A Simplified Guide to Small Marine Craft Navigation.

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Chapter 5 (v.1) - Indicating Direction at Sea.

Submitted: December 03, 2016

Reads: 198

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Submitted: December 03, 2016

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Indicating Direction at Sea.

 

To the navigator of a craft out on the open sea with no landfall in sight, the horizon is a circle drawn about the vessel he is on, and there is nothing to distinguish one point on that circle from another. Before a navigator can take his vessel from say, New York to Port Florida, he must first ascertain the direction in which Port Florida lies from the New York, so that by steering in that direction, he may arrive at Port Florida.

Direction is thus determined by the point on the horizon on which a place lies, or towards which a vessel is moving.

A distinction is made in navigation between these two aspects of direction. The direction in which a place or point of reference lies is called the bearing of the place or point. The direction in which a vessel moves in still water is the direction of her fore-and-aft line, which is called the course of the vessel.

Although the course of the vessel may well be the same as the bearing of her destination, this distinction is necessary because it is frequently required to indicate the direction of places, points, even other vessels or floating objects, other than the vessel's destination.

Thus, a vessel's course may be East when Port Florida bears South and a large bulk carrier on the port beam bears North from the vessel. Three directions have been expressed, one the vessel's course and the other two bearings from the vessel.

Those new to navigating often confuse these important navigational terms so it is essential to be quite clear from the outset as to their meaning:

The COURSE of a vessel is the direction indicated by the projection of the vessel's fore-and-aft line in the direction in which she is proceeding, or required to proceed.

A BEARING is the direction in which a place, point or object lies with reference to the vessel.

More precise definitions of Course and Bearing will be given later, but the above will suffice for this preliminary discussion of Direction.

It was stated earlier that the four directions, North, South, East and West, abbreviated to N., S., E., and W., respectively, are the cardinal directions. All other directions can be referred to the two adjacent cardinal directions; for example North-West, or NW, is the direction midway between North and West, or 45° to the left of North; North North-West (or NNW) is the direction midway between the directions of North and North-West, or 22%° to the left of North, see illustration 1.

illustration 1

 

However, in modern navigational practice this method of referring to direction by naming points has been largely superseded by the three-figure notation system by which any direction can be described by angular measurement relative to the direction of North.

If a circular protractor, see illustration 2, is placed so that the 0° mark points towards the North Pole, the graduations will indicate the true direction as measured from the center of the protractor. Under  the three-figure notation system, if you are looking towards the North Pole you are said to be facing due North or zero degrees true , 000°T. If you turn to your right through an angle of ninety degrees, you will face due East or 090°T. If you turn a further 90° to the right you will face due South or 180°T.; turning a further 90° to the right brings you to face due West or 270°T., and turning yet another 90° will bring you back to face due North again.

You will thus have turned through 360° in a clockwise direction, every degree of which can be used to indicate a specific direction. For instance, the direction 229° True lies between 180° and 270° and therefore between South and West; the direction 053° True is between 000° and 090° and therefore lies between North and East, and so on.

It is important to appreciate that although angles in general can be referred to as, say, 45° or 90°, when they are indicating direction in the three-figure notation system there must be three figures, hence NE is written 045° and not 45°., and E is written as 090° and not 90°.

 

illustration 2


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