Danger Bearings and Danger Circles

What the rule requires

The line originates at the object, not at the vessel. A danger bearing keeps a vessel clear of an off-lying hazard using a single charted object: a line is drawn from the object tangent to the edge of the hazard, or tangent to a danger circle scribed at a safe distance around it, and labelled NMT (not more than) or NLT (not less than) that bearing. So long as the object's observed bearing stays on the safe side of the danger bearing, the vessel remains in safe water. Bowditch Ch. 7 §706

Two constructions, and the choice between them is the first decision in the problem.

• Tangent to the charted edge of the hazard. Use this when the hazard has a definite plotted limit — the outer edge of a rocky patch, the enclosing danger curve — and you are content to clear that edge.
• Tangent to a danger circle. Scribe a circle about the hazard at the distance you have decided to keep off, then run the tangent to the circle instead of to the hazard itself. The clearance is built into the geometry, so the limit bearing already contains your margin and you are not eyeballing an offset while the bearing is changing.

The label is the whole of the working instruction, and it is decided by which side of the line the navigable water lies on. The device in common circulation among instructors is danger to the left, NMT; danger to the right, NLT, taken as you look along the bearing to the object: if the hazard lies to the left of your line of sight, standing toward it swings the object's bearing higher, so the limit is NMT. It is sound as far as it goes, and it fails in two places worth knowing before you rely on it. First, it depends on you having the hazard genuinely on one side of the sight line rather than dead ahead beyond the object. Second, it collapses when the limit bearing sits near 000°, where the numbers wrap and "more than" stops meaning "farther from safe water."

The check that never fails: pick a point in water you know to be safe, measure the bearing to the object from there, and compare it with the limit. Bearing from safe water lower than the limit, the label is NMT. Higher, the label is NLT. That takes fifteen seconds on the chart and it is immune to how the geometry happens to lie.

What the limit does not give you is a position. Danger bearings and clearing lines supplement the fix to keep the vessel off charted hazards between fixes, and visual piloting still fixes the vessel by bearings and ranges to positively identified charted objects. Bowditch Ch. 7 §707

You are standing in toward an unfamiliar anchorage with a shoal on your port hand. From a position you know is in clear water the light bears 143°T; the tangent drawn from the light to the danger circle gives a limit bearing of 151°T. How do you label the line, and what does a subsequent bearing of 156°T tell you?

NMT 151°T. The bearing taken from known safe water, 143°, is less than the limit, so safe water lies on the low side and the limit is not-more-than. A bearing of 156° is on the wrong side of the limit and the vessel is standing into the hazard; alter to bring the bearing back below 151° before doing anything else.

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Telling it apart

The criterion is what the line entitles you to conclude about your position.

• Danger bearing — a limit, not a position. It answers one question, which side of the line you are on, and it answers it from a single object. Misfiled as a fix by candidates who read the labelled bearing off the chart and treat the point where it crosses their course line as where they are.
• Line of position — a line along which the vessel is known to lie at a given moment from a single observation, and a fix is the intersection of two or more LOPs taken simultaneously. Bowditch Ch. 7 §701 One bearing to one object is an LOP; you are somewhere on it. A danger bearing is drawn before you observe anything and stays put.
• Cocked hat corner — where a three-bearing fix produces a triangle lying near a hazard, the corner nearest the danger is assumed for safety. Bowditch Ch. 7 §705 That is a rule for reading a fix pessimistically, not a limiting line, and it applies only when you have three simultaneous LOPs to well-separated objects.
• CBDR — compass bearings of an approaching vessel are the canonical method of detecting risk of collision, and a constant bearing with decreasing range means risk of collision exists. Bowditch Ch. 7 §704 A steady bearing means opposite things in the two cases. On an approaching vessel it is the warning. On a fixed charted object with the bearing steady on the safe side of the limit, it is confirmation you are holding clear.

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Working a question

Gull Point Light stands on the shore ahead. Cutler Ledge, a rocky patch, lies about 2 miles west of the light and rather less than a mile north of your intended track. You are making 090°T along the coast and you want to leave the ledge no closer than 0.5 mile.

1. Identify the object and the hazard, and confirm both on the chart. Gull Point Light is the only object of use; the ledge is the hazard. If you cannot positively identify the light you are looking at, you have no danger bearing, because visual piloting depends on objects positively identified on the chart. Bowditch Ch. 7 §707
2. Decide which construction applies. You have specified a clearance of 0.5 mile rather than "clear of the edge," so the danger circle governs: centre it on the ledge and scribe it at 0.5 mile. Bowditch Ch. 7 §706
3. Establish which side of the ledge you are passing. You are standing east and passing to the south of it. Safe water is southward; onshore, to the north, is the danger. That decides which of the two tangents from the light you want — the one grazing the southern edge of the circle.
4. Lay the tangent from the light to that edge of the circle and extend it westward, out through the water you will actually be in.
5. Read the bearing in the sense you will observe it. The line runs out from the light on 278°T. You will be looking at the light from seaward, so the bearing you compare against is the reciprocal, 098°T.
6. Label it. Take a bearing from a point on your track that is plainly clear of the ledge — well south — and it comes out lower than 098°, in the low 090s. Safe water is on the low side, so the line is labelled NMT 098°T.
7. Monitor it. Take the light regularly as you close. 094°, 096°, still on the safe side. A bearing of 103° puts the vessel north of the limit line and inside the danger circle; the ledge, not the arithmetic, is what you are now dealing with.
8. Act on the limit as a limit. Come to starboard, to seaward, and hold until the light's bearing has come back below 098° with a margin, then resume. The correction is not "a little" — it is until the bearing reads on the safe side.

If the passage plan took you north of the ledge instead, nothing about the method changes except the tangent used and the label, which becomes NLT to the other limit bearing.

Working the same chart, a candidate lays the tangent correctly, reads 278°T off the plot, and labels the line NMT 278°T. Where does that put the vessel?

Into the ledge, or at least blind to it. 278°T is the direction from the light outward along the line; the bearing observed from the vessel is the reciprocal, 098°T. A hand-bearing compass alongside will never read anything near 278° for an object ahead on the port bow, so every observation appears to satisfy the limit and the limit stops working. Label the line with the bearing you will actually read, and note the reciprocal alongside it if you want both.

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Where candidates lose the point

• Reversing NMT and NLT. The classic wrong pick is the option that gets the geometry right and the sense backwards, and it attracts because both options quote the same limit bearing. Settle it by measuring from water you know is safe: bearing lower than the limit means NMT, higher means NLT. Do not settle it by which end of the line looks nearer the shoal on the chart.
• Drawing the line from the vessel. A line laid from your DR position tangent to the hazard is a bearing that changes with every mile you run, so it cannot be a limit. The line comes from the charted object and stays fixed. Bowditch Ch. 7 §706
• Comparing the plotted limit against a bearing taken in a different reference. A limit measured off the chart and a bearing read from a compass are only comparable if both are in the same reference. Write the reference into the label and read every subsequent bearing in that same one.
• Treating the limit bearing as a fix. A candidate who reads the limit as a position will answer that being on NMT 098°T tells them where they are. It tells them which side of one line they are on. A fix is the intersection of two or more simultaneous LOPs. Bowditch Ch. 7 §701
• Taking one bearing and stowing the compass. The protection lasts only while the bearing is being observed; the wording is conditional on the observed bearing staying on the safe side. A set or a sheered course between fixes is exactly what the danger bearing exists to catch. Bowditch Ch. 7 §707
• Using NMT and NLT across 000°. Where the tangent runs near north, a bearing of 358° is "less than" 002° arithmetically while being on either side of it geographically depending on the case. Where that happens, write the limit as a side instead — keep the light east of the line — and do not trust the numbers.

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Check yourself

You intend to pass a wreck no closer than 0.75 mile using a single lighthouse. What do you construct on the chart, and what do you draw the tangent to?

Scribe a danger circle of 0.75 mile radius about the wreck, then draw the line from the lighthouse tangent to that circle on the side you will pass. Tangent to the charted edge of the wreck would only clear the wreck itself and would leave your 0.75 mile margin to be judged by eye. Bowditch Ch. 7 §706

The hazard lies to the left of your line of sight to the charted object. Is the limit NMT or NLT, and why?

NMT. Standing toward a hazard on the left of the sight line swings the object's bearing higher, so higher bearings are the dangerous ones and the limit is not-more-than. Confirm it against the chart by measuring the bearing from known safe water, which will be the lower figure.

Your danger bearing is labelled NLT 216°T. Successive bearings of the light read 224°, 220°, 217°. What is happening and what do you do?

You are in safe water and closing the limit. The bearing is falling toward 216° and will cross it if the trend holds, so the set or the steering is carrying you toward the hazard. Alter now to bring the bearing up and hold a margin above 216°, rather than waiting for the bearing to reach the limit.

You have a good danger bearing running and no other charted object in sight. Can you report a fix?

No. One object gives one line, and a fix is the intersection of two or more LOPs taken simultaneously. Bowditch Ch. 7 §701 With a single object you can obtain a running fix by taking a bearing, running a timed distance on a steady course and taking a second bearing, then advancing the first LOP along the DR track — less reliable, because it assumes course and speed were held exactly. Bowditch Ch. 7 §708

A three-bearing fix puts you in a cocked hat whose nearest corner is a quarter mile off a shoal. Which part of the triangle do you work from?

The corner nearest the danger. Where the triangle lies near a hazard, that corner is assumed for safety; the centre is taken only when the fix is small and clear of danger. Bowditch Ch. 7 §705

The bearing of a vessel fine on your starboard bow has held steady for six minutes while the range has closed. Does your danger bearing bear on this?

No. The danger bearing concerns a charted hazard and a fixed object. A constant bearing with decreasing range on an approaching vessel means risk of collision exists. Bowditch Ch. 7 §704 A steady bearing is the warning in that case and the confirmation of safe water in the other; keep the two uses of "steady bearing" separate.

Standing in toward a harbour, the tangent from the entrance light to the danger circle around a reef comes out on 001°T as observed from seaward. How do you label it?

Not with a number alone. Across 000° the arithmetic of "more than" and "less than" no longer tracks which side of the line you are on, so label the line by side — keep the light to the east of the limit, or to the west, whichever the safe water is — and steer to that. Establish the sense from a bearing taken in known safe water before you need it.