| Onboard ship, there are
three principal references for direction: the ship’s longitudinal axis,
the magnetic meridian, and the true or geographic meridian.
Bearing: The horizontal direction of one terrestrial point from another, expressed as an angle from 0000 clockwise to 3600 . Relative bearings (abbreviated with an R following the bearing): Bearings measured with reference to the ship’s longitudinal axis. Magnetic bearings (abbreviated with an M following the bearing): Bearings measured with respect to magnetic north. They are measured with a magnetic compass. True bearings (abbreviated with a T following the bearing): Bearings that are measured with respect to true or geographic north. They are measured with a gyrocompass of known error. Ship’s head, or heading: A special bearing denoting the direction in which the ship is pointing. It can be be expressed with reference to magnetic or true north. True bearings are only plotted
on chart. Magnetic or Relative bearings must be converted to True in order
to plot on chart.
Shipboard Compasses; Used to obtain precise information on headings and directions. .....Gyrocompass;Used the most onboard ship, provides you with true bearings. .....Magnetic
compass;Used
as a backup because it requires no electricity to operate, is the primary
means of checking gyrocompass.
Magnetism
Magnetic North Pole
..........Caused primarily by the fact that the earth’s magnetic and geographic poles do not coincide. Compass align with magnetic lines of force flowing from north and south magnetic pole. Also caused by the magnetic abnormalities in earth’s crust. Some locations have similar values of variation as at other locations. Isogonic lines – line along which measured variation is the same. ..........Example:
isogonic line chart
How to determine Local Variation
Locate the compass rose nearest to the area in which the ship is operating Locate the variation and annual increase/decrease from the center Locate the year from the center of the compass rose Subtract the year indicated from current year Multiply the number of years times the annual change (sum) Add the sum(or subtract if decreasing) to the variation in the center of the compass rose Round the total off to the closest ½ degree
Example Magnetic Compass CNO requires that each self-propelled ship and service craft of the USN be equipped with one or more magnetic compasses suitable for navigation Exception of nuclear-powered submarines, all ships and craft must have a magnetic compass at the primary steering station Many ships have more than one magnetic compass Primary magnetic compass is called the steering compass Normally located on the centerline in the ship’s pilothouse where it can be best seen by the helmsman Readings from the steering
compass are labeled “per steering compass” (PSTGC)
Standard and Steering Compasses Secondary magnetic compass is called the standard compass Normally located on the centerline at the secondary conning station Readings from the standard compass are labeled “per standard compass” (PSC) Newer U.S. Navy ships will
typically have one steering compass due to fact that ships are being outfitted
with two redundant gyrocompass systems
Magnetic Compass Cautions Magnetic compass cannot be expected to give reliable service unless it is properly installed and protected from disturbing magnetic influences Precautions to observe in vicinity of magnetic compass Compass should not be placed near iron or steel equipment that will be moved frequently Immediate vicinity should be kept free of sources of magnetism, particularly those of changing nature No source of magnetism should
be permitted within a radius of several feet of magnetic compass
Magnetic Compass Operation Small bar magnet freely suspended in the magnetic field of earth will always align itself parallel to the lines of force of that field, establishing direction U.S. Navy standard No. 1, 7-inch magnetic compass Components: Circular card graduated in degrees from 0 to 359 Bowl of compass fluid that supports the floating card Bar magnets correct and align compass card Gimbals act as pivots that rest in metal ring, allowing compass to remain level despite motion of ship Binnacle is housing/stand
for compass
Magnetic Compass Advantages and Disadvantages Advantages: Backup in case of gyro failure Simple, self-contained mechanism Operates independent of electrical power supply Requires little or no maintenance Not easily damaged Disadvantages: Seeks magnetic meridian instead of true meridian Cannot be used near earth’s magnetic poles
Magnetic Compass Error Before using magnetic compass onboard ship, must first correct for the magnetic influences that make the compass deviate from geographic north Variation Deviation
Deviation Deviation is defined as the amount that the compass is deflected from the magnetic meridian because of the effects of the ship’s iron Expressed in degrees East or West Caused by the interaction of the ship’s metal structure and electrical currents with the earth’s magnetic lines for force and compass magnets Permanent magnetism – created in the ship’s structure during the building process Gains its own unique magnetic field based on the angle that the keel is laid Induced magnetism – varies according to the intensity of the component of Earth’s field Amount of deviation varies
as the ship changes course and with equipment alterations
Shipboard Degaussing System Also has an effect on deviation Degaussing system - electrical installation designed to protect ships against magnetic mines and torpedoes When a ship is close to a magnetic mine or torpedo, the magnetic field of the ship actuates the firing mechanism Purpose – counteract the ship’s magnetic field and establish a condition such that the magnetic field near the ship is, as nearly as possible, just the same as if the ship were not there Degaussing installation consists of permanently installed degaussing coils wrapped around ship on underside of hull, control unit to control the coil current, and compass compensating equipment to prevent disturbances to mag compasses Coil is a large diameter electrical wire A, F, L, M, Q Coils
Deviation Ship’s magnetic effects may be corrected by the proper placement of various correctors Process for correcting deviation error is called swinging ship Swing the ship through 360 degrees, stopping each 15 degrees and comparing the compass heading to the properly functioning gyrocompass Results are recorded on magnetic compass deviation table “Deviation Tables” – provide a means for knowing the deviation of the magnetic compass for any heading Information is crucial if the gyrocompass fails Updated annually and posted
on/near magnetic compass
Sample Deviation Table Top portion: name of ship, location of compass, binnacle type, and compass type Middle section: ship’s heading every 15 degrees and deviation data DG OFF – degaussing off DG ON – degaussing on Bottom portion: information
on magnet and bar placement that corrects for excessive deviations
Deviation Tables
Example: Your ship is on course 090 degrees true and the OOD now wishes to make good course 117 degrees (magnetic course) by magnetic compass Determine if DG ON or OFF Locate the course nearest to your desired course on the deviation table Nearest course is 120 Read the deviation 2.0 W Apply the deviation correction to the ordered course Westerly deviation means
compass reads less than it should = add WEST or subtract EAST
117 degrees.............................2
degrees W.............................119
degrees
Compass Error Calculations Three lines of reference have been established: True heading - direction of true north Magnetic heading - direction of magnetic north Compass heading - direction
of north point of compass
Ship’s head
True North
Mag North
Compass North
Variation Deviation
Compass Error
Converting from Compass to
True
When converting from steering compass heading to true heading, navigator must take into account variation and deviation Sequence of conversion: Apply deviation to steering compass heading to obtain magnetic heading Apply variation to the magnetic heading to produce the desired true heading Westerly errors subtracted and easterly errors added The following memory aid is used to help remember the steps in converting steering compass heading to true heading: Can Dead Men Vote Twice At Elections Compass Deviation Magnetic Variation True +East head head head The most challenging calculation is determining the correct deviation to apply. Standard deviation is based
on ship’s head magnetic. Due to this fact, when converting from compass
heading to true heading, it is necessary to interpolate twice if the ship’s
head lies between two magnetic headings listed on the deviation tables.
Converting from Compass to
True
Can Dead Men Vote Twice At Elections Compass Deviation Magnetic Variation True +East head head head First interpolation – steering compass heading can be considered an approximation of the magnetic head Second interpolation – magnetic head computed again as better approximation than steering compass heading Example: A ship’s heading
is 305 p stg c. What is the ship’s magnetic heading if DEG OFF?
Example: Ship’s compass head is 3050 with degaussing OFF. 3000 1.00W 3050 3150 2.50W The desired deviation is 5/15 or 1/3 of the difference between 1.00 W and 2.50W: 5/15 * (2.5 - 1.0)=.5; D= 1.5W This value is subtracted from 3050 to get a 303.50M. The first interpolation gives a good estimation of ship’s head, so a second interpolation can be performed in order to more accurately account for deviation: 3000 1.00W 303.50 3150 2.50W 3.5/15 * (2.5-1.0)=.4; D=1.40W The required deviation , rounded to the nearest .50, is 1.50W. Results in ship’s magnetic
head 305 - 1.5W = 303.5M
Shipboard Compasses
Converting from True to Compass: It may be necessary to convert a true heading to a compass heading in the event a gyrorepeater fails and a certain desired course is to be steered. In order to do this, corrections are applied in a reverse order according to sequence: T V M D C A W True Variation Magnetic Deviation Compass + West head
head
head
*Only one interpolation is
required when converting from true to compass
Shipboard Compasses
True to Compass Conversion
Example: While steaming on a heading of 1490T , the ship’s gyro tumbled. What steering compass course should be steered to keep the ship on the same true course? Assume a variation of 9.00E, with degaussing OFF. T V M D C 1490T 9.00E 1400M Calculation for deviation: 1350 1.50W 1400 1500 0.50W
5/15 * -1.0 = -.33, rounded to -.5, applied to 1.5 ; D=1.00W
Note: Only one interpolation is required when converting from true to compass. T V M D C 1490T
9.00E 1400M
(+) 1.00W 1410W
Gyrocompass
Main source for determining direction thus indicating true north Gyroscope – rapidly spinning body having three axes of angular freedom Must be lit off a minimum of 4 hours prior to use Checked for error at least once daily while ship is underway Proper function if error is 2 degrees or less Powered by electricity and consists to two main components Master gyrocompass consists of a control cabinet, power supply, speed unit, alarm unit, and transmission unit that is located within the ship’s hull where it is least affected by pitch and roll Repeaters receive signal transmitted from master gyro for real-time data Relative bearings on outside circle True bearings on inside circle Normally found at all ship’s control stations: pilothouse, bridgewings, aftersteering Additional spaces: CO’s cabin,
CIC
Gyrocompass Advantages and
Disadvantages
Advantages: Seeks true meridian instead of magnetic meridian Can be used near the earth’s magnetic poles Not affected by surrounding material Signal can be fed into integrated navigation systems and automatic steering systems Extremely accurate, highly reliable, and easy to use Disadvantages: Highly complex instrument requiring periodic maintenance by qualified technicians Dependent on electrical power supply Subject to electronic and
mechanical failures of its component parts
Gyrocompass Error
Several sources of error caused by the transmission network but error is small Most normally functioning gyrocompasses will not have an error of more than 2.0 degrees East or West Must take error into account during plot At sea, QMs must determine gyrocompass error at least one a day via the following methods Observe a natural or artificial range. A bearing is shot to the range when lined up, then compared to the charted bearing. The difference is equal to the gyro error. If the ship is at a known location, such as a pier or an anchorage, a gyro error can be obtained by comparing a known bearing to an object ashore, as measured on a chart. Comparing the ship’s heading
while pierside to the known heading of the pier will give gyro error
If the ship is not underway, a trial and error adjustment of three or more simultaneous lines of position until a point fix results. If the lines initially meet at a point, there is no gyro error. If they form a triangle, they are adjusted by successive additions or subtractions of 10, then if necessary, .50 to the bearings until they meet at a point fix. The total correction applied to any one LOP is the gyro error. Compare the gyrocompass to another gyrocompass of the same error. At sea using sun as reference
Shipboard Compasses
Compass Error
“Compass Best, Error West” If the gyrocompass bearing is higher than the actual bearing, the error is west “Compass Least, Error East” If the gyrocompass bearing
is lower than the actual bearing, the the error is east
G.E.T. - Gyro + East = True
Gyro Gyro Error True 180 degrees ? 182 degrees 062 degrees
?
060 degrees
Homework
Read Marine Navigation Chapter 11 on Tides Bring Marine Navigation to class Workbook problems Chapter 9 Section 3: 1, 2A, 2B,
7
|