Magnetism & Compass Flashcards
Examples of permanent magnetism?
The steel of which ship is built of. Hull, ships structure, bulkheads, cranes, engines, anchors
Examples of electro magnetism?
Added machinery, speakers, instruments
What is variation?
The angular difference between True north and Magnetic north
When should the magnetic compass be adjusted? (5 Factors)
- Every two years the compass must be swung and adjusted by a compass adjuster
- When major yard work has been conducted which may have changed the permanent magnetism
- If new equipment has been added
- Damage has been done to the vessel
- If a noticeable change has been seen
What is deviation?
The angular difference between Magnetic north and Compass north
What are permanent magnets used for?
To counteract ship’s permanent magnetism
How do we counteract Induced magnetism?
Soft Iron
Horizontal magnetism = Kelvin balls
Vertical magnetism = Flinders bar
Which solas chapter and annex is magnetic compasses?
Solas chapter 5. Annex 13
Name some reasons for a change in deviation in the long term? (2 factors)
Refit period. If major works are undertaken causing vibrations to the hull.
Long voyages on repeated heading
Reasons for a change in deviation in the short term? (4 factors)
- Changes of electrical equipment.
2 .Heavy shocks to vessel- pounding. - Lightning strikes
- If lubber line is incorrectly aligned.
What does a fluxgate compass do?
Detects the Earth’s magnetic field electronically.
Advantages of fluxgate compass? (4 factors)
- Small with no moving parts
- Self correcting
- Transmits data to other units.
- Can be fitted anywhere.
Disadvantages of a fluxgate compass? (2 factors)
- Must remain horizontal so must be fitted with gimbals/ additional vertical correctors.
- Requires power to drive the reference coils.
What is induced magnetism?
The short temporary effect on the compass due the heading or temporary changes due to i.e cargo
How do we counteract permanent magnetism? (3 factors)
Hardcore magnets
1. Heeling error corrector magnets on a pendulum
2. Fore-and-aft corrector magnets
3. Athwartship corrector magnets
List places where we can find value of variation?
- Compass rose adjusted for year
- Interrogate ENC
- Variation charts
- Compass error
When shall the OOW check the compass error?
Before departement and At least once every watch
How does a gyroscope work?
Free spinning gyro allowed 3° of movement damped in tilt (immersed in gyro fluid) allows gyro to stay fixed to true north
What gyro errors are there?
- Speed error
If the vessel accelerates or changes course aggressively this can cause the gyro to slightly deflect from its spin axis - Latitude error
A deflection of the gyro off its spin axis depending on the latitude (and therefore how fast the earth is rotating beneath the vessel)
Which input does an gyro have to improve accuracy?
GPS
What is a Pelorus?
A method of taking bearings relative to ship’s head or to the compass via the brass wheel
What is an Azimuth mirror?
Goes on top of the compass and lets us take bearings
What are the operational- / pre departure- checks of a magnetic compass? (9)
Freedom of movement of the gimbal
Card floating freely and level, and rotating without any friction
Compass card clear and sharp with no distortion or discolouration
Liquid clear and free of bubbles
No liquid leaks around seals or filler plugs
Optical system adjusted and clean
Azimuth reading devices and means of illumination in working order
No tools or speakers close to the compass
The compass error log book and deviation card for each magnetic compass installed should be checked
What are the operational-checks of a gyro compass? (8)
Calibration
Check for errors
Cleaning and lubrication
Power supply and backup
Mechanical alignment
Regular inspection
Routine testing
Record keeping
What is on the deviation card? (9)
Location and date of swing
Name and CoC of adjuster
Name and CoC of Master
Name of Harbour Master
Location of all correctors
Residual Deviation
Flinders Bar
Heeling Magnets
Information to re-create the swing
