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Clearing hazards with a danger bearing

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What is the issue?
Whether you are navigating by instrument or eyeball, what lies beneath the water is invisible in most places. And sometimes, the main concern of navigation is to avoid these hidden dangers such as extensive reefs or shoals.

A good example that can be used to illustrate the problem is the Shingles Bank, on the western entrance to The Solent on the south coast of England. It is marked by several lighted buoys on the southeast side, or channel side, but none on the north side. Given its numerous shallow patches and rapidity of the tides, it has all the ingredients to put any vessel which might have the misfortune to come up on it in peril. But likewise there is deep water to the north with settled conditions and a good visibility cut between North Head Bank and the Shingles, keeping Hurst Castle open of its own width of Fort Victoria - see Solent Coastal Description. The problem with this is that the dangerous Shingles Bank lies close by which is mostly covered making it risky.

Why address this?
Operating a boat in the vicinity of hidden dangers makes for a high degree of anxiety, or the potential of a collision. Keeping a vessel in safe water and clear of a hidden dangers will keep anxiety levels down, the vessel safer, and makes the odd shortcut more usable.

How to address this?
Establish a danger bearing as presented above. A danger bearing is a navigation technique generally used to help a vessel avoid underwater obstructions and is used when sailing near reefs or shoals that are unmarked by navigational aids. It is a limiting lien of bearing for a specific hazard, the passing of which takes the vessel out of safe water and causes it to run into the danger.

Hurst Castle as used in this approach to The Solent
Photo: Neil Howard via CC BY-NC 2.0

Establishing a danger bearing is one of the easiest ways to ensure that your vessel and crew stay in safe water. All it requires is a few minutes to perform the simple steps below.

  • 1. Pick a prominent point, lighthouse, or stationary object located on the chart outside of the danger area that will be clearly visible. In the case of the above example the entrance to the Solent, the tower of Hurst Castle.

  • 2. Draw a line, as above, on the chart or plotter from the point that leads clear of the hazard. In this case, a line if 251°T, reciprocal 71°T, from ½ a mile out to 2½ miles, is well clear of the danger. Write ‘safe water’ on the appropriate side of the line ‘danger’ on the opposite.

  • 3. Pop a waypoint on the tower of Hurst Castle on your chart plotter.

  • 4. From that point on you have one number to observe, in this case, 71°T, and a real-time feed of its relative bearing to the vessel. If the vessel starts to drift towards anything less than 71°T, in this example, you are in danger. 71°T and above and you are in safe water.

  • 5. Use your waypoint to maintain a steady watch on your danger bearing and steer your course so as to keep on the safe side of the line. This makes it easy to steer clear of the determined danger.

Once a danger bearing has been established it is a simple one number you have to remember, less than that or greater than on the GPS, keep the vessel clear of the hazard or in danger as the case may be. If you do not want to use the GPS and a waypoint, the same principle works with a hand bearing compass. You will need to convert the true bearing to a magnetic compass bearing by first applying the local variation. Likewise, the ship’s compass also allowing for deviation.

However, when sailing local waters we tend not to set a waypoint, and use a plotter as a back-up to eye-ball navigation so it is a resource that is ready to be used. Likewise, when we do set a destination waypoint we tend to use it for bearing to the destination, calculating how long you have to go ('Estimated Time of Arrival' or ETA) and speed made good towards it ('Velocity Made Good' or VMG). None of these will suffer from being deactivated for a brief period whilst being in the vicinity of a hidden hazard.

Either way, with a few minutes it takes to establish a danger bearing, you can slide past the hazards in safe water without making numerous position plots and enjoy the sailing with some peace of mind.

With thanks to:
Michael Harpur, Yacht Whistler.

Coastal Navigation; Danger Bearing

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