SHORT NOTE ON MAGNETIC COMPASS

INTRODUCTION

  • The magnetic compass is fitted on the upper bridge(also called the monkey island). And it is placed exactly on the centerline of the ship.
  • It is referred to as the standard compass because it is considered the primary means of direction indication.
  • There are two types Dry card and Wet card.
    • The dry card was used in the olden days, But it was too sensitive for steering so now we have wet card. Now dry card is not found on any merchant ship.

WET CARD COMPASS

The wet card is made of mica and is only about 15 cm in diameter. Graduations are printed on it. In Modern wet card compasses, the directive element is a ring magnet fitted around the base of the float. Ring magnet is used as it provides less resistance and turbulence. By immersing the card in liquid, oscillation caused by vibration, Rolling and pitching are damped.

THE LIQUID

The bowl is filled with a mixture of distilled water and pure ethyl alcohol so that the mixture has following properties.

  • Low freezing point about -30’C
  • Small coefficient of expansion.
  • Low relative density about 0.93
  • Does not discolor the card.

THE BOWL

Wet card is about 15cm in diameter, The bowl is 23cm so that the card rotates comfortably. The top of bowl is transparent glass. The bottom is of frosted glass to diffuse the light coming from the bulb below.

The wet compass bowl -image credits cruising.coastalboating.net

What is the Lubber line in the above image?

On the inside part of the bowl, there is a small projection with a line marked on it. This line is called the Lubber line and it represents the direction of ship’s head.

THE BINNACLE

It is a cylindrical container made of non-ferrous metal. Olden days teak wood was used.

  • The Bowl is slung inside the top portion of the binnacle. The middle portion is accessible by a door and contains corrector magnets.
  • Corrector magnets – There are a number of horizontal holes, both fore and aft and athwart for hard iron or permanent corrector magnets which are used to adjust undesirable, disturbing magnetic effects caused by ships steel hull or other metal body onboard. This correction is done by qualified compass adjuster.
image credits – owaysonline.com

Quadrantal Corrector – There are two soft iron spheres that are fitted in brackets, one on either side of the binnacle. This can be adjusted by sliding it towards or away to get the desired compass adjustment.

The helmet is just a cover made up of non-ferrous metal to provide protection to the compass bowl from sunlight, rain, and spray. Generally, it is also covered with canvas in practical onboard vessels.

Care and maintenance of Magnetic compass

  1. The door to access the corrector magnets should always be closed and keys must be kept in safe custody.
  2. Soft iron sphere (Quadrantal Corrector ) and brackets should be painted to prevent rusting.
  3. The Brass part of binnacle should be polished regulary.
  4. Magnetic material, Electric machinery, Electric wires should be kept away from the compass.
  5. Helmet should be closed when not needed.
  6. In rare case, A bubble may develop in the wet comapss bowl. This has to be removed at earliest opportunity.

Explain importance of Taking & Recording Compass Error?

The compass error should be checked each watch and on every new course to measure the error of the gyrocompass, which is the angle the gyro north makes with the True north to establish trends in deviation on different courses at varying latitudes.

To ensure a compass is in good working condition, it is important to check performance of magnetic compasses particularly after: a vessel has been operating on short voyages for a long period of time then relocates, which results in a large change in magnetic latitude