9 NAVIGATION The compass
The first written reference to a magnetic compass goes back nearly a thousand years and there’s good reason to believe they were in use well before then.
The basic compass is a very simple device. A magnetised needle is allowed to pivot freely and will align itself with the Earth’s magnetic field – more or less north and south. Unfortunately, ‘more or less’, isn’t really good enough for navigation. The compass is pointing to the magnetic north pole, currently located (it moves slowly with time) about 600 miles from the true north pole. This produces an error known as ‘variation’. Variation isn’t constant but differs from place to place. The amount for each locality (and its rate of change, usually small enough to ignore) is noted on your chart’s 360° ‘compass rose’.
Another form of error is called ‘deviation’. This is caused by magnetic disturbances caused by the boat itself: cast iron engine blocks, electrical equipment and so on. Steel boats have a particular problem. It’s possible to measure the deviation by ‘swinging the compass’ but most of us simply minimise its effects by mounting the compass as far away as is practicable from any object that might disturb it. Incidentally, electronic fluxgate type compasses can identify deviation for themselves – a very useful procedure. Whatever the source of the error, corrections must be made before we can truly know what course we are steering. An easily remembered rule can help us here – based on a single word:
CADET
This reminds us that when converting from compass (C) to true (T) we have to add (AD) east (E) – and of course west must be subtracted.
BASIC CHARTWORK
A common belief is that charts are only for offshore vessels. Nothing could be further from the truth. A ship crossing an ocean could navigate between two points without referring to a chart at all – there’s nothing out there, after all. It’s inshore that we need the most information. The purpose of charts is twofold. Firstly, they warn us of any dangers and, secondly, they allow us to practise the art of navigation – which we can think of as travelling safely from one place to another. As a bonus to anglers, charts also show us where wrecks, shallows and other good fishing spots are to be found.
Position finding
Much of navigation relies on knowing where we are. Although these days there’s a growing reliance on GPS (Global Positioning System) it’s still important that we don’t depend on it entirely. The traditional method
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Fig 9:1
RYA Seamanship for Sea Anglers
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