A Simplified Guide to Small Marine Craft Navigation.

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Chapter 21 (v.1) - Required Chart Instruments for Navigation.

Submitted: May 15, 2017

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Submitted: May 15, 2017

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Required Chart Instruments for Navigation.

 

 

The Parallel rule, or one of its substitutes, is essential an instrumen for measuring and laying-off courses and bearings on a chart. Initially the budding navigator will require only one of the following types of parallel rule, and the best plan is to choose one and stick with it, because a navigator will be less likely to make errors if they become thoroughly familiar with one instrument and its use, rather than if they  chop and change between a selection of them. Experience is the only guide in making a final individual decision as to which one to use.

A parallel rule, or a substitute thereof, is made from either transparent acrylic, which is preferable, boxwood or brass, and its detailed use will be described later in the guide.

Expanding or Bar-Type Parallel Rule comes in four lengths 15”, 18", 21” and 24”. The ideal size for small marine craft navigational work is the “18" version, although the 15” size may be necessary for very small chart tables with little or no elbowroom.  The Parallel Rule is placed flat on the chart and allows courses or bearings to be transferred across the chart by opening out the hinges and then ”walking“ the two halves of the rule over the chart.

Parallel Rules may be plain but a variant titled “Captain Field's Improved Parallel Rule” is an excellent type. This is engraved with degree marks along its edges which enables the navigator to dispense with the compass rose on the chart, or a protractor.

Roller-Type Parallel Rule is probably the easiest type to use, but suffers from the disadvantage that it is very prone to roll off the chart table altogether if not returned to its rack immediately after use. Once more, the 18” or 15" sizes are the most suitable for small marine craft.

The Roller Rule is a wide straight edge fitted with a roller at each end. The rollers have nonskid knurling and make it possible to slide the rule across the chart so that it remains parallel to its original position thus enabling lines to be transferred parallel from any position on the chart.

The combined Protractor and Parallel Rule, is a square protractor available in two sizes, 5” and 10", of which the latter size is the one most recommended. It is transparent and graduated from 000 to 359 both clockwise and anticlockwise. Its underside has a matt surface on which pencil lines can be drawn. It can be used instead of a parallel rule for laying-off and reading courses and bearings, but tends to be a little cumbersome and time-consuming in its use.

 

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Protractor. This is an instrument of which there are many available variants, and is for drawing angles of a specific size, or to draw a line in a given direction on a chart

Dividers are required for measuring distances and scales on a chart, and to avoid frustration when navigating they should be at least 7" long.

Pencil-compasses, at least 6” long, are required for marking off ranges of lights and for general chart-plotting work. It pays to obtain a good pair with soft pencil-leads incorporated, rather than the school-type into which an ordinary pencil is inserted, as they are unsuitable for accurate navigational plotting.

Soft pencils only must be used for chart work, as they make for easy erasure all lines and marks, drawn lightly, after use.

Soft India rubber eraser, should be used to clean off a chart after use, for good quality charts are expensive, and when properly cared for they can be used over and over again.

Pencil sharpener. A quality version and a spare should always be ready to hand since soft pencils break easily and frequently need re-sharpening.

Chart magnifier. Preferably, a hand-held magnifying glass and not a frame mounted glass, which is used for reading the small details printed in minute size on many charts.

Chart weights. Small paperweights used for holding down the corners of charts and keeping them flat

Charts.  As with land maps, they are classed as the most important instrument of navigation and as such require the most careful care. Importantly, again as with land maps, only fold charts in their original folds; however, they could if a situation demands it be folded in reverse with the printed sides out and by only using the original folds.

If it is necessary to reduce the size of charts because of the limitations of a chart table or navigation space, roll the charts from the top or from the side with each chart lightly tied with a ribbon style binding to stop it from springing out. A navigator should never store charts rolled up together in a single bundle.

 

 


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