Si-tex SP38, Installation And Operation Manual, Page: 46 / 193

Sitex SP38 Installation and Operation
Document PN 29010101 V1r0
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Electrical Connections
Route the Compass Transducer cable from the compass to the location where the SPU will be. Avoid
running the cable parallel to antenna cables, AC power, or DC wires/cables carrying heavy current.
Excess cable should be coiled up. The cable may be shortened if desired to avoid excess coils. If the cable
is too short, additional cable may be ordered from your Sitex Dealer.
See the Compasses section for further details on connecting the cable to the SPU.
Magnetic Compass and Sensor
The Sitex Magnetic Sensor can be mounted to Sitex’s standard Magnetic Compasses, and also to a wide
variety of other brands of externally gimballed compasses.
The advantage of using a Magnetic Compass with a Magnetic Compass Sensor attached to it is that the
Autopilot is sensing the position of the magnets on the compass card, rather than sensing the earth’s
magnetic field directly.
Since the card of the compass has physical inertia, it does not react immediately to the changes in the
magnetic field it is immersed in, but only to those changes which persist long enough to overcome the
resistance of the compass card to movement. This means that the compass is a more stable heading
reference than a Fluxgate Compass. The Magnetic Compass will largely ignore changes in the magnetic
field in the plane of the compass card caused by vessel motion, to which the Fluxgate Compass may react.
This is particularly important in high latitudes, (such as Alaska, Labrador or the North Sea), where the
horizontal component of the earth’s magnetic field (which is the portion that the compass is trying to detect)
is very small relative to the magnetic field of the vessel.
The disadvantage of the Magnetic Compass is that in very rough weather with the vessel going to
windward, it is possible for the vessel motion to upset or “tumble” the compass card, disabling the Autopilot.
Because of the relative insensitivity of the Magnetic Compass to both the Earth and the vessel’s vertical
magnetic fields (due to its inertia), it is recommended for use only at latitudes higher than 50° (North or
South), and for use only on steel vessels when optional quadrantal spheres are included.
Locate and mount the compass in a position that minimizes magnetic interference. The compass should be
at least 1 metre (about 3’) away from such equipment as radios, radars, depth sounders, and engine
instruments.
To reduce the possibility of the card ‘tumbling’ in rough weather, mount the compass as close as practical
to the center of pitch and roll. Mounting the compass far forward under a bunk or similar location is a poor
choice for rough weather performance.
Caution! It is important to remember that the compass is a vital part of the autopilot system.
Locating it properly, particularly on steel-hulled vessels, is essential to ensure proper operation
of the autopilot. Sitex strongly recommends that the services of a qualified compass adjuster be
used to select the best installation location, and to compensate the compass properly for
deviation, including that caused by heeling error.
Care must be taken not to place compensating magnets too close to the compass, as this will cause the
Compass Sensor (mounted underneath the compass) to read incorrectly. To correct compass deviation on
steel vessels, or on other vessels with magnetic compensating problems due to interference from adjacent
ferrous masses such as an engine block or winches, the use of a pair of compensating quadrantal spheres
is recommended.
Note: the Sitex Magnetic Sensor should not be attached to an internally gimballed compass
(that is, where the compass card is gimballed and the bowl of the compass is not), as this will
result in large errors between the compass and the autopilot. This is because the compass card
does not remain parallel to the sensor when the vessel rolls or pitches.