted radar-conspicuous objects for danger bearing monitoring. The obligation to use radar properly 33 CFR §83.07 applies to both tasks simultaneously.

Sectored Lights as Exam Questions

The exam may present a scenario where a vessel observes a light change from white to red. The candidate must recognize that the vessel has crossed into the danger sector — the equivalent of crossing a pre-plotted danger bearing. Light List — light-characteristic (sec) The correct action is to alter course back toward the safe (white) sector immediately.

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Common Pitfalls

Confusing NLT and NMT. The label depends entirely on the geometry. If the hazard is to port of the track and the reference landmark is ahead and to starboard, the danger bearing will be NMT (the bearing must not exceed the limit). Candidates frequently apply the wrong label because they do not sketch the geometry before answering.

Assuming a bearing change means safety. Rule 7 explicitly states that risk of collision may exist even when an appreciable bearing change is evident, particularly when approaching a very large vessel or a tow or when approaching a vessel at close range. 33 CFR §83.07 The same logic applies to a danger bearing: a bearing that is changing may still be moving toward the danger limit, not away from it.

Relying on scanty radar information. Assumptions shall not be made on the basis of scanty information, especially scanty radar information. 33 CFR §83.07 A single radar bearing to a charted object is not sufficient to confirm the vessel is clear of a danger circle. Systematic, repeated observations are required.

Ignoring the deviation table. The compass bearing used to monitor a danger bearing must be a corrected compass bearing. The deviation table or compass comparison record is required to be current and in the wheelhouse. 33 CFR §164.35 An uncorrected bearing may place the vessel inside the danger arc without the navigator realizing it.

Treating a danger marker as a danger bearing. A danger marker buoy marks the hazard's location. Light List — Danger Marker It does not by itself define the safe side or the limiting bearing. The navigator must still construct the danger bearing from the chart, using a charted landmark as the reference point — not the buoy itself, which may drag or be off-station.

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Quick Check

Q1 — Under Inland Rule 7, when is risk of collision deemed to exist with respect to compass bearings?

Risk of collision shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change. 33 CFR §83.07

Q2 — A danger bearing to a lighthouse is labeled NLT 312°T. You observe the lighthouse bearing 308°T. Are you in safe water or standing into danger?

Standing into danger. The observed bearing (308°T) is less than the NLT limit (312°T), meaning the vessel has closed inside the danger arc. Alter course immediately to bring the bearing back to 312° or greater. Bowditch Ch. 7 §704

Q3 — You are using the doubling-the-angle technique. Your first relative bearing to a lighthouse is 030° and your second relative bearing is 060°. You ran 1.8 nm between observations. What is your distance off the lighthouse at the time of the second bearing?

1.8 nm. When the angle on the bow doubles, the distance run between the two bearings equals the distance to the object at the time of the second bearing. Bowditch Ch. 7 §703

Q4 — You are transiting at night and observe a sectored light change from white to red. What does this indicate and what action is required?

The vessel has crossed from the safe (white) sector into the danger (red) sector, meaning it has crossed the equivalent of a pre-plotted danger bearing into dangerous water. Alter course to return to the white sector. Light List — light-characteristic (sec)

Q5 — Rule 7 states that risk of collision may exist even when an appreciable bearing change is evident. Under what circumstances does the rule specifically note this?

Particularly when approaching a very large vessel or a tow, or when approaching a vessel at close range. 33 CFR §83.07

Q6 — What bridge equipment does 33 CFR §164.35 require for plotting relative motion, and why is it relevant to danger bearing work?

33 CFR §164.35 requires equipment on the bridge for plotting relative motion. 33 CFR §164.35 This is the plotting equipment — chart table, parallel rulers, dividers — used to construct danger bearings and danger circles on the chart and to record successive bearing observations that confirm the vessel remains on the safe side of the limit.