Your deviation should be recorded graphically on a deviation curve, and
kept handy for future reference. To ensure ongoing accuracy, your
compass deviation should be checked and updated once a year.
How to plot a deviation curve
Check that the compensation screws are in a neutral position (according
to the compass model, the screw slot will either be horizontal, or aligned
with a dot.
Although simple, calculating your compass deviation must be done
carefully. Deviation can be checked effectively by comparing different
headings on your compass, with those obtained from a hand bearing
compass held well clear of any interference. Standing at the stern of your
boat is usually the best position, provided it is a "non magnetic" area.
Example: if the hand bearing compass reads 30° and the steering
compass reads 34°, deviation on a course of 34° is - 4°.
•
Step 1 : Find a position that is well away from any source of
interference
Choose a nice day with smooth water. Select a distant object or landmark
(at least 3 miles away from the boat), whose bearing is known.
Sight the landmark with the hand bearing compass and while running your
boat under power, steer the boat slowly around in a circle, while
maintaining the sight of the landmark.
If the bearing remains constant, it means that you are in an area well
clear of any interference. If not, repeat the operation, re-situating yourself
at another place on your boat until the bearing remains constant through
a 360 degree circle.
•
Step 2 : Compare headings from hand bearing and steering
compasses
Reversing your circle direction, compare the 2 compass’s headings at 30
degree intervals (North, 30°, 60°, East, 120° etc.). Any difference noted
is due to the deviation on the steering compass.
•
Etape 3 : Establish the deviation curve
Plot the deviation (positive or negative) noted in Step 2 above on your
compass deviation card.
3.
Compensation
Compasses are generally supplied with built-in compensation, but some
models can accept optional compensation modules.
Compensating a compass consists of adjusting the position of the 2 built-
in compass magnets, in order to affect the horizontal component of the
compass card with respect to the North/South and East/West deviation.
The compensation procedure is a delicate operation and should ideally be
carried out by a professional and qualified compass service centre.
