Swinging a compassNov 26, 2013 10:47 AM
As much art as it is a science
A properly mounted compass aboard a Western Towboat vessel in Puget Sound.
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“It is the ever-present fallibility of electronics that keeps the compass adjuster in business,” Capt. George Barber said when asked to explain the science and art of swinging a compass. Three years into his retirement, the 75-year-old mariner, teacher, compass adjuster and businessman displayed a half-dozen compasses — both assembled and in parts — to show how it all works.
Barber started by providing a basic overview of the construction of a magnetic compass. The compass card, in the most common configuration, has an inverted cone under the center. This will typically have a jeweled fitting to receive the compass needle that is mounted on the base of the compass bowl. The U.S.-made Dirigo Compass that has a needle mounted on the card represents a variation of this. The needle turns in a cavity at the top of the little shaft coming up from the base of the bowl. In the center of the card, above this assembly, a bubbled section, which can be seen from above, contains a liquid that helps the card to float as lightly as possible in the fluid of the bowl. The fluid serves to dampen the motion of a marine compass when the vessel is at sea. Compasses built in Europe or Japan have a 50/50 mix of distilled water and alcohol, while those built in the U.S. have a light oil. A setscrew in the side of the compass bowl allows the fluid to be topped up.
Capt. George Barber, 75, uses a training device to demonstrate the swinging of a compass. He has been a compass adjuster for 23 years.
“When the compass is cold,” Barber explained, “its fluid shrinks, creating a suction force inside the compass that could cause a bubble if a compass bowl leak source was present. So once you have topped the cold compass up with fluid as need be (then re-fastened its filler plug setscrew), when the compass has rewarmed is a good time to check it again for leaks, this time with the fluid as newly pressurized.”
Most compasses have an accordion of flexible metal in the bottom, or a fluid-tight air chamber to allow for expansion and contraction of the fluid with temperature changes. If Barber goes aboard a vessel to swing the compass and finds that there is or has been a fluid leak, he could require the owner to replace the compass or have it factory-serviced as need be.
Assuming the compass appears to be in sufficiently good mechanical order, Barber will take his primary instrument out of his pocket. This is a typical little steel tape measure available in any building supply. “When the metal tape is coiled it will not influence the compass with much force,” he said, before pulling out a 10- to 12-inch length of the tape. With the ship stationary (before the ship departs would be a good time), he would slowly approach the mounted compass with the slightly extended tape, until the dial assembly was deflected two degrees only, in each rotational direction. After the magnetic deflector tape is removed following each deflection, where the compass settles is carefully observed. “For a compass to demonstrate absolute freedom from pivot-and-jewel friction, it must return consistently and completely from each two-degree rotational deflection. Otherwise the compass proves it can’t provide correct heading information, no matter what adjustment process is attempted.”
Two types of compass bowls with a valved glass bottom at left and accordion metal body at right, plus the corrective magnets.
The first magnetic measurement, Barber noted, is of the vertical gradient at the compass position. “This is also usually best checked and resolved before the ship leaves her dock.” To measure this, Barber would remove the compass from its gimbals and place his vertical force instrument in its place. This device has a magnetic bar balanced in a round dial the size of an alarm clock with the magnet set parallel to the keel of the vessel. The magnet would register any downward pull and corrective vertical magnets could be placed under the compass.
Then, the actual swinging of the compass, or more accurately swinging the ship with the compass and correctors attached, is a relatively straightforward combination of precise measurement and a steady eye. Directing the vessel to an open area some distance from shore, Barber will have it stopped. He affixes a small plastic mount to the compass glass over the center of the compass rose. He then takes a very thin rod of about 18 inches from a protective case and fixes it to the plastic holder, creating a “center pin” for taking horizontal bearings. Stepping back from the compass he visually aligns the rod, first with the compass lubber’s line and the ship’s center mark forward, thus checking compass fore and aft alignment. The sight pin is used to determine a fixed distant vertical mark, on land as far away as visually possible. Eleven miles is considered a good distance, allowing a bigger turning circle, although a nearby building’s vertical edge could be used.