A professional compass adjustor will "swing" the compass by checking it against known references, such as a proven range or ranges on structures or geographic features. Once on the range, he'll have the boat slowly motor along the cardinal directions-north, south, east, and west-as indicated by the boat's compass while he reads the bearing on the range with a pelorus. Knowing the range's true bearing and the local variation, he can calculate the compass's deviation. During the process, he'll adjust compensator magnets around the compass as necessary to minimize deviation and balance it around the compass rose. He'll then once again establish the remaining deviation, on at least the eight 45-degree points of the compass, and use those numbers to draw a deviation card. (See "A Sample Deviation Card," above.)
Posted at the navigation station, the deviation card allows the navigator to convert a course laid on a chart to a course to steer. Conversely, it allows him to correct a bearing taken by the ship's compass on a landmark to a magnetic bearing. That magnetic bearing can then be plotted on a chart using the magnetic compass rose.
Once the ship's principal compass has been swung and adjusted, have the adjustor also swing and make up a deviation card for any other compass that might be used for navigation, including the one that supplies heading data to the autopilot.
In the absence of a serious problem that entails moving equipment (cockpit stereo speakers, for example, that are too close to the compass), the process takes about an hour once the boat is on station at the adjustor's chosen location.
Deviation by GPS
Prior to the widespread use of GPS, you took compass bearings off recognized features in the landscape and plotted them on a chart to figure out where you were. Your GPS knows where you are, so you take a bearing on a headland you don't recognize, plot it from your known position on the chart, and voilà: "Honey, that's Dangerous Point off our port bow!"
Because your GPS knows where it is, it can also calculate where it is in relation to any other location in terms of range (distance off) and bearing. And because it comes loaded with magnetic-variation information for the globe, it can tell you that bearing in magnetic notation as well as in true notation. This means that you can use your GPS to check your compass.
Create a waypoint in your GPS with the coordinates of an accurately charted and identifiable feature, say a lighthouse. Set the waypoint as your destination and steer toward it. The farther you are from the waypoint the better, but a couple of miles provides sufficient accuracy. With your boat's bow on the mark, note the compass heading and the magnetic bearing toward the waypoint from the GPS (not your COG because this will be affected by current). The difference between the heading and the bearing is your compass's deviation-on that heading. To check deviation all the way around the compass, approach your waypoint from several directions. You may need to use several waypoints to complete the circle.
Some sailboats and a good many powerboats have two helm stations, each with a compass. Only one of these can be the master compass, so you have to designate one. On a sailboat, this would probably be the one you use when motoring. Swing both compasses and adjust both, if necessary. Make a deviation card for the master, then create a relative deviation card for the other by sailing the cardinal and 45-degree points on the master and noting the difference in heading between the two compasses.
Another issue is parallax. This arises when a boat's steering station is off-center or, on a sailboat, when you steer from the side of the cockpit. If you're standing behind the starboard helm station and looking over the bow, you're not sighting down the boat's centerline. If you aim the bow toward a mark, the boat won't be heading toward that mark. When swinging the compass, therefore, you should create a sight forward (it could be as simple as a sail tie on the lifeline) the same distance off centerline as the compass. By lining up that sight (let's call it a lubber mark) with the compass lubber line, you'll eliminate the parallax error. You might even want to establish a permanent lubber mark to avoid the "lubber's loop." The parallax error is significant. On a 40-foot sailboat with twin wheels, the wheels might be 30 inches off centerline and 36 feet from the bow. The resulting parallax angle is 4 degrees.
These are just a few of the ways to get more out of your compass, which to my mind is the single most underrated piece of equipment on any cruising boat.
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The U.S. Coast Guard is asking all boat owners and operators to help reduce fatalities, injuries, property damage, and associated healthcare costs related to recreational boating accidents by taking personal responsibility for their own safety and the safety of their passengers. Essential steps include: wearing a life jacket at all times and requiring passengers to do the same; never boating under the influence (BUI); successfully completing a boating safety course; and getting a Vessel Safety Check (VSC) annually from local U.S. Coast Guard Auxiliary, United States Power Squadrons(r), or your state boating agency's Vessel Examiners. The U.S. Coast Guard reminds all boaters to "Boat Responsibly!" For more tips on boating safety, visit www.uscgboating.org.