Page 9 - Electronic Navigation Cyber Book

easterly and westerly precession. The resulting mean-error is, therefore,
zero If the ship is on an intercardinal course the force exerted by the
mercury (or pendulum) must be resolved into north/south and east/west
components.

The result of the combined forces is that precession of the compass
occurs under the influence of an effective anticlockwise torque. Damping
the pendulum system can dramatically reduce rolling error. In a top-heavy
gyrocompass, this is achieved by restricting the flow of mercury between
the two pots. The damping delay introduced needs to be shorter than the
damping period of the compass and much greater than the period of the
roll of the vessel. Both of these conditions are easily achieved.

Electrically-controlled compasses are roll-damped by the use of a viscous
fluid damping the gravity pendulum. Such a fluid is identified by a
manufacturer's code and a viscosity number. For example, in the code
number 200/20, 200 refers to the manufacturer and 20 the viscosity. A
higher second number indicates a more viscous silicone fluid. One
viscous fluid should never be substituted for another bearing a different
code number. Additionally, since roll error is caused by lateral
acceleration, mounting the gyrocompass low in the vessel and as close
as possible to the center of the roll will reduce this error still further.

Maneuvering (ballistic) error
This error occurs whenever the ship is subject to rapid changes of speed
or heading. Because of its pendulous nature, the compass gravity control
moves away from the center of gravity whenever the vessel changes
speed or alters course. Torque's produced about the horizontal and
vertical axis by maneuvering cause the gyro mechanism to process in
both azimuth and tilt. If the ship is steaming due north and rapidly
reducing speed, mercury will continue to flow into the north pot, or the
gravity pendulum continues to swing, making the gyro spin axis north
heavy and thus causing a precession in azimuth.

The decelerating vessel causes easterly precession of the compass.
Alternatively, if the ship increases speed the compass precesses to the
west.

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