Compasses and sextant

Question 1

When should the Magnetic Compass be adjusted?

Answer 1

Magnetic Compass – Adjustment

• Adjust the magnetic compass when it is first installed.
• If the compass becomes un-reliable.
• Any structural repairs or alterations to the ship may affect the vessel’s permanent and induced magnetism. Adjust the compass on completion of repairs or alterations.
• If any electrical or magnetic equipment has been fitted near to the compass, adjust the compass.
• If more than two years have passed since the last compass adjustment took placed (and a record of compass deviations has not been maintained)

Question 2

When should Performance checks be carried out on the magnetic compass?

Answer 2

Magnetic Compass – Performance Checks

The performance of the magnetic compass should be checked:
• After the vessel has carried a cargo with magnetic properties.
• If any electromagnetic lifting appliances have been used to load or discharge the vessel.
• If the ship has been involved in a collision of incident, which may have caused major structural damage.
If the vessel has been laid up for a period of time.

Question 3

What are the regular operational checks you should carry out on the magnetic compass?

Answer 3

Magnetic Compass – Regular Operational Checks

• Check the gimbal is moving freely.
• Check the compass card is clear and sharp. Card should float freely and rotate without any friction.
• Check the liquid is free of bubbles and clear. No liquid leaks around the seals or filler plugs.

Question 5

How would you monitor the performance of the magnetic compass?

Answer 5

Magnetic Compass – Monitoring

• Record compass deviations in the Compass Deviation Book
• Take a compass error after every large alteration of course.
• Record a compass error at least once a watch.
• The purpose of taking a compass error is to identify any excessive deviations, which may indicate the need for repair.

Question 6

Which vessels are required to have Gyro Compass repeater?

Answer 6

All ships over 500gt should have a heading repeater at the helm and emergency steering positions, and, as far as possible for vessels under 1600gt, shall be fitted with a bearing repeater capable of 360 degrees

Question 7

What are the corrections for the magnetic compass?

Answer 7

Magnetic Compass Corrections

Flinders Bar – Vertical soft iron corrector. A soft iron bar correcting for vertical soft iron abaft of the compass. Most common vertical soft iron on the ship is the funnels.
Spheres – Quadrantal soft iron correctors. The Flinders Bar produces a small quantity of fore and aft, horizontal and vertical effect on the compass. The soft iron spheres correct for these errors.
Heeling Error – Compensated by permanent magnets set vertically in a bucket beneath the compass.
Horizontal Magnets – Permanent fore and aft corrector magnets. Permanent athwartships corrector magents.

Question 10

Describe the Error of Perpendicularity found in the Sextant and how to correct it.

Answer 10

Perpendicularity

The Index Mirror is not perpendicular to the plane of the instrument.

To correct: Hold the sextant horizontally, arc away from the observer’s body. Set the index bar half way along the arc. Look into the index mirror so that the ‘true’ image of the arc and the reflected inamge can be seen. The ‘true; image of the arc and the reflected images should be in alignment. If they are not in alignment, there is an error of perpendicularity. Correct using the index mirror screw.

Question 11

What regular checks should be made of the Gyro Compass?

Answer 11

Gyro Compass Checks

• The datum line at the base of the repeaters is aligned with the fore and aft axis of the vessel. Perform any realignment before the ship sails.
• All repeaters should be within 0.50 of the gyrocompass.
• Take a compass error and record it in the compass error book at least once a watch.
• When the vessel is on passage, correct the gyro for latitude and speed error.

Question 12

Describe the Index Error found in the Sextant and how to correct for it.

Answer 12

Index Error

The Index Mirror and Horizon Mirror are not parallel to each other. It is the error remaining after perpendicularity and side error have been corrected for.

To correct: set the instrument to zero. Look at the horizon. Adjust the sextant reading so both images of the horizon come into line. The sextant reading when the horizon is in line is the Index Error. A positive index error should be subtracted and a negative error should be subtracted at the start of your calculations.

Question 13

Describe the Side Error found in the Sextant and how to correct it.

Answer 13

Side Error

The Horizon Mirror is not perpendicular to the plane of the instrument.

To correct: hold the sextant vertically. Observe the altitude of a celestial body. Adjust so that the reflected and the direct images of the horizon appear as a continuous line across the horizon glass. Tilt the sextant. If the reflected portion appears above of below the direct part the glass is not perpendicular. Adjust using the two screws behind the horizon glass.

Question 14

What are the Non-Correctable Errors found in the Sextant?

Answer 14

Non-Correctable Errors of the Sextant

Prismatic – Two faces of the mirror not parallel to each other,
Worm & Rack – Caused by wear on the gearing rack
Graduation – May be found on the arc, Vernier of micrometre scales.
Shade Error – Faces of the shades not parallel to the ground.
Collimation – Axis of telescope not parallel to the plane of the sextant.
Centering – Pivot of the index arm is not in the exact centre of the curvature of the arc.