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DIY Compass Adjusting

Check your Compass using the CompassAdjuster Pelorus Compass Adjuster Pelorus

Checking for errors in your compass is not difficult and is well within the scope of a prudent mariner. The compass can be compared against leads/ranges ahead or distant objects of known bearing ahead. The following method is simple and allows for all the headings to be checked. 

The first step is to print the CompassAdjuster Pelorus card to make the pelorus using a CD and CD case. (Use A4 size paper and check the printer has not distorted the compass card.) The second step is to glue and trim the card face to a CD disc. The final step is to locate the centre of the CD with a sighting peg. 

Conventional peloruses are used to determine a bearing of an object with the knowledge of the vesselís heading. The CompassAdjuster Pelorus is used to determine the vesselís heading with the knowledge of a bearing. 

The CompassAdjuster Pelorus is graduated counter-clockwise to allow for expedient determination of the vesselís heading.


Checking your Compass using a known distant bearing.  

  1. align the CompassAdjuster Pelorus with the vesselís fore & aft axis. This can be done aligning the case with a fore & aft or athwartship structure such as a hatch or heading the vessel directly at the sun and align the case so the shadow from the sighting peg falls on the reciprocal lubberline.

  2. establish the magnetic bearing of a distant object, either from the chart or by hand bearing compass.

  3. rotate the CompassAdjuster Pelorus card so that the value of the distant objectís bearing is on the lubberline.

  4. observe the distant object through the sighting peg and note the vesselís heading off the CompassAdjuster pelorus card.

Checking your compass using the sunís shadow

  1. place the CompassAdjuster pelorus in a position it will be in sunlight for most of the vessel's swing and ensure the pelorus is level and the sight peg perpendicular..

  2. place the vessel on a know magnetic heading, be it on a set of leads or towards a distant object of known bearing.

  3. rotate the card on the pelorus until the shadow falls on the value of the vesselís magnetic heading

  4. the vesselís heading will now be indicated by the shadow of the sighting peg.

When the sun is low, its change of bearing is slow. If you take some time in checking your compass, you can resume a known heading and reset the card. If unable to establish a known magnetic heading, set up the pelorus on a compass heading, any deviations observed can be re-calculated later when on a known heading.

With knowledge of the sun's magnetic bearing, rotate the card until the sunís magnetic bearing for the time is on the reciprocal lubberline (or the reciprocal bearing on the lubberline). The vesselís magnetic heading will be indicated by the shadow of the sighting peg.

"Sun Surveyor Lite" is a very useful free Android & iPhone app that shows the sun's magnetic bearing for where you are. "EZ Celestrial" is a purchased iPad app that, amongst other navigational functions, provides the sun's data specifically for use with the CompassAdjuster pelorus.

The following two links may be of used to find the sun's true bearing and magnetic values for your location. 

Basic Compass Adjustment - for compasses with inbuilt correctors 

(Note - The resulting deviations are only valid whilst the vessel is upright. Vessel heel may introduce significant deviations resulting from the vertical magnetic force at the compass)   

  1. Steady the boat on magnetic north - if the compass doesn't read north, adjust the athwartship magnetic field (labelled N/S or Co-eff C) until compass north and magnetic north are the same.
  2. Steady the boat on magnetic east - if the compass doesn't read east, adjust the fore & aft magnetic field (labelled E/W or Co-eff B) until compass east and magnetic east are the same.
  3. Steady the boat on magnetic south - if the compass doesn't read south, adjust out half the difference with the athwartship magnetic field (labelled N/S or Co-eff C)
  4. Steady the boat on magnetic west - if the compass doesn't read west, adjust out half the difference with fore & aft magnetic field (labelled E/W or Co-eff B).
  5. Complete a full swing noting the magnetic heading against the compass heading. There is a deviation card template with the compassadjuster pelorus download or you may consider using the deviation table & curve or deviation rose template from this site.

A more detailed description of compass adjusting can be found at "Handbook of Magnetic Compass Adjustment".



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