Marine Compasses

An inside look at these intricate instruments.

The compass is the last piece of equipment a boat buyer thinks about when giving a new boat the eyeball test. On the water, it’s taken for granted. That is, until it’s needed. Ritchie, Suunto and Plastimo all make fine compasses, and if one didn’t come standard on your new ride, you’d be wise to install one. But how do they work? Just like the magnetized needle you floated on a cork in elementary school, the magnets inside seek out magnetic north.

The Magnets
Magnetic North: Most marine compasses have two sets of magnets. One set is tuned, or "balanced," to seek out magnetic north — not the North Pole but the location of earth's north polarization, right now close to Ellesmere Island, Canada, near Greenland. The compass rose on nautical charts shows the difference, called variation, between true north and magnetic north for navigation.

Corrector Magnets: The second set of magnets is called the corrector magnets, which correct deviation, error caused on board by electronic instruments or ferrous metals. The correctors are connected to adjustable brass rods installed in the bottom of the compass housing, enabling you to compensate, or "swing," your compass card to adjust for deviation. Variation, being constant for a location, is accounted for in computing navigation formulae or can simply be added or subtracted "in your head" for noncritical navigation.

Cards: The dial, called the card, rotates on a steel pivot mounted on a sapphire jewel "movement" that helps with smooth turning. Front-reading compasses have a semi-sphere card, good for eye-level readings, but the flat-card style is more stable in rough water and allows you to take bearings relative to your course. Some cards have the elements of both types combined.

Mineral Spirits: The dial moves submerged in mineral spirits, a lightweight fluid that doesn't slow down its movement. A baffling system underneath the dial helps the liquid flow without sloshing — much like in a fuel tank — to help prevent the dial from bouncing around. A diaphragm in the bottom of the housing keeps the fluid from escaping and expands and contracts to prevent air bubbles inside.

The three most common ways to mount a compass are with a bracket mount, a surface mount or a flush mount. The bracket mount is the least stable, followed by a fixed deck mount. The most stable way to install a compass is by cutting a hole in the fiberglass and flush-mounting it at the helm. It's imperative that the compass be level and aligned with the keel of the boat lest you install built-in error.