Coastal Navigation, How to Double the Angle on the Bow

The most useful technique anybody making a coastal passage should learn is how to double the angle on the bow. If your response to my statement is: "Why should I do that? I've got a GPS." Think again! Not only is there sense in knowing the basic techniques of navigation - you might well need them in an emergency - but there's great satisfaction.

Doubling the angle on the bow will show you a position when you have only one object such as a headland, a tree on a headland, or something of that kind to use.

The system is to take a bearing on the desired object when it reads a whole angle, say 40 or 30 or 20. You will be somewhere along that position line. As you take the bearing, note the boat's speed and course and the log reading.

When the angle doubles to 80 or 60 or 40, take another bearing to the same object and plot it on the chart. At the same time or very quickly afterwards, read the log again.

You now know how far the boat has travelled and in what direction. That can be plotted off from any point along the first position line. The line should be drawn only as long as the distance travelled, and the second position line then transferred to the end of the line marking the vessel's position by dead reckoning. Transferring simply means drawing a line parallel to the observed bearing through the dead reckoning position.

If, during the time between the first and second observation, some observation has been made to establish whether the boat is being affected by current, that should be taken into account in plotting the dead reckoning position. So if a current of say one knot from dead ahead is established, then the amount of foul current suffered during the time between the two bearings must be calculated.

For example, if the time was 45 minutes and you're travelling at four knots, then the boat will have travelled three nautical miles between the two positions, but it will also have been hindered by the current, which in 45 minutes at a rate of one knot will have held the boat up by three-quarters of a nautical mile. So the actual position will be three-quarters of a nautical mile further back along the direction of travel. This is where the transferred position line must cut and this will be the boat's position at the time of the second bearing.

If you show your crew how to do it, it will help them pass the time and give them a sense of achievement. It may also stop them from asking the otherwise inevitable questions: "Where are we?" and "Are we nearly there?"


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