Cross bearings
With compass bearings we can determine our estimated position, for example by means of a cross-bearing. We make a compass bearing of a point that we can recognize both on the chart and along or on the water. We convert this compass bearing into a true bearings. We draw that bearing on the chart as a line to the point on the chart. We then know that we are at least on that line. We just don’t know where on that line yet. So we need a second bearing line at another recognizable point. The intersection of two bearing lines is our position. It is important that we make the angle between the two bearings as perpendicular as possible, through the correct choice of recognizable bearing objects. Otherwise the bearing is not reliable. It is even better to make 3 bearings. If the 3 lines on the chart do not pass through approximately the same point, something is wrong. When we make a bearing, don’t forget to account for the variation and deviation (only if the bearing is made on the steering compass). If we make the bearing with the hand bearing compass, we assume that the deviation is 0, because we do not have a deviation table for the handheld compass.
Luminous range diagram
The nominal range of a lighthouse can be found in the list of lights and in the chart. That nominal range applies at a visibility of 10Nm. If visibility is better, then a light can be seen from a greater distance. That distance/range determined by visibility is called the “luminous range” and we can look it up in the chart below.
If you were to draw a vertical line at the nominal load of 12Nm to the graph of 10Nm visibility and from that intersection point draw a horizontal line to read the luminous range, you will immediately notice that luminous range is equal to the nominal load, which is 12Nm. This is because the nominal range applies at 10Nm visibility.
Example 1: Luminous range
Suppose a lighthouse has a nominal range of 18Nm. The visibility is 20Nm. What is the luminous range?
Answer: 30Nm
Example 2: Luminous range
Suppose you want to use a light to make a bearing and need a luminous range of 5Nm to do so. The nominal range of the light is 6Nm. What is the minimum visibility you need to make a bearing on the lighthouse?
Answer: minimum 7.5 Nm
Compass bearings to stay clear of hazards
Background bearing
For example: if you just can no longer see between two islands, you know which line in the chart you are on. A light line or guide line is also a kind of background bearing. So if you see on the chart that it is safe sailing on a line of 2 points that are in line with each other, you can use that background bearing to stay clear of hazards.
NMT or NLT
We can also draw a bearing line on 1 object in the chart that separates a safe and a hazardous sector. We then determine whether the bearing
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- should not be more than x degrees (No more than)
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- should not be less than x degrees (no less than)
To stay in safe waters. We note NMT or NLT at the bearing line in the chart.
