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Chapter 25
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Navigation Instruments

Dividers

   Dividers are used for measuring or marking off distance on a chart.  They are usually bronze and should be rust proof.

Parallel Rules

   These rules are used to transfer course lines from the compass rose on a chart.  They can be difficult to use in the restricted space of a small chart table.

Set Squares

   A pair of set squares fitted with large handles for easy movement are often more convenient than parallel rules for the transfer of course lines.

Plotter

   Most yachts carry a plotter, which is usually made up of a compass rose etched into a transparent square, with a straight edge which swings about the centre of the compass rose.  This is a simple and convenient means of plotting courses and bearings.

Log

   Most yachts are fitted with mechanical or electronic logs which provide distance and speed, derived from the rate of revolution of a hull mounted impeller or sensor.

Lead Line

   This consists of a weighted line marked at intervals to show depths.  It is now almost entirely replaced by the electronic echo sounder, which is quicker, easier and more accurate, although a good skipper will check his echo sounder with a lead line from time to time.

Deck Log

   The information contained in the yacht's Deck Log is a most important and useful aid to navigation, for examples: the hourly record of barometric pressure will give indication of weather tendencies, a record of course and distance run is essential information.

Steering Compass

   Every yacht has a steering compass, normally fixed in a position easily visible to the helmsman.  Deviation and Variation errors are explained in the section on chartwork.

Hand Held Compass

   Most instruments listed in this section need no further explanation, but obtaining accurate bearings using the hand held compass is so fundamental to safe navigation that its properties and use are described in more detail.  Most position lines drawn on a chart are obtained using this instrument.  It should be treated with care.

   Many styles of hand held compass can be obtained.  All are similar, and simple to use, and most can be read at night by means of built in illumination.

   The compass normally consists of a body enclosing a damped compass card marked in divisions of 1o with numerical notation every 5o.  It has marks or grooves whereby an observer can align the compass body with an object on which a bearing is to be taken, and at the same time read off the bearing from the card, usually through a prism.

   Practice in use of the compass is essential if accurate bearings are to be obtained.  Errors result from:

    Trying to take bearings to quickly.

    Not holding the compass body level.

    Not aligning the compass body properly in relation to the line of sight.

    Allowing metal objects - rigging, radios, contents of pockets - to deflect the card.

 THE HAND HELD COMPASS

Ch 25 01

Horizontal, steady and properly aligned.

 

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