BRIDGE EQUIPMENT Flashcards
What is a sextant?
Instrument used to measure the angle between 2 objects on the same plane. Can be used for celestial navigation, vertical sextant angles, horizontal sextant angles.
How would you use a sextant for celestial navigation?
A sextant allows us to measure the angular distance a celestial body and the equator, this measurement is known as the sextant altitude. With this measurement we can go into the Nautical Almanac, with the aid of the Sight Reduction Tables.
From a meridian passage or a Polaris sight, we can obtain a line of latitude.
From the sun, moon and planets we can obtain multiple position lines and carry out a running fix
From the stars we can take multiple sights simultaneously and obtain multiple position lines which intersect as a 3 or 4 point fix for example.
Parts of a sextant
Index mirror, shades, telescope, horizon mirror, arm, micrometer drum, frame, graduation arc.
What are the errors of a sextant?
Correctable errors
• Perpendicularity. Index mirror not perpendicular to the plane of the instrument. Hold sextant horizontally, with the arc away from the observer. Set the index bar half way along the arc. The true image of the arc should be aligned with the reflected image provided by the index mirror. If they are not, adjust the index mirror screw.
• Side error. Horizon mirror not perpendicular to the plane of the instrument. Set the index arm to zero. Hold the sextant almost horizontally. Observe the horizon through the telescope. If error is present. Adjust the screw furthest away from the plane of the instrument. Adjust so that the reflected and direct images of the horizon appear continuous. Rock the sextant slightly.
• Index error. Index mirror and horizon mirror not parallel to eachother when the index arm is set to zero. To reduce the index error, set the index arm to 0 and observe the horizon with the sextant held upright. Adjust the screw on the horizon mirror which is closest to the frame until there is no visible step in the horizon. Otherwise. Set instrument to zero, look at the horizon. Adjust index bar so that both images of the horizon are aligned. The resulting sextant reading is the index error. A positive reading is subtracted, A negative reading is added.
Tip: PSI.
Non correctable errors.
• Prismatic. 2 faces of the mirror not parallel to eachother
• Worm & Rack. Caused by wear on the gearing rack
• Graduation. Error on the arc, vernier or micrometre scales
• Shade Error. Faces of the shade not parallel to the ground
• Collimation. Axis of the telescope not parallel to the plane of the instrument
• Centering. Pivot of the index bar is not in the exact centre of curvature of the arc
Tip: PSCCWG
What are the carriage requirements for compass?
Magnetic compass required on all commercial vessels regardless of size. Must be capable of being corrected to True. Must be provided with a pelorus for taking bearings
Gyrocompass required on all commercial vessels over 500GT. There must be a repeater at the emergency steering position. Must be able to take bearings over 360 degrees of the horizon.
What are the principles of a magnetic compass?
What are the errors?
The Earth’s Magnetism is caused by convection currents of molten metal in the Earth’s core creating a self-sustaining dynamo, with an associated magnetic field. The magnetic compass relies on the fact that a freely suspended magnetic needle will align itself with the Earth’s magnetic field and point towards magnetic north.
Variation. Effect on the magnetic compass caused by the angular difference between true geographic axis/poles and the magnetic axis/pole. Variation is the angle between the geographic meridian and the horizontal component of magnetic force at any point on the Earth’s surface. It is measured East or West of true north. Magnetic variation is represented by isogonals on small scale charts (produced every 5 years) and on large scale charts on compass roses, indicating variation for a given year along with the annual rate of change.
Deviation. Effect on the magnetic compass of the ship´s own magnetic field. Deviation is caused by 2 types of Magnetism. Permanent and induced.
Permanent magnetism is a property of hard iron materials. These permanently retain their magnetism despite being removed from a magnetic field (e.g. steel) A ship under construction will acquire permanent magnetism which, after being launched, will reach a relatively stable condition.
Induced magnetism is a property of soft iron materials, which acquire magnetism when placed in a magnetic field, which changes as the field changes.
A ship consists of both typed of materials, it therefore has its own magnetic properties and a magnetic field, which causes the needle on the compass to no longer align itself with magnetic north. The induced magnetic field of the ship changes as the ship changes course, consequently the deviation varies on different headings.
What are the compass correctors?
- Horizontal magnets. Horizontal Hard iron magnets stored inside the binnacle to correct for horizontal permanent magnetism
- Heeling error bucket. Vertical Hard iron magnet inside the binnacle to compensate for vertical permanent magnetism
- Kelvins spheres. Quadrantal soft iron correctors positioned athwartships of the binnacle. Corrects for induced magnetism
- Flinders bar. Vertical soft iron corrector positioned forward or aft of the binnacle. Corrects for the angle of the earth´s magnetic field.
What is a Vertical force instrument?
Instrument used to measure the vertical compinent of the magnetic force at any point on the earth´s surface
When is compass adjustment required? By who?
- When the vessel is new or when the compass is first installed
- Major structural repairs or alterations
- Electrical or magnetic equipment modified in close proximity to the compass
- When the compass becomes unreliable
- When 2 years have elapsed (unless records kept)
Describe how you would swing a compass. Can it be done by the master?
Swing the ship through 360 degrees. Ascertain the deviations on the core points within the 260 degrees. Apply compass adjusters to minimise the deviations. A master mariner may correct a compass, however a master mariner may not carry out the full compass swing.
What are the principles of a gyrocompass?
A gyrocompass relies on the principle of gyroscopic inertia. A gyroscope or spinning disc with an axis will point continuously at a fixed point in space. The gyroscope can be dampened or weighted in order to point to a particular direction. This is known as precession. Gyrocompasses can be dampened by weight or by solenoids either side.
Gyrocompasses should be accurate up until 1.5 degrees.
What are the potential errors of a gyro?
- Steaming error. The fact that the vessel is underway means the relative rotation of the earth is different. This must be corrected with a speed input from the GPS
- Latitude error. Gyrocompasses are dampened in latitude to account for the speed of rotation of the earth being different at different latitudes.
- Ballistic deflection. As the vessel alters course the gyrocompass applies corrective forces to compensate for the force exerted by the course alteration which can cause errors.
- Ballistic dampening. When a ship alters course quickly, the dampening effect of the gyrocompass in response to ballistic deflection can cause the gyro reading to lag behind the ship´s head.
- Rolling (quadrantal) error. Gyrocompasses which are weighted to provide horizontal precession are affected by rolling and pitching.
What are your actions in the event of loss of gyro?
- Reduce speed
- Engage hand steering and navigate using the magnetic compass
- Close Watertight doors
- Fix position
- Be aware that the following input has input from the gyro: ECDIS, Radar, GPS, Autopilot R.O.T indicator, repeaters, AIS.
- It is possible to use the COG from the GPS on the ECDIS.
- Manual plotting techniques on radars using relative bearings.
- Do not be distracted by the amount of alarms
What checks would you carry out on the compass prior to departure?
- Freedom of movement on the gimble
- Card floating freely and level, rotating without friction
- Liquid free of bubbles and clear
- No liquid leaks
- Compass card clear and sharp with no distortion or discolouration
- Optical system (If any) correctly adjusted and clean
CLOG
When should a compass be checked?
- Prior to departure
- after a large alteration of course
- once per watch
