A Simplified Guide to Small Marine Craft Navigation.

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Chapter 6 (v.1) - Understanding The Marine Compass.

Submitted: December 05, 2016

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Submitted: December 05, 2016




Understanding The Marine Compass.


In order to refer to or proceed in a particular direction at sea it is necessary to refer to some datum line or fixed direction. The most convenient datum is the North-South line through the vessel's position and the Mariner's Compass is the instrument that provides this datum line for reference. It can be one of two different types, either a gyrocompass or a magnetic compass.

The principles on which each of these compasses works are entirely different. The gyrocompass is a sophisticated piece of electronic and mechanical activity based on the directive properties of a gyroscope, and it is very accurate, indicating the direction of the geographical or true North Pole but its dependence on electric power and its very high price make it impractical for the vast majority of small craft.

The magnetic compass, is a comparatively simple mechanical device which makes use of the directive properties of the Earth's magnetism; it is not so accurate as a gyrocompass and indicates a direction known as Compass North, which is not always the same as True North and for which navigators must therefore make due allowances.

Where and when a magnetic compass was first used is not known.  Early compasses were somewhat primitive and crude and it was not until 1750 that Dr. Gowan Knight made what could be claimed as the first scientific compass. The bulk of research and compass improvement belongs to the 19th Century, and particularly to the illustrious Sir William Thomson, later Lord Kelvin, whose compass was a precision instrument that has remained essentially unchanged to the present day.

It had been found in very early times that small pieces of certain iron ores had the property when freely suspended or pivoted of settling with one axis in a particular direction. This indicated not only that such ores were natural magnets, but that the Earth itself has the properties of a huge natural magnet.

The magnetic field of the Earth is similar to that which would be produced by a short magnet situated near the Earth's center with its axis passing through the neighborhood of Hudson's Bay in the North of Canada and South Victoria Land in Antarctica, these points being known as the Earth's magnetic poles. They do not coincide with the true North and South poles of the Earth.

If a magnetized needle is poised horizontally on a pivot, the needle will swing back and forth and finally settle at rest in one direction. If deflected, it will swing back and forth and then return to rest in this same direction no matter how many times the needle is deflected. This direction will be found to be approximately North, but it is not as a rule the True North because the Earth’s magnetic poles, to which the ends of the magnetized needle are attracted, do not coincide with the geographical or True North and South poles.

The direction in which the magnetized needle lies when affected solely by the Earth's magnetic force and no other disturbing influences is termed the magnetic meridian, and for the same reason as given above, the magnetic meridian will not necessarily coincide with the true meridian N.-S., line at any place.

If you attach to a magnetized needle freely poised as described above a circular card graduated in degrees and, or, points of the compass, so that the North andSouth points of the card lie exactly over the North and South ends of the needle then you will have the basis of the marine magnetic compass.  It is important to appreciate that in a marine compass the graduated card is attached to the compass needle, or needles and the two are free to rotate together. The needle does not pivot above a fixed card.

There are two principle types of magnetic compass, known as the dry card compass and liquid compasses respectively. Compasses for small craft are almost invariably of the liquid type because the rapid movement, jolting and vibration experienced by small craft in a seaway would be too severe for a dry card compass which is consequently more often found on large vessels.

The modern precision liquid compass consists of a system of magnetic needles ,usually two, attached to a graduated compass card made of mica so that they lie parallel to the North-South graduations on the card and equidistant from the N-S' line. The card is mounted on a central pivot within a bowl that is completely filled with a low freezing mixture of distilled water and alcohol.

The compass card almost but not quite floats, so that the friction between the card and the pivot is reduced to a minimum and the liquid dampens down most of the tendency of the card to oscillate from side to side when disturbed. The bowl is sealed but has a clear glass top through which to read the graduated card. Compensation for expansion and contraction of the liquid owing to temperature changes is incorporated into the construction of the bowl.

For steering purposes, a line or pointer is painted on the inside of the forward side of the compass bowl, in some makes the line may be represented by a black pin or wire, called the lubber line. The compass bowl, must always be carefully mounted in its bracket, its stand, pedestal, or binnacle so that a straight line from the center of the compass card to the lubber line is exactly in, or parallel to, the fore-and-aft line of the yacht.

The boat’s heading or compass course is read from the graduated compass card against the lubber line. So that the compass bowl will remain horizontal despite the heel and pitch of the boat, the bowl might be suspended in a brass ring arrangement called gimbals. In the simplest types of compass the gimbals are merely bracketed to some part of the boat's structure in a convenient position for the compass to be read for steering and, or, observing bearings.

Some yacht compasses have a transparent dome to the compass bowl which contains a gimbaled ring in which the compass card and its magnets are mounted, so that the bowl can then be mounted without any further provisions of gimbals. The domed bowl heels with the yacht, but a cage in which both the compass and the lubber line are mounted, can tilt the bowl to remain level at all times. An additional feature of this type of compass is that the dome magnifies the card, enabling it to be read from a distance.

Other types of compasses such as the sestral moore are designed for fitting at eyelevel, for example on top of a boats coach roof or cabin hatch. The compass card has a turned down circumferential edge, and two lubber lines instead of the usual one on the forward side of a compass. This compass can be viewed either from above, in which case the card graduations on the upper surface are read against the forward lubber line, or from eye level or below in which case the graduations on the turned-down edge which are slewed round 180° from those on the upper surface, and are read against the after lubber line.

The compass can be fitted with sight vanes so that compass bearings of distance objects can be taken by aligning the sight vanes on the on the turned-down edge, and which are slewed round 180° from those on the upper surface, are read against the after lubber line. The compass can be fitted with sight vanes so that compass bearings of distance objects can be taken by aligning the sight vanes on the object and reading the bearing on the flat upper surface of the card.

The illumination of a steering compass is necessary. Some compass cards are sufficiently marked with luminous paint, but if they are not then provision for an electric or paraffin oil light must be made. The light must be no more than a glimmer and preferably orange in color, otherwise the helmsman's night vision will be impaired. A small bulb controllable by a rheostat would be ideal.

Many navigators prefer the many refinements to compass design afforded when the bowl and gimbals are housed within a binnacle. The binnacle provides protection for the compass against damage, and is fitted with various hoods, magnifiers and lighting devices for easier compass reading and night use, and with provision for compensating the compass for errors caused by the boat‘s own magnetism.

 A reduced size binnacle compass for small marine craft is the Standard Binnacle and Admiralty Pattern Compass and the overall height of the binnacle is only 18 inches. and its diameter 7 inches. It has a domed removable hood fitted with a hinged front shutter, sliding nightshades, and a lifting handle. The compass bowl is suspended in knife edge gimbals and has a 3 inch diameter 0° - 360°card which can have a special reading device making the image for steering equal to that of a 6 inch card. The binnacle has sliding doors for access to electric lighting and corrector magnets.

It is important to remember, that any magnetic substance that is close to it can influence the functioning of a magnetic compass considerably so it is housed in, or mounted on, non-magnetic material, and any magnetic material must be kept as far away from the compass as practicable. For that same reason, anyone using the compass must first remove his knife, keys, watch of any other magnetic article he may have about him.

© Copyright 2019 Sergeant Walker. All rights reserved.


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