The Compass

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 while manoeuvring.

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.

Alignment

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.

Maintenance

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 air bubbles

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 compass.

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:

  Deviation =   Reading-Hand-Bearing-Compass
              - Reading-Steering-Compass

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 conditions. 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).