Compass Flashcards
What is on a deviation card?
- Date and location
- Name of compass adjuster
- CoC and signature of Compass adjuster
- Name and signature of master
- Location of all correctors
- Residuals coefficients
- Residual deviation
- Heeling magnet
- Flinders bar
What are the operational checks for the Magnetic compass?
The performance of all magnetic compasses, including spares, should be
checked as follows:
• Freedom of movement of the gimbal.
• The card floating freely and level and rotating without any friction.
• Liquid free of bubbles and clear.
• Compass card clear and sharp (able to be read) with no distortion or discoloration.
• Optical system (if any) correctly adjusted and clean.
• Azimuth reading devices and means of illumination in working order.
• No liquid leaks around seals or filler plugs.
• No Tools left in the area or speakers etc
The compass deviation records and the deviation card for each magnetic compass installed
on the ship should be checked.
ANNEX 20
When should a magnetic compass be checked?
They are first installed;
They become unreliable;
The ship undergoes structural repairs or alterations that could affect its permanent
and induced magnetism;
Electrical or magnetic equipment close to the compass is added, removed or altered;
A period of two years has elapsed since the last adjustment and a record of compass deviations has not been maintained,
or the recorded deviations are excessive
or when the compass shows physical defects.
What are the properties unique to a free gyroscope?
~GYROSCOPIC INERTIA (rigidity in space)
Gyroscopic Inertia is that property of a gyroscope that causes it to maintain the position of
its axis in space regardless of how the supporting base is tuned and to resist any force tending to turn its spinning axis in a new direction. When you try to change the orientation
of the gyroscope or apply a torque, it will try to maintain its original orientation.
~PRECESSION
One of the most interesting properties of a gyroscope is gyroscopic precession. If you apply a force to tilt the spinning disc in one direction, the gyroscope will not immediately respond by tilting in that direction. Instead, it will experience a phenomenon called precession.
Precession is a rotational movement perpendicular to the applied force, and it occurs because of the conservation of angular momentum. The direction and speed of precession depend on various factors, such as the rate of spinning and the applied force.
Why do Gyro Compasses not work at the poles?
Gyro compasses lose all horizontal directive force as the poles are approached and are unusable north or south of 85° of latitude. They are generally reliable up to 75 ° but there after should be checked by the azimuths of celestial bodies often.
What are the operational checks for a gyro compass?
*Ensure Maintenance schedule as required by the MANUFACTURER
~Regular Inspection
Physical damage, loose connections, signs of wear and tear
~Calibration
Aligning gyrocompass with true north
~Check for errors
Checked against expected readings
~Cleaning and Lubrication
Follow the manufacturer’s guidelines!
~Power supply and back up
~Mechanical Alignment
Ensure it is correctly mounted
~Routine Testing
Performance checks
~Record Keeping
Maintenance activities, calibrations and any adjustments made.
Identifies trends, potential issues and compliance with regulations.
Describe the following:
~ azimuth mirror
~ pelorus
Azimuth mirror
-Point the arrow upwards to take bearing or a heavenly body.
-Point the arrow downwards to take a bearing of terrestrial object.
-Prism must always be horizontal.
Pelorus
The pelorus, which enables the navigator to obtain bearings of shore objects, is an
alternative to the azimuth mirror. It is particularly useful when the line of sight of the Azimuth mirror on the standard compass is obscured. Being a portable instrument, it can be transferred from bridge wing to bridge wing, so that the line of sight need never be impeded by such obstructions as the funnel. It is lined up for and aft, and then it can give relative
bearing from the ships head.
It has a graduated 360 degree compass card inside. It gives relative bearings to the ships head.
If required and the ships head is known, then it could be used to take compass bearings.
How do you monitor compass performance?
Compass performance should be monitored by frequently recording deviations in the compass deviation book.
Compass errors should be determined after every large alteration of course, and at least once every watch when there have been no major course alterations.
Checking the compass deviation regularly may show the need for repair, testing or
adjustment.
In addition, compasses should be inspected occasionally by a competent officer or compass adjuster.
- CHECKING YOUR COMPASS
Via Amplitude of the sun
- Take a bearing of the sun at sunrise or sunset ONLY
*REMEMBERING that the bearing is said to be when the sun is half a diameter above the horizon.
- Declination of the sun for the day from the nautical almanac
- Latitude from the GPS
- True bearing of the sun from Nories Nautical Tables @ sunrise and sunset
- Compare these to your observed bearing to give compass or gyro error
- CHECKING YOUR COMPASS
via Transit
When two well-charted objects are in transit, a compass bearing is taken.
- The true bearing is taken from the chart using parallel rulers.
- A comparison of the compass bearing and the true bearing allows us to work out the Compass Error.
- Variation can be found from the chart leaving us with the actual deviation we are
experiencing. - We then compare our actual deviation and the compass card to confirm that the card is accurate.
- CHECKING YOUR COMPASS
Via True Azimuth of a heavenly body
- Take a bearing of the sun. Note the time.
- Find the GHA and declination of the sun for that time.
- Work through chosen longitude and LHA.
- Go to Vol. 1 or 2 of the Sight Reduction Tables with the argument; LHA, LAT and
declination. - Extract the Azimuth (True Bearing) of the sun.
- Compare the Azimuth of the sun against your gyro or magnetic compass bearing.
Do we need a compass
It is required under Solas Chapter V
Regulation 19.2.1
All ships, irrespective of size, shall have:
a properly adjusted standard magnetic compass, or other means, independent of any power supply, to determine the ship’s heading and display the reading at the main steering position;
This is then bought into UK law via the Merchant Shipping (Safety of Navigation)
Regulations 2020
Parts of a compass
Departure checks of a compass
What is variation
Magnetic and Geographic Poles-
There are two ‘North’s’: True North and magnetic north.
-True north is the centre of rotation of the earth. Bearings and lines of longitude given on charts are based on true north.
-Magnetic north is the name given to the direction in which the “North” end of a magnetic needle, would point to.
“Variation is the angular difference between True North and Magnetic North measured East or West”
