Swinging The Compass
compass is one of the navigation tools on the ship, whose existence is very important, like other ship navigation tools the compass also often experiences an error problem called swingging.
Deviation is that the problem of compass error thanks to structure of the vessel, proximity of metal objects, magnetic fields or electrical equipment. It may be influenced by loading cargo (e.g. canned fruit) or having repair work done (new metal parts or electronics). Deviation also varies with the ship's head (way it's pointing) as this changes the position of the magnetic flux s within the boat relative to the earth's magnetic field as well as it's relationship with interfering objects onboard.
Read: Binnacle on Ship
Individual vessels have individual deviation cards that show the adjustment required for changes in position of the vessel's head. The process wont to produce a deviation card is named 'swinging the compass'. There are several methods for doing this but in general terms the procedure is as follows:
- The vessel is anchored securely in midstream
- A line of known bearing (magnetic bearing from the chart) is established
- The vessel moves through each of eight compass bearings (cardinal and inter-cardinal points)
- for every of the eight points the bearing of the line is taken and any discrepancy between this compass bearing and therefore the known chart bearing is noted. If the compass bearing is bigger the deviation is west, if less the deviation is east.
Attachment may be a sample table of deviation values
This can be plotted as a deviation curve and intermediary values read off the graph if required.
Total compass error is that the combination of both deviation and variation (declination) and therefore the rule is:
For example, a variation of 15° east and a deviation of 5° west, gives a complete compass error of 10° east. Whereas a variation of 11° west and deviation of 3° west, gives a complete compass error of 14° west .
