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Choosing the right Compass Option
Comnav Newsletter
COMPASS SELECTION AND PLACEMENT
The source of heading information and it's accuracy is one of the most important pieces of information in determining how well an autopilot will hold a course. Proper compass
selection and placement must be taken into account. Wood, Fiberglass and Aluminum Vessels: Since these boats do not normally have excessive magnetic
interference, compass selection is generally not as difficult. It is mainly up to the dealer's and owner's preference of choice between a fluxgate and a card compass. Some factors that
may influence the selection are the speed of the boat and how far north the owner plans to travel. If it is a fast boat, or if the owner is planning to travel
north of approximately sixty degrees, the compass selected should probably be an externally gimballed magnetic type. Steel Vessels In general a fluxgate compass should
not be installed on a steel vessel. Since most steel vessels have a definite amount of magnetic interference, the best choice is an externally gimballed, binnacle mounted, magnetic
compass with compensating spheres. If the wheelhouse is also of steel construction the compass may have to be remotely mounted above the cabin in a non-magnetic, weatherproof box. A good
example of this is on most ocean freighters, they have their magnetic compass mounted in a brass or wood stand approximately four feet above the roof of the cabin. They then use a
periscope type arrangement to view the compass heading from below. HINTS AND TIPS One of the best compass inputs for any autopilot on any boat is a "TRUE" gyro
compass. Due to the price and the physical size and weight of these units, this is not always a practical solution. Fluxgate compasses are generally mounted low and in the centre of
the boat to minimize the rolling effect and provide best performance. Magnetic compasses may also be mounted in a similar position but are usually positioned so as to provide the
operator with a visual steering compass as well. In a lot of northern or fast boat applications, a rate assisted, digital gyro may be a wise choice although it again may have to be
remotely located. This type of compass is a good choice for wood, fiberglass and aluminium applications as well but they may still be prone to some northern application errors. For
proper compass operation, a professional compass adjuster must always compensate a magnetic compass. When this has been accomplished the course holding ability of the autopilot will be
greatly improved. Magnetic material such as tools, radios, speakers and any other metallic items should always be kept away from any compass to avoid magnetic interference. Most marine
electronic equipment has a "Compass safe Distance" either on an attached label or stated in the manual, as much as possible this distance should be observed. If bubbles should appear
in the magnetic compass it may have developed a leak and should be repaired by a professional as soon as possible. The air bubbles may restrict the movement of the card and therefore
affect the operation of the autopilot. There are no hard and set rules; this information is intended only as recommendations for compass selection, positioning and
maintenance.
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