Navigation and Seamanship

2.1.7. Construction of Magnetic Compass

Based on the construction the magnetic compass can be divided in to two types. They are

1) Dry card

2) wet card

Dry card compass

The card base

           A light weight aluminum ring with 254 mm dia. is attached to a hub by a silk cord (like cycle wheel with spokes). Silk cord is used because it does not shrink or stretch due to moisture or changes in atmospheric temperature. The compass card is made of rice paper because it is very light and unaffected by changes in temperature.

The card support

           The bottom of the hub is attached with a small cap fitted with a sapphire. This arrangement provides a practically frictionless support for the compass card and requires no lubrication. This rests on a pivot tipped with iridium which is a hard non magnetic metal

The directive element (magnets)

           The magnets are attached to the card base, that ensures the card is not only north seeking but also remain horizontal even when ship tilts with rolling / pitching. It consists of 3, 4, 6 or 5 pairs of needle magnets symmetrically fixed.

The compass bowl

             It is a cylindrical brass bowl having a transparent glass top.

The lubber line

            There is a small line marked on the inner rim of the compass bowl. This line is called Lubber line and it represents the direction of the ships head. If the compass is fitted on the fore and aft centre line of the ship the lubber line shows the direction of the ship’s heading. The reading of the compass card, which coincides with the rubber line, is the compass course of the ship.

Suspension of bowl

              On the outside of the compass bowl there are 2 projections, called gimbals at the same level as the compass card. If the ship rolls or pitches, the bowl would remain horizontal because its centre of gravity below gimbals. The gimbals are triangular in cross-section apex downwards. These gimbals rest on ‘V’ shaped depressions in a horizontal ring called the gimbals ring which encircles the compass bowl. The gimbals ring itself is pivoted at its forward and after sides. The top of the verge ring is grooved to accommodate the azimuth mirror.

The Binnacle

The binnacle is a cylindrical container(a protection cover) made of teakwood and brass. Non-magnetic materials are used in its construction. The compass bawl is swing inside the top portion of binnacle.

The helmet

Large brass helmet placed on the top of the binnacle is called helmet. This protects the compass bowl from direct sunlight, rain, spray, dew, frost etc.