A Compass Course
Planning an extended voyage? Add a compass adjustment to your checklist.
As with many products that we take for granted today, the magnetic compass was once a revolutionary technology. Early mariners knew how to navigate their vessels using a knowledge of basic astronomy, but in overcast conditions they had no way of knowing how to steer a course until the invention of the compass.
The first recorded use of a magnetic compass comes from the Chinese writer Zhu Yu in the early 12th century: “The ship’s pilots are acquainted with the configuration of the coasts; at night they steer by the stars, and in the daytime by the sun. In dark weather they look at the south-pointing needle.”
How were the Chinese to know, back then, what a profound impact their little “south-pointing needle” would have on commercial shipping, naval warfare and recreational yachting in the centuries to come?
In the 13th century, the adoption of the magnetic compass quickly advanced dead-reckoning methods and the development of portolan charts. Mariners’ newfound ability to sail in overcast weather enabled them to operate during winter months, increasing traffic across world trade routes, including the highly valuable ones between the Mediterranean and northern Europe.
The new technology, however, was simply a magnetized needle floating in a bowl of water, and it’s easy to understand the problems encountered while bouncing around in heavy weather. Imagine trying to prevent water from sloshing out of an open dish even in relatively calm conditions. To solve this problem, the dry mariner’s compass was developed. Consisting of a wind rose with a needle pivoting on a pin, all enclosed in a glass-covered box, the dry compass became the standard for maritime use for centuries to come.
The marine compass of today, the liquid compass, arrived in the 19th century. Designed to protect the magnetized compass needle or card from excessive swing or wobble by adding a dampening fluid, it was widely adopted by everyone except the British Navy. Apparently confident in their proven ability to blast any threatening fleets out of the water at will, the British Navy saw no particular reason to change anything until, they too, eventually adopted the liquid compass, in 1908.
Demand for high accuracy soon increased. Clipper ships racing to deliver cargoes from the Far East needed to steer the most direct courses to bring their goods to market ahead of the competition; fighting ships demanded accurate bearings to train their guns on the enemy; coastal vessels needed better bearings to improve position fixes and provide greater safety.
In the era of wooden ships, it was a relatively uncomplicated task to compensate a compass for accuracy but, with the advent of steel vessels, the job became much harder and the role of the “compass adjuster” became essential. “Compass adjusting is the analysis of a vessel’s magnetic personality and compensating for it at the binnacle, so that the earth’s magnetic field can pass through the compass without being deflected out of its normal path,” explains Captain Keith Sternberg, a lifelong mariner and highly acclaimed compass adjuster operating in the Puget Sound area.
The best way to interpret compass deviations is to first ‘swing ship’ and record the deviations on eight headings. The needs for compensation can then be calculated. The record (facing page) is kept for future reference.
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As Sternberg explained, compass errors caused by the ship’s magnetic character are called “deviation.” Deviation is complex because a vessel’s magnetic properties affect the compass in different ways as the ship changes course. If it were possible to see the earth’s magnetic field passing through an unadjusted compass as the vessel turns a circle, you would see the lines of magnetic force wobble from side to side as the ship turns, weaken in strength, and even stand on end vertically. In such conditions a compass may be unsteady even on those headings which don’t have large errors, and the compass will not do a good job of steering.
“I get little demand from yachts these days,” says Sternberg. “Fiberglass construction provides a relatively clean vessel magnetic profile, and modern yacht compasses do a pretty good job with their built-in magnetic compensators.”
Anyone planning an extended voyage, however, would be well served to get their compass checked out and properly
Using a combination of knowledge, skill and, as some might say, ‘black magic,’ today’s professional compass adjusters deliver a remarkably high level of compass accuracy.
adjusted prior to departure.
“I believe in the worth of the magnetic compass and in the value of personal skills and self-reliance,” says Sternberg. “GPS is not a seamanlike substitute for a compass. It tells you the direction you have been progressing, but that may not be the same as your heading, and it is useless for a heading if you’re not moving.”
Working across multiple disciplines—and using a combination of knowledge, skill and, as some might say, “black magic”—today’s professional compass adjusters deliver a remarkably high level of compass accuracy. They’ve brought us a long way from the days when, as Jacques de Vitry wrote in 1218, “An iron needle, after having been in contact with the loadstone, turns itself always toward the northern star, which, like the axis of the firmament, remains immovable, while the others follow their course, so that it is very necessary to those who navigate the sea.”