What is Compass Adjusting?


Compass adjusting is analysis of a vessel's magnetic personality and compensating for it at the binnacle, so that the earth's magnetic field can pass through the compass without being deflected out of its normal path. Compass errors caused by the ship's magnetic character are called Deviation.  Deviation is complex, because a vessel's magnetic properties effect the compass in different ways as the ship changes course.  If it were possible to see the earth's magnetic field passing through an unadjusted compass as the vessel turns a circle you would see the lines of magnetic force wobble from side to side as the ship turns, weaken in strength, and even stand on end vertically.  In such conditions a compass may be unsteady even on those headings which don't have large errors, and a pilot compass will not do a good job of steering.
     To the untrained eye, compass deviations can appear to be wild and random; East on one heading and West on another heading, without an obvious pattern.  A trained compass adjuster knows how to interpret these deviations and separate them in to manageable components so they can be compensated.  The best way to do this is to first "swing ship" and record the deviations on eight headings. The needs for compensation can then be calculated; magnets can be placed and quadrantal spheres adjusted as needed.  This preliminary swing is good procedure, but large ships usually expect me to do the whole job with one turn of the ship, and I have methods for doing that.
      There are two distinctly different types of problems for the compass adjuster to solve:  1)Finding the errors of the compass with accuracy; 2) removing deviation.  Determining the errors of the compass is a work of piloting and seamanship, spherical trigonometry and practical astronomy.  Removing the deviation is a problem of physics and magnetism.
      There are several ways of finding the errors of the compass.  My preferred method is to take bearings directly from the compass.  Seagoing ships have a standard compass above the pilothouse, where it should always be in a steel ship, and I have several types of azimuth instruments which fit most of these compasses. Weather permitting, I use the sun, a star, or planet.  Azimuth is found at any moment with a kind of slide rule I have which solves the astronomical triangle graphically. The factors entered are local hour angle, latitude, and declination.  For local hour angle I use a railroad-grade pocket watch adjusted to the local time of the body being used. For the sun, this is called Local Apparent Time.  If a clear view of Mount Rainier or Mount Baker is available, that works well too.  Horizontal bearings are calculated from geographical coordinates. Calculated bearing or azimuth is always a true bearing,  then converted to magnetic bearing by subtracting the local variation.
     Aboard vessels which don't have a standard compass binnacle I take bearings with a pelorus. I have bearings worked out from points all around the Puget Sound area; mountain peaks, prominent hills, and city streets.  The pelorus tells me what our heading is, either true or magnetic. It also works with the sun or a star or planet.  I have an interesting pelorus which solves the azimuth problem mechanically, using latitude and local apparent time.  This instrument is a patented compass adjuster's tool called the "London Polaris". A photo will be found under "Instruments" in this website.
     Sometimes I use a ship's gyro compass to bring me to magnetic headings, on occasions when I have nothing available for bearings. Using the gyro is convenient but I don't like to use it because the gyro error might not be stable. However, there are times when I have to do a job in overcast darkness or in the midst of rain squalls and just can't see anything.  On rare occasions I have to "bootstrap" a job on a small vessel. That's when none of the usual methods can be used, such as on a foggy day or in a small bay or inlet on a cloudy day.  Shelton, for instance, or LaPush on a rainy day.  I pick the best tree I can see work out the pelorus bearing by trial and error.  This may sound crude, but think of it this way: if the compass bearing of some object is the same on all headings, it is clear that the compass is reading the earth's magnetic field only, and not reacting the the magnetism of the boat as it changes heading.  Therefor the compass is adjusted.  Errors of alignment do not show up with the "bootstrap" technique, however, so it is not to be preferred over a known bearing.
     I never use GPS to find compass errors.  No matter how accurate it might be for position and course made good over ground, it isn't capable of reading out an instant heading as a vessel is turning.  The GPS-powered satellite compass works fairly well, once its errors of alignment are determined, though I have heard mixed reports from fishermen about them. 
     To compensate for the magnetic disturbances around a vessel's compass, magnets are used to nullify the effects of permanent magnetism and cast iron used to compensate for soft-iron effects. Vessels built of non-magnetic materials, wood or aluminum, don't always need the cast iron correctors, but if there is an anchor windlass close to the compass there will be a need for cast iron quadrantal spheres. Without them, NE and SW headings will be low and SE and NW headings will be high. I have in mind the old-time Puget Sound purse seiner or halibut schooner,  with the anchor windlass only a few feet forward of the compass.  Magnets take several forms, and I always carry a selection of magnets with me on a job. Small craft compasses are often fitted with built-in adjustable magnets. With built-in correctors, even a non-magnetic vessel still needs a compass adjustment, because the built-in correctors can cause a lot of deviation.
     Another source of compass errors is misalignment; such as a lubber line displaced to starboard or port of the ship's head.  I find that off-center compasses are usually out of alignment. Gyro repeaters are sometimes out of line.  Bearings taken with an out of line repeater result in miscalculations of gyro error and magnetic compass error, when ship's mates calculate compass error based on gyro bearings. I always examine the alignment of gyro repeaters.
     Changes in latitude can cause large compass errors. I know of a ship whose deviations on E and W headings changed 40 degrees through a change of latitude from 35° S to 40° N.  The proper amount of Flinder's bar can correct this type of problem.  The bar is cast iron, or should be, and mimics induced magnetism caused by the dip of the earth's magnetic field.  To make a judgement on the amount of bar I read through the ship's compass observation book. Second mates who might be reading this; please recored deviations on E and/or W headings at different latitudes, preferably in both hemispheres and at the magnetic equator.
      I can work to an accuracy of a half-degree if the vessel steadies up well, giving the compass a chance to catch up. On small craft this requires a good helmsman.  I try to reduce deviations to one degree or less, and can usually do that on ships which have a proper standard compass binnacle, and on non-magnetic hulls.  Compasses inside of a steel pilot house can be difficult; some respond well but some are drifty. It is not reasonable to expect the earth's magnetic field to control a compass mounted on a steel plate inside of a steel pilot house.
      The last detail of a compass adjustment is the writing up of the deviation card.  It shows the remaining deviations and the courses to steer.  I can supply a frame to fit the card, so that the card can be posted in the pilot house.

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