The Steering Compass
The steering compass on sailing yachts normally
consist of a permanently installed high-quality magnetic compass located
in the cockpit near the steering wheel or steering helm. This compass is the
primary navigation instrument for the helmsman. It is used to determine and
to check the direction the ship is heading for.
Steering compasses should have cards with an increment of 1-5 degrees.
The larger the compass card is, the more accurate the compass course can be read.
For night usage, the compass should be equipped with an electrical light.
The illumination should be dimmed and suited for
night usage, such that the helmsman can keep his eyes adapted to the darkness.
Since this adaptation process may take up to 15 minutes, the compass illumination
is a safety issue!
On boats with steering wheels, dome compasses are popular.
Dome compasses are mounted on a horizontal surface such as a
pedestal or directly on the deck. They can be advantageously installed on
the mounting pedestal of the steering wheel. There the compass is easy to
read for the helmsman and well out of the way of sheets, lines and crewmen
Bulkhead compass are mounted on a vertical surface such as the back wall
(bulkhead) of a cabin and are installed on boats that have no convenient
location for a dome compass.
Steering compasses have a "head marker" for precise
reading of the course from the compass card. The line defined by this marker
and the pivot point of the compass card should be aligned exactly parallel
to the keel line of the vessel.
A compass does not require expensive maintenance.
Beside keeping the compass clean to allow for correct reading, some regular
checks should include the following:
- the compass reading should be checked regularly with some known bearings
(e.g. Sun azimuth angles)
- the damping fluid in the dome of the compass should always be free of
When not navigating, the compass should be protected from intense sunlight exposure.
The Deviation Table
For permanently installed magnetic compasses,
such as the steering compass or the fluxgate compass of the autopilot, a
deviation table should be elaborated to determine the deviation error of the
The deviation error depends on the direction the
vessel is headed to. It is measured by "swinging the boat" through 360°
and taking the (absolute) bearing to a known direction.
The bearing can be done directly using the steering compass or in combination
with a pelorus. A pelorus is a dumb compass in the form of a disk with
a graduation in degrees, clockwise from 0° in the reference direction
to 360°. The pelorus is mounted such that the forward keel line is the
reference direction (0°). In this way relative bearings referring to
the keel line (longitudinal axis of the vessel) can be measured.
The elaboration of a deviation table can be done in different ways.
An easy and quick method consists of comparing the reading of the steering
compass in different directions with those of a hand bearing compass which
would be aligned to the keel line and used in a location free of magnetic
deviation. How to find such a location is described in the description
of "The Hand Bearing Compass".
Turn the vessel round in different directions and
compare the course readings of the steering compass with the readings from
hand bearing compass, which should be aligned along the longitudinal axis
of the vessel. Any difference is due only to deviation of the steering compass
and should be recorded in a sheet similar to the following table:
If the steering compass has a "higher" course
than the hand bearing compass, this means that the deviation is west (negative).
If the steering compass has a "lower" course than the hand bearing compass,
the deviation is east (positive).
Assuming the hand bearing compass is free
of deviation showing the magnetic course:
Deviation = Reading-Hand-Bearing-Compass
This deviation table can be used in both directions:
from Compass Course (CC) to Magnetic Course (MC) and vice versa.
Example: while converting Course-over-Ground
to Compass Course, the resulting Magnetic Course is 27°.
The value closest to 27° for Magnetic Course in the deviation table is 31°
with a corresponding value of
+1° for deviation.
The resulting Compass Course is 27° -
( +1° ) = 26°.
If the deviation for any direction exceeds 10°,
the steering compass must be checked for compensation!
Frequent, careful observations should be made to
determine the constancy of deviations and results should be systematically
recorded. Significant changes in deviation will indicate the need for readjustment
and for the elaboration of a new deviation table.
One reliable method to check the compass - based
on celestial navigation - is to calculate the azimuth angle of the Sun and
compare this azimuth angle with the compass reading when "heading for the
Sun". This is described in more detail in the
"Notes on compass check with precalculated azimuth angles".
Steering a Compass Course
Holding a defined compass course is difficult, especially at night in heavy weather
Getting accustomed to the natural movement of the compass card takes a little while.
Sometimes, like when sailing close-hauled in shifty winds, holding a compass course
is just about impossible.
However, in many conditions such as in thick fog, holding a steady course is crucial.
Here are some practical tips:
Pick a spot on the horizon: Steering while looking down at the compass
is difficult. Try to pick a spot on the horizon that seems to line up with
the desired compass course, and then look down to the compass only periodically
to confirm your heading.
Try to steer an average course: In big waves, when the compass
card is swinging around, try to bracket the desired heading by steering no
more than, about 5 degrees on either side.
Pay attention and report your course: In tricky conditions, pay
attention to your average course and report that to the navigator in regular
intervals (30 minutes - more often if the conditions and situation require it).