How to Avoid Hazards with Danger Bearings

Avoid Hazards with Danger Bearings

Imagine that a dangerous rock-strewn island lies ahead to starboard.

The currents in this area are notorious for sweeping small craft down onto the shoal.

We need to make the anchorage on the other side before sunset.

How can we do this easily and safely?

When approaching an area with navigational hazards, you can stay safe by plotting bearings to a single object.

This danger bearing is a reference point to tell us whether we are passing safely by the hazard.

Choose a charted object that lies between you and the danger and on the side of the danger on which you wish to pass.

 

An alternative is to take a tangent
bearing to the steep sides of an island or cliff s that lie ahead of the danger.

Always check the chart before you make a decision. The accompanying illustration and instructions demonstrate using a danger bearing to pass the hazard safely.

As long as the bearing to the tank reads more than 045 degrees magnetic, you will clear the shoal to starboard. If the bearings start to decrease, turn left to cross the danger bearing line and return to safe water.

1. Choose a danger bearing. Pick a charted, visually prominent object such that a line drawn through the object will skirt the shoal’s near edge at its far end and pass near your approach point at its near end. This is your danger bearing. Draw short tick marks along the line pointing toward the side of the
danger (see illustration). This helps the danger bearing stand out from other lines, such as your trackline.

2. Determine the direction of the danger bearing. Measure the direction to the object twice to confirm the bearing. Convert a true bearing to a magnetic bearing by applying variation (add westerly or subtract easterly variation). Label the top of the danger bearing line with the magnetic danger bearing (045°M in the illustration).

3. Prefix the danger bearing. If the danger lies to starboard of the bearing, use the prefi x NLT (not less than) in front of the danger bearing. If the hazard lies to port, use the prefix NMT (not more than). In the illustration we write NLT in front of the danger bearing of 045 degrees magnetic. As long as your
bearing to the tank reads more than 045 degrees, you’ll pass safely. If it reads 045 degrees or less, you need to turn left until you cross the danger bearing.

RADAR DANGER RANGES AND CIRCLES OF POSITION

Using a radar-range circle of position to clear a dangerous pass.

Radar-equipped vessels oft en use predetermined range settings to clear shoals.

As discussed earlier, lines of position are actually parts of circles. Compare a radar beam from your antenna to a pebble dropped into a tub of water.

Concentric circles emanate from the pebble’s splash point (your antenna), bounce off the tub sides, and return to the point of origin.

So when we plot a radar range as an arc on the chart, we are actually plotting a segment of a circle of
position.

The following steps show how to use this secret to plot radar ranges
to clear any danger in your path:

1. Determine from the chart a range from a prominent point, cliff ,
headland, or other feature that will keep you clear of off -lying shoals.
If passing between two headlands, pick the one with the more prominent point. This gives a sharper radar picture. In the illustration, we chose the prominent point to starboard.

2. Draw a line parallel to your courseline and tangent to the prominent point. Measure the distance from the tangent line back to your courseline (0.5 NM in the illustration).

3. Adjust the variable-range marker (VRM) on your radar to that distance. The VRM measures distance (range) on a radar. The illustration shows what the radar will look like as you pass between the two shoals.

Keep the VRM circle tangent to the prominent point to starboard to
clear both shoals

Be safe Sailors ⚓

Be aware with Marinesthing

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