Set it Straight

While the conga line of new electronics jockeying for position at a boat’s helm may seem endless at times, prudent boaters will always reserve space for a magnetic compass. The humble compass often takes a backseat to modern navigational gear, but it still remains the only piece of equipment that shows direction when there’s a total loss of onboard power.

Compasses come in a variety of mounting styles. The bracket-mount style of compass comes with a removable bracket that is bolted or screwed into the mounting surface where the compass will be placed. The bracket is adjustable, so the surface can be horizontal, vertical or angled. Bracket-mounted compasses are the simplest to install, but they’re also less stable than units installed using cutouts in the helm or bulkhead.

Surface-mounted units are easy to install. They come with a mounting base that can be glued, bolted or screwed to any flat, horizontal location. Flush-mounted units are similar to surface-mounted units, but they require a cutout in the console or mounting area, which must be horizontal, and that cutout accepts the body of the compass, allowing it to sit flush.

Bulkhead-mounted units are mounted directly to a vessel’s bulkhead, and like the flush-mounted compasses, they require a cutout to accept the body of the unit, though in this case it is vertically oriented. Dash-mounted compasses are a space-saving option for smaller powerboats with a vertical console. They are similar to bulkhead-mounted compasses but much smaller, allowing them to be mounted at the helm, much like console gauges.

When selecting a location for a compass, ensure the unit will be level and aligned parallel with the vessel’s centerline. It must be free from deviation — the unintended effect of electrical or magnetic items located near your compass — or located where it can be compensated for.

Check proper orientation by temporarily mounting the compass at the chosen location and then sight along the unit’s lubber lines and center pin, to verify that they line up properly with the fore-aft line of the vessel.

Next, check the area around the temporary installation for potential sources of deviation. Stereo speakers are common troublemakers due to their large magnets, but any metal item or electronics within six to eight feet, sometimes farther, can cause problems. Be sure to test the effects of the electrical or electronic equipment when it is in both the on and off position. The same goes for the engine. Check the compass while it is off, idling and at various rpm.

The steps outlined so far will help reduce the effects of deviation, but getting rid of all magnetic influence is impossible. That’s why every compass requires a deviation card that indicates how many degrees have to be added or subtracted from the boat’s heading in order to obtain the correct magnetic reading. A deviation card addresses deviation as well as variation, which is the difference between magnetic and geographical poles.

Many methods have been developed to “swing the compass,” which means to verify its accuracy. The simplest one I’ve seen is listed on the Ritchie Navigation website.

To ensure accuracy on all headings, check for deviation every 30 degrees and record any deviation on a deviation card. Ritchie, a compass manufacturer, recommends checking at the start of each boating season for changes in deviation. If you feel that the deviation on your boat is of an unusual nature, the services of a professional compass adjuster will be a wise investment. For more information, visit