Check
your Compass using the CompassAdjuster 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 vessel’s heading. The CompassAdjuster Pelorus is used to
determine the vessel’s heading with the knowledge of a bearing.
The
CompassAdjuster Pelorus is graduated counter-clockwise to allow for expedient
determination of the vessel’s heading.
Checking your Compass using a known distant
bearing.
align
the CompassAdjuster Pelorus with the vessel’s 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.
establish
the magnetic bearing of a distant object, either from the chart or by hand
bearing compass.
rotate
the CompassAdjuster Pelorus card so that the value of the distant object’s
bearing is on the lubberline.
observe
the distant object through the sighting peg and note the vessel’s heading
off the CompassAdjuster pelorus card.
Checking your compass using the sun’s
shadow
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..
place
the vessel on a know magnetic heading, be it on a set of leads or towards a
distant object of known bearing.
rotate
the card on the pelorus until the shadow falls on the value of the
vessel’s magnetic heading
the
vessel’s 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 sun’s magnetic bearing for the time is on the reciprocal lubberline
(or the reciprocal bearing on the lubberline). The vessel’s 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)
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.
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.
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)
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).
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.
