Compasses

Question 1

How to check magnetic compass error with a transit

Answer 1

Establish true bearing of transit from chart

Take compass bearing of transit with Polaris or azimuth ring
Difference equals compass error

Determine variation and subtract from error

Remaining difference is the Deviation for that particular ships heading.

Compare to deviation card and record in compass observations book.

Question 2

How often should the compass be checked

Answer 2

Substantial course change
At least every watch or 4 hours
Pre departure and arrival
Deviations after being calculated need to be recorded in the compass deviation book

Question 3

When must a compass be swung

Answer 3

When installed
Every 2 years
If there have been any major modifications done to the vessel
If Electrical or magnetic equipment has been installed or removed in the vicinity of the compass.
If the vessel has been in a collision or fire
Compass shows physical defects

Question 4

Where are compass checks recorded

Answer 4

Compass observation book

Question 5

What will happen if the gyro fails

Answer 5

• Radar will loose heading and will have to revert to either ground stabilized with gps input or to heads up display.
• ECDIS will Loose heading information
• Auto pilot

Question 6

What is permanent magnetism, induced magnetism and retentive magnetism.

Answer 6

Permanent Magnetism: Is caused by the hard iron component and is acquired during the vessels construction. It remains constant through the life of the vessel. It is corrected on a compass by placing permanent magnets around the compass aligned fore and aft, athwart ships and vertically.(Fore and Aft correctors, Athwartship correctors and Heeling bucket)

Induced magnetism: It is caused by the soft iron component in the vessel. This changes whenever the vessel changes course or position especially when large changes in Latitude. The effect is lessoned in a compass by placing soft iron correctors around the compass. Two spheres (kelvins balls) placed athwart ships either side and a flinders bar vertically orientated on the fore end of the compass.

Retentive magnetism: created when a vessel spends long periods on same course. Retentive magnetism is temporary and may remain from a few minutes up to a few days. May give rise to unusually large deviations.

Question 7

Checks of a magnetic compass

Answer 7

1. Freedom of movement of gimbal
2. Compass card floats freely and level and rotating smoothly
3. Compass card legible and not distorted
4. No bubbles
5. No Leaks
6. Optical system correctly adjusted and clean
7. No. Magnetic equipment near the compass.
8. Compass deviation card readily available on display
9. Compass observation book stored in an easily accessible location and up to date.

Question 8

What is on the compass deviation card

Answer 8

1. Uncorrectable residual deviation on set headings.
2. Location of correcting magnets
3. Date, time and location of Compass adjustment
4. Signature of compass adjuster

Question 9

What are the required inputs into a gyro compass

Answer 9

Lat

SOG

Question 10

Actions if gyro not working

Answer 10

Silence alarm
Hand steering
Switch to magnetic compass
Check connections
Notify DPA
Non- conformity
ECDIS, RADAR, Autopilot and AIS will have error.
Log Incident

Question 11

What to check if notified of inconsistent deviation reading

Answer 11

• Check Calculations
• Physical inspection:
  Damage
  Electrical equipment,
  Check corrector magnets in place according to deviation card
  Lubber line still aligned
  Compass card pin
• Observations book: Look for a trend
• Magnetic anomalies
• Raise non-conformity
• Notify DPA
• Notify flag, dispensation

Question 12

What is true north

Answer 12

The point around which the earth rotates at the North Pole

Question 13

What is magnetic north

Answer 13

The point at which the earths magnetic fields exit and enter the earth. Current location

Question 14

What is retention error

Answer 14

Temporary magnetism retained by the ships structure when on a long voyage on the same heading for a long period.
Can remain for a few minute to a few days.
To rectify, short shake down on different headings will rectify

Question 15

How often is Maintenace required on a gyro

Answer 15

According to manufacturers specs
Sherry marine 18 months
Others 1 year

At annual service, fluids changed, inner surface cleaned, cantering pin changed if necessary

Five year:
Gyro-sphere and cantering pin changed

Question 16

Operational check of a gyro compass

Answer 16

• All repeaters in measure same heading as master. +- 0.5 deg out
• All repeater aligned whit fore and aft centre line
• Check compass error by most appropriate method
• Check all mechanical parts and structure of repeaters
• If alignments are out repairs must be carried out prior to sailing.

Question 17

How to check compass by amplitude of the sun

Answer 17

Gyro or Compass bearing of Sun at sunrise or sunset

Must be half diameter above horizon

Get Declination of Sun for that day. To the closes degree

Take ships Lat from GPS

Norries, True azimuth and rising and setting

Line up Dec and Lat to find Amplitude (true bearing) for that day.
Compare to compass and calculate error

Question 18

Compass check by azimuth of a celestial body

Answer 18

Compass bearing at UTC
Day tables
GHA and Dec
GHA - Long = LHA
LHA + Dec + Lat into Sight reduction tables (LHA greater than 180 then Zn=Z or LHA less than 180 then Zn= 360-z)
Azimuth of the sun
Compare against compass bearing.