Revised by BoatUS editors in April 2012
Power or Sail
Sailboats heel and powerboats vibrate and pound - your compass must compensate for these conditions. Most sailboat compasses have an internal gimbal so the card will stay level when the boat is heeled. Powerboat compasses compensate in one of four ways. First, by using heavy dampening fluid. Second, by installing a weight under the card to slow its motion. The third, designed by Aqua Meter, relies on an air chamber to pivot the card, which floats in the compass fluid at almost neutral buoyancy. Fourth, Aqua Meter's Navagrid System uses lubber and bearing lines floating above the compass card to provide a cushion of dampened fluid over the card.
Size of Card
Recreational marine compass cards range in size from about 2 1/2-7" dia. The larger the card, the more accurate your course is likely to be because a larger card is more stable and easier to read. Smaller cards are sufficient for short runs or occasional use, and they're popular because small compasses are easy to stow below. You should select the largest card your space and budget can handle. On some compasses, the card will appear larger due to the magnification of the dome. This is referred to as apparent diameter.
Type of Card
There are three basic types of compass cards: the dish or back-reading card; the dome, or front-reading card; and the Ritchie CombiDial, a combination dish/front-reading card. The dish card and the Ritchie CombiDial are most often used on larger boats while the dome card is suited for sailboats with tillers so the helmsman is able to see the bulkhead-mounted compass at or about eye level.
Fluxgate compasses feature a digital or electronically reproduced "analog" display rather than a card or dish. Instead of a rotating compass card, the fluxgate compass uses a stationary electronic sensor mounted below deck. This sensor detects minute changes in electrical voltage caused by a deflection of its north/south orientation. In other words, it "senses" its position relative to the earth's magnetic poles and reports it, via hundreds of readings per second, to a micro-computer. This microcomputer averages the readings to arrive at a single accurate and steady electronic display.
The accuracy of your compass, once installed, will be affected by its proximity to iron and other metals on your boat. You should verify its accuracy on points north, south, east, and west by aiming your boat at structures on land, several miles distant, with known bearings from a navigational aid. If your compass readings are more than a few degrees off in any one direction, you should have a professional adjuster correct it.
Balancing Compass Dials For The Southern Hemisphere
The dials in magnetic compasses should be balanced to compensate for dip caused by the earth's magnetic field. All Ritchie compasses come balanced for Zone 1 (essentially all of the Northern Hemisphere). If you need a compass calibrated for the Southern Hemisphere, contact our Special Order Center at 800-568-0319.