A Simplified Guide to Small Marine Craft Navigation.

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Chapter 9 (v.1) - Taking Compass Bearings.

Submitted: December 14, 2016

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Submitted: December 14, 2016

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Taking Compass Bearings.

 

There are many occasions when a navigator requires finding the compass bearing from his craft to one or more distant objects in order to determine the crafts position. Where a compass is sited high enough to provide all-round vision for fixing a position the means of reading the bearing against the compass card may be a sight vane of the sort found on the Moore compass. A more sophisticated sighting arrangement is the Azimuth Sight Ring, which consists of a circular stand that fits exactly round the rim of the compass bowl, around which the Azimuth Sight Ring is then hand rotated.

The optical arrangement consists of a glass prism, and that is turned about a horizontal axis by means of milled knobs on either side. An object on the horizon can then sighted across the top of the prism, which is adjusted to bring the reflection of the graduations on the compass card up to eye-level. In addition, observations can be taken of celestial bodies, and the Azimuth Sight Ring is provided with shades for use when observing the Sun.

Unfortunately, the location of a steering compass on a small marine craft is rarely suitable for that compass also to be used for observing bearings. Any bearing must be taken from a position where there is good all-round visibility, and unfortunately, the steering compass is usually located low down in the cockpit or wheelhouse. Where this is the case, a navigator has the choice of one-or other of two solutions: either a Hand Bearing Compass or a Pelorus.

A Hand-Bearing Compass, is as its name implies, it is a small easily portable compass, usually with a prism fixed over one edge. This prism is used in exactly the same way as in the Azimuth Sight Ring described above, to reflect and magnify the edge of the compass card, against which the bearing of any desired object can be read. The best quality instruments have liquid or oil-filled bowls, and cards graduated with either 1° or 2° divisions to allow accurate bearing sights to be taken.

For night use, hand-bearing compasses usually have either a self-contained flashlight with the battery in the handle, or glowing luminous material. When not being used, the handle type hand-bearing compass should always be housed in brackets that are made especially for it to be secured to a convenient bulkhead.

In addition, there are lightweight mini hand-bearing compasses available which the object to be viewed is done so through a hole in the edge of the compass. When correctly aligned, the scale on the compass card comes into view immediately below the object being sighted and can be read very accurately with the eye focussed at infinity, the same as the object. A further advantage of this type is that being small it can be worn around the neck, for which a sling is provided, or alternatively carried in the pocket.

The second solution to the bearing problem is a Pelorus, presumably named after the pilot of the ship that carried Hannibal away from Italy, and whose name was Pelorus. Its use is for the taking a bearing of an object which is obscured from view at the steering compass position.

A Pelorus is effectively a dummy compass in which the compass card is moved by hand instead of by magnetic forces. It consists of a brass plate or azimuth circle graduated as a normal compass in degrees and pivoted at the centre so that it can be rotated by hand, and a clamping screw is provided for locking the plate in any desired position. There is an outer ring, which carries a lubber mark and a set of sighting vanes is free to rotate either on this ring or on a central pivot.

To use a Pelorus, the instrument must be placed so that the lubber-line and central pivot are either exactly in the fore-and-aft line of the craft or exactly parallel to this line. A convenient place may be on top of the wheelhouse and permanent markings could be made here so that the Pelorus can be quickly aligned whenever required.

With the instrument correctly aligned to the fore-and-aft line, the base plate is then turned by hand until it reads the same as the steering compass. The object of which a bearing is required is sighted with the sight-vane while the helmsman calls out when he is exactly on the course to which the Pelorus has been set.

As both Steering Compass and Pelorus are reading the same heading, the bearing taken by Pelorus will be the same as if taken with the Steering Compass. If the helmsman is required to steer a new course the Pelorus base-plate is unlocked and realigned to the new heading then re-locked. When the helmsman reports he is exactly on his new course, the sight-vane is directed onto any required object and the Pelorus bearing taken and recorded as before.


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