Navigation and Seamanship (1+1)

Procedure

1. Understand the working principle of magnetic compass

2. Study the construction aspect of magnetic compass

3. Study the construction aspects of both wet card and dry card compass

4. Observe the following parts of compass

The Card base:

Aluminium ring is attached to brass hub centre piece by a silk cord. Silk cord doesn’t shrink or stretch due to moisture or changes in atmospheric temperature.

The Card support:

The bottom of the hub has a small cap fitted with a sapphire bearing surface.

The Card:

The compass card is made of rice paper because it is very light and unaffected by changes in temperature. The card is made of mica for wet card compass.

The Directive element:

Two or three pairs of needle magnets

The Compass bowl:

A glass hemisphere containing a mixture of alcohol and water in the case of wet card and empty in the case of dry card compass . In wet card compass, oscillations are damped without loss of accuracy by immersing the card in a light. The card therefore has a dead beat movement.

The Lubber line:

Inside the compass bowl, there is small projection with a line marked on it. This line is called “lubber line” and it represents the direction of the ship’s head.

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 Binnacle:

The binnacle is a cylindrical non-magnetic container made of teakwood and brass.

The Helmet:

The top of the binnacle is provided with a large brass helmet.