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.
First download the CompassAdjuster Pelorus
card to make a pelorus using a CD and CD case.
You will need to print and glue the card face to a CD disc. (Use A4 size
paper and check the
printer has not distorted the compass card.) The final
step is to locate the centre of the CD with a sighting peg. Alternatively a
printed disc can be purchased through Compass Adjuster. To buy a printed disc, send a message to organise
payment and delivery.
The
CompassAdjuster Pelorus is graduated counter-clockwise to allow for expedient
determination of the vessel’s heading.
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.
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. However, if you are slow in
checking your compass, you can always resume a known heading and reset the card.
If unable to establish a known magnetic heading, set up the pelorus on a compass
heading.
(CompassAdjuster.com.au
is working on the ability for you to download the sun’s magnetic bearing for a given
location, time and time increments. In the meantime, the following two links may
be of help to find the sun's
true bearing and magnetic
values for your location. With knowledge of the sun's magnetic bearing, simply rotate the
card until the sun’s bearing for the time is on the reciprocal lubberline and
the vessel’s magnetic heading will be indicated by the shadow of the sighting peg.)
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.
