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This includes the Compass theory for the STCW assessment portion of this course. Compass errors must also be understood.
Theory of Magnetism
Magnet -A body, which produces a magnetic field around itself. It has the property of attracting certain materials capable of being magnetized. Each magnet has a North Pole, which is the point where the magnetic field starts and radiates out of the magnet. This pole is represented by the color Red. It also has a South Pole, the point where the magnetic field converges to enter the magnetic. This pole is represented by the color Blue. Opposite Poles attract & like poles repel each other.
Magnetic Field -Any space or region in which magnetic forces are present, as in the Earth's magnetic field, or in or about a magnet, or in or about an electric current.
Magnetic Force -The strength of a magnetic field a.k.a. Magnetic Intensity.
Permanent Magnetism -The magnetism, which is acquired by hard iron, which is not readily magnetized by induction, but which retains a high percentage of magnetism acquired unless subjected to a demagnetizing force. The strength and polarity of this magnetism in a craft depends upon the heading, magnetic latitude, and building stresses imposed during construction. A ship will be pointing the same direction, in the same location, for many months, as it is being build. The steel of the ship is being acted on by the Earth's Magnetic Field in the exact same manner for months on end. By the time the ship leaves the yard it will have developed a magnetic field around itself that is similar to the angle of the Earth's Magnetic Field that has been acting on the steel for so long.
Induced Magnetism - The magnetism acquired by soft iron while it is in a magnetic field. Soft iron will lose its induced magnetism when it is removed from a magnetic field. The strength and polarity of the induced magnetism will alter immediately as its magnetic latitude, or its orientation in a magnetic field, is changed. The induced magnetism has an immediate effect upon the magnetic compass as the magnetic latitude or heading of a craft changes.
Residual Magnetism -Magnetism which remains after removal of the magnetizing force.
Hard Iron -Iron or steel which is not readily magnetized by induction, but which retains a high percentage of the magnetism acquired.
Soft Iron -Iron or steel, which is easily magnetized by induction, but loses its magnetism when the magnetic field is removed.
The Earth's Magnetic Field
Magnetic Poles
Either of the 2 places on the surface of the Earth where the magnetic dip is 90 degrees.
Either of those 2 points of a magnet where the magnetic force is the greatest.
North Magnetic Pole -The magnetic pole in the northern Hemisphere, at Lat. 78.2N, Long. 102.9W (approx. in 1980) (it is in Canada). This relates to the actual Magnetic field of the Earth. The term should not be confused with North Geomagnetic Pole.
Geomagnetic Poles -Either of 2 antipodal points marking the intersection of the Earth's surface with the extended axis of a powerful bar magnet assumed to be located at the center of the Earth and approximating the source of the actual magnetic field of the Earth.
The color blue represents the North Magnetic Pole. The North Magnetic Pole is actually a South Pole of a magnet. It is the point in which the Earth's Magnetic Field enters the Earth. This pole will attract the North Poles of a Magnet. The north end of a magnet will always point to the Earths North Magnetic Pole because it is really a South Pole & opposites attract.
The South Magnetic Pole is represented by the color red. It is actually a North Pole of a magnet because it is the point in which the Earth's Magnetic Field leaves the Earth. This pole will attract the South Poles of a Magnet.
Magnetic Equator -That line on the surface of the Earth connecting all points at which the magnetic dip is zero.
Aclinic line -a line drawn on a chart connecting zero dip.
Isoclinic lines -lines drawn on a chart connecting equal magnetic dip.
Magnetic Meridian -A line of horizontal magnetic force of the Earth. A compass needle without deviation lies in the magnetic meridian.
Magnetic Dip -Angular distance between the horizontal and the direction of a line of force of the Earth's magnetic field at any point. Also called DIP & Magnetic Inclination.
Directive Forces of Magnetic Compass -The force tending to cause the directive element of a compass to line up with the reference direction. It is also, the value of this force. Of a magnetic compass, it is the intensity of the horizontal component of the Earth's magnetic field.
Isogonic Lines -Having equal angles. A line connecting points of equal magnetic variation.
Agonic Lines -A line connecting points of zero variation.
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The Magnetic & Gyro Compasses
The Standard Compass (psc) A magnetic compass designated as the standard for a vessel. It is normally located in a favorable position with respect to magnetic influences. (Located on the flying bridge, away from all the electronics of the bridge.)
The Steering Compass (pstgc) A magnetic compass by which a craft is steered. The expression is sometimes used to refer to gyro repeater similarly used, although for this usage Steering Repeater is preferable. Prior to the invention of the Gyrocompass, steel ships often had a Steering Compass at the wheel & another Magnetic Compass, as backup, away from anything that would increase the deviation (electronics) of the compass. Now, a standard compass is located on the flying bridge with a periscope down to the steering stand so you can see the magnetic compass heading.
Magnets -usually 4,they are lined up with the Earth's Magnet field & are fixed under the card.
Compass Card -That part of a compass on which the direction graduations are placed. It is usually in the form of a thin disk or annulus graduated in degrees, clockwise from 000 at the reference direction to 360, and sometimes also in compass points. A similar card on a pelorus is called a pelorus card.
Annulus - A ring shaped band.
Pelorus - A dumb compass, or a compass card without a directive element, suitably mounted and provided with values to permit observation of relative bearings, unless used in conjunction with a compass, to give true or magnetic bearings.
Cardinal Points/Intercardinal Points - Indicate North, South, West & East / NE, SE, SW, NW. 32 points on the compass just like there are 32 points of relative position around the ship.
I'll start it out for you.
North, 90 away is East, half of that (45) is NE, half of that (22.5) is NNE or ENE, half of that (11.25) is NxE, NExE, ExN and NExN. The BY or X points always start with the Cardinal or Intercardinal that it is next to and then is BY(x) the Cardinal point it goes toward.
See the "Favorite Links" Page for more info.
Lubber's Line - Indicates the course the vessel is on. The Lubber's Line must be lined up with the centerline of the ship exactly to get the real heading of the vessel. Compass Repeaters are often not on the centerline of the ship but they must still be lined up, parallel to the centerline to remove any possible error for bearings. The bow of the vessel, on the centerline is the reference point of the vessel's course & the reference point for relative bearings.
We do not want a compass that constantly Hunts (oscillates.) A dampening fluid is put in the compass in order to slow the movement & also dampen the effects of the ship rolling. This fluid used to be almost pure alcohol but it was discovered that the seamen were drinking the fluid out of the compass. The fluid is not mineral oil or distilled water with a non-freezing solution. It is important to use the Fill Plug to fill the compass and keep it free of air pockets/bubbles in the bowl. It is designed to be full. Also, use the proper solution, improper solution can render the compass useless by melting the paint off the compass card. The supports of the compass are also designed to keep the compass as steady as possible so that it is happy and will not hunt. It is quite obvious what this means on a rough day verses a calm day.
Gimbals - A device for supporting anything, such as an instrument, in such a manner that it will remain essentially horizontal when the support tilts. It consists of a ring inside which the instrument is supported at 2 points 180 degrees apart, the ring being similarly supported at 2 points 90 from the instrument supports.
The Binnacle - The stand in which a compass is mounted. For a magnetic compass it is usually provided with means of mounting various correctors for adjustment and compensation of the compass.
Quadrantal Correctors - Masses of soft iron placed near a magnetic compass to correct for quadrantal deviation. Spherical quadrantal correctors are called Quadrantal Spheres. 2 Quadrantal spheres are placed next to the compass; these spheres are hollow, soft iron.
Quadrantal Deviation - Deviation which changes its sign (E or W) approximately each 90ï‚° change of heading. It is caused by induced magnetism in horizontal soft iron.
Flinders Bar - A bar of soft, un-magnetized iron placed vertically near a magnetic compass to counteract deviation caused by magnetic induction in vertical soft iron of the craft.
Heeling Magnet - A permanent magnet placed vertically in a tube under the center of a marine magnetic compass, to correct for heeling error.
Heeling Error - The change in deviation of a magnetic compass when a craft heels, due to the change in the position of the magnetic influences of the craft relative to the Earth's magnetic field and to the compass.
Heel - to incline or be inclined to one side. Caused by list or the vessel rolling.
Heeling adjuster - A dip needle with a sliding weight that can be moved along one of its arms to balance the magnetic force, used to determine the correct position of a heeling magnet.
Degaussing a ship - removing the magnetic field from the ship. This gets rid of the permanent magnetism. Navy Vessels often do it so they cannot be detected as easily & they will not disturb the magnetic field near mines.
Azimuth Circle (Bearing Circle) - sits on top of the compass and allows the navigator to take bearings of objects & celestial bodies.
Gyrocompass - anything that spins can be considered a gyro. A child's toy top, a quarter spinning on its end. The spinning object resists the force of gravity and stays standing up. The gyrocompass continues to turn because an electrical current is added to make it spin. The gyrocompass is unaffected by magnetism and orientates itself with the Earth's axis of rotation, pointing to true North. Even a gyrocompass is not perfect. Adjustments must be made for speed and latitude so that the gyro will continue to point to True North. A flaw of the gyrocompass is that it is big, expensive and needs a continuous electric source to properly operate. Even a momentary lapse of power can cause the gyro to topple. We can "cage" the gyro or grab it so it doesn't topple.
Other sources of errors in the gyro compass, beside latitude and speed being improperly set, are power surges, power outages, wear & tear on the gears, friction, dirt, etc. The newer gyro compasses are less affected by these errors.
Bridge Equipment
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Measurement of Direction
Bearing: the horizontal direction of one terrestrial point as measured from another, expressed as an angle from 0 to 360 degrees.
There are three references for bearings: Relative, Magnetic, Gyro & True.
Magnetic Compass - The magnetic compass points to Magnetic North and is influenced by changes in the magnetic field around it.
Gyrocompass - A compass having one or more gyroscopes as the directive element. Points to True North and requires electrical power and periodic calibration.
Gyrocompass Repeater
A device that displays at different locations the indications of the master gyrocompass. Follows the motion of the Gyrocompass. Has no direction finding ability of its own, must be lines up with the gyro heading before it can be used.
Bearing Circle
Azimuth Circle – metal ring placed on a repeater with “cross hairs” that allow you to get an object “in your sights and shoot” the bearing. It also has mirrors to allow the navigator to reflect the bearing of the sun onto the repeater. This part is used in celestial navigation.
Alidade – a metal ting that gets placed on the repeater but it looks more like a scope on a rifle to shoot bearings.
Course Recorder – a repeater that prints the vessel’s heading onto a piece of paper. It can be used in court to determine what the vessel’s heading was at the time of an accident.
Only tells the vessel’s heading at a specific time. |
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GPS - Global Positioning System – A series of satellites that orbit the Earth and send out signals of their position. These satellites are operated by the Dept. of the Defense of the U.S. The receiver on the vessel interprets the signals to “triangulate†your position.
3 Satellites can determine Latitude, Longitude & Altitude. To get an accurate fix you need 4 satellites. DGPS – Differential GPS – Uses land based towers to correct for any errors that the DOD introduced into the signals sent out.
Loran-C - A Terrestrial Based Radio Navigation System. Radio towers send out signals, the unit receives the signals & gets a “time delay†that can be plotted on the chart.
Master Station & Secondary Stations in the “Chainâ€. Being phased out throughout most of the World.
ECDIS - Electronic Chart Display Integrated System. Charts are displayed on a computer screen. Computer receives input form the Gyro, GPS, Doppler Speed Log, Wind Indicator, Depth Meter, etc.
Radar - Used for distance (range), Not normally used for bearings because it is less accurate than other bearing measurements. Lets you see through the dark & fog. Shows the location of objects around your vessel. Radar sends out radio signals that get reflected back to the ship after “bouncing†off nearby objects.
Doppler Speed Log – determines Speed through the water (Speed of the water as it pass over the hull of the ship from bow to stern.) Transducer in the hull electronically measures the speed as it passes over the sensor. For true speed, you need to take into account currents, direction & engine speed.
Fathometer – Depth meters that display the depth of water under the hull of the vessel.
Electronic Readouts with storage capability & graphic display. Paper displays – graphic display of depth on paper (older units)
Depth of water = Fathometer Reading + Draft
Anemometer - Measures Relative Wind Speed. Takes into account both True Wind Speed and the Vessel’s Speed. True wind can be calculated from the relative wind measurements. It is like “speed through the water†for wind. Use a wind wheel, vectors or Bowditch tables to determine the true wind speed.
Engine Order Telegraph (EOT) - Sends a signal from the Bridge to tell the Engineers the desired speed. This does not actually control the engines.
Bridge Control Throttle - Normally the EOT is used to inform the Engineers of the desired speed so they can adjust the RPM’s using the throttle. Modern ships have the control of the throttles for the Bridge. Engineers stand by in the Engine Room for Emergencies only when the Ship is in “Bridge Control.â€
GMDSS - Global Maritime Distress & Safety System - Communications for the vessel. All Mates must be licensed GMDSS Operators. Provides warning information to Navigators concerning weather, safety and navigation aids. Allows vessels to send distress messages when they need help. Radios can talk to each other electronically to identify the location of surrounding vessels.
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