Compass work Flashcards
Define variation
The angular difference between True North and the earths magnetic North. Named for East and West of True North
Define Deviation
The angular difference between magnetic North and Compass North. Affected by materials such as metal and electrical components on board ship, as well as the heading of the vessel . Named East or West of Magnetic North
What IS a magnetic compass?
A bar magnet freely suspended in the horizontal plane and acted upon by both the Earth’s Magnetic field and the magnetic properties of the ship
What does a compass adjuster do
Swings the ship around a 360 degree arc of the horizon, whilst taking observations.
As they do this, they adjust the correctors and make notes of the whatever deviation they are unable to correct for. This is produced on a compass deviation card which is kept in the bridge by the Compass
Four steps in finding compass error
- Take a compass bearing
- Calculate the True Bearing
- Find the difference between the two (Compass error)
- Label it east if lower than T, West if higher than True
Further steps necessary to check the deviation card is correct
- Take an observation and note the ships head
- Calculate and remove variation
- Remove this from the compass error to find the observed deviation
- Compare this with the DEV card for the same ships Head
How is a free gyroscope made into a gyroscope?
It is gravity controlled to make it North Seeking
Then Damped to settle on the Meridian
Steaming error with a gyro, What is it and How is this corrected?
Course/Latitude/Speed Error
Input of Latitude and speed for automatic correction
Other than using a transit to check gyro error, what may we use?
Rising and setting amplitudes of the sun
Two things which make for good watch keeping practice in keeping check on Compass performance
Comparing gyro and magnetic compasses
Taking Compass error
How often should we check our compass error?
At least once per watch
Or after a large alteration of course (20-30 degrees)
Coming into Port
Leaving Port
How can we check compass error?
Transits
Azimuth calculation ( Any time you see a celestial body) Longer calculation
Amplitude of the sun
Can only be done twice a day (sunset and sunrise)
Quick and easy calculation
What is declination?
The angle between the celestial and terrestrial equators
Always labelled North or south
What is found on a Compass Card?
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
Residual coefficients
Flinders Bar
Heeling Magnets
Information to re-create the swing
Where do we find Variation?
Variation Charts
Standard Navigational Charts (SNC)
GPS screen
ECDIS
In a magnetic Compass Binnacle, which magnets correct for Hard Iron (permanent magnetism)
Fore and aft magnets
Athwartships magnets
Heeling magnet (both Hard and Soft Iron)
In a magnetic Compass Binnacle, which magnets correct for Soft Iron (Induced Magnetism)
Flinders Bar
Spheres (Kelvins Balls)
Heeling magnet (both soft and hard iron)
What is retention error in a magnetic compass?
Error caused by a magnetic compass retaining some deviation after being on one heading for some while
Operational (Departure) Checks of a Magnetic Compass
a) Freedom of Movement of the Gimbal
b) Card floating freely and level
c) Liquid free of bubbles and clear
d) Compass card clear and sharp, no distortion or discolouration
e) Optical system (if any) clean and adjusted
f) Azimuth reading devices and means of illumination in working order
g) No liquid leaks around seals or filler plugs
h) No tools, phones, equipment left near the binnacle
What size vessels are required by SOLAS to carry a gyro compass?
Within what latitudes is a gyro compass reliable?
≥ 500gt
76° N - 76° S
Under what Principle does a Gyro function?
Gyroscopic inertia, stabilising the gyro in space
It remains fixed to a position via precision
When must a magnetic compass be adjusted?
On instillation
When they become unreliable
Every 2 years
The ships undergoes structural repairs (damage etc)
When Electronic or magnetic equipment close to the compass is added, taken away or altered
Regular Maintenance of a Gyro Compass
Regular Inspection
Calibration
Error Checks
Cleaning and Lubricating
Power supply and Backup power checked
Mechanical Alignment
Routine Testing
Record Keeping
Maintenance under the Manufacturers Guidelines
What is a Pelorus?
An alternative to the Azimuth Mirror
Gives bearing relative to the Ships Head
Fitted on the bridge wing, with the locater key lining it up with the lubber line
