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Coastal navigation

Limiting course – which course can the other vessel be on?

At night you often see nothing of another vessel but its lights. The combination of visible navigation lights and your own true bearing to the vessel gives a fixed sector of possible courses. NauticTools2 works out the two limiting courses and shows the sector in a plot.

• Navigation lights
• Sidelight · sternlight
• True bearing
• Course sector
• Plot
• Slider

The six combinations of lights

Sidelights show 112.5° each from right ahead, the sternlight 135° astern. Their overlaps give six combinations, each with a fixed sector width:

• red and green – 6°
• green only – 104.5°
• green and white – 10°
• white only – 125°
• red and white – 10°
• red only – 104.5°

How to use it

1. Choose the visible lights

   Select the combination you can see on the other vessel.

2. Enter the bearing

   Enter your true bearing to the other vessel.

3. Calculate

   NauticTools2 gives the 1st and 2nd limiting course and draws the sector. The true course lies somewhere in between; for a sailing vessel the wind direction can narrow it down further.

4. Move your own position

   The slider below the plot moves your own position around the other vessel – the bearing runs from 0° to 360°. The limiting courses and the plot update immediately while you drag, and the − and + buttons move it by 1° at a time. This makes it easy to see how the possible course sector turns with the bearing.

Example

Related tools

• Bearing Conversion – from relative bearing to true bearing
• Course Conversion – from compass course to course over ground