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What shipboard device, developed centuries ago to navigate the seas,
is still an essential item on all sea-going craft?
Recent years have seen dramatic developments in modern navigational equipment
used in yachts and ships. Loran, earth satellite navigation (Satnav &
GPS), radio direction finding, precise depth finding, digital computer charts,
and doppler speed measurement, are among electronic devices widely available.
It's not unusual today to see yachts equipped with all these and more.
But it's extremely rare to find a boat without a magnetic
compass. The compass, used since ancient times to find a way at sea,
probably remains the most used navigation instrument on seagoing vessels
today. Amazingly, the magnetic compass is also the most dependable of on-board
navigational tools. It needs no electric power to do its job, unusual among
navigation instruments. Thus, after a loss of electricity, the compass still
has full ability to show the vessel's heading. It shows direction even when
the boat is tied fast or anchored (in contrast with Loran and GPS,
which will indicate course only after the boat is making way and steady
on some heading.)
Today's compass, though based on the old principle of a magnetized needle,
uses long life permanent magnets, with a low friction alloy pivot in a jewel
bearing, to be very sensitive to small changes in magnetic field. The compass
card floats in a special optically clear oil solution which damps erratic
motion in rough seas for a more settled reading. Regardless of manufacturer,
compasses with larger diameter compass cards tend to be steadier and easier
to read, and worth the added price. Compass brand names commonly in use
include Ritchie, Danforth, Plath (German), Plastimo (France), Suunto (Finland),
and Sestrel (Britain).
A new compass device, developed in recent years, and often serving as a
secondary compass, is the electronic
"fluxgate" compass. While disadvantaged by the need for electrical
power to operate, its advantages include an ability to digitally supply
compass heading to remote instruments such as course indicator, autopilot,
digital plotter and others. The fluxgate compass can be mounted far from
the steering station, to a location where magnetic influences in the yacht
may be small.
Magnetic compasses have two correctable errors. The first, magnetic variation,
is a compass error caused by the earth's lines of magnetic flux varying
from True meridiens (can be thought of as the magnetic and True north poles
being in different locations). Variation changes with the location of a
vessel on the earth. Variation also gradually changes at a given location
each year. The correction for variation is shown on your current navigation
chart's compass rose.
Magnetic deviation is the second correctable error. Deviation error is caused
by magnetic forces in your particular boat. Pieces of metal, such as an
engine or an anchor, can cause magnetic forces in your boat. Stereo and
other speakers, if too close to the compass, introduce large errors in compass
heading. Some electrical equipment and wiring also contribute to deviation.
Errors in your magnetic compass from magnetic deviation can be measured.
After measurement, most, if not all deviation can be "compensated"
or its effect removed with small adjusting magnets. This job of "swinging
the compass" is often done by a professional compass adjuster. Your
compass adjuster also can adjust many fluxgate compasses.
An accurate compass helps provide peace of mind when navigating at sea.
When out of sight of land in your boat, take a moment to reflect on your
magnetic compass. It's probably the instrument you take for granted and
look at more often than any other. Though improved with modern materials,
it is an ancient device, invented by man long before any other instrument
on your vessel. If you're like most skippers, the compass is the navigation
tool you expect to rely on if a major electrical failure or lightning strike
render electronics unusable.
So I wish you and crew good luck, smooth sailing, and safe, accurate navigation
with your compass, a valuable relic of the ages.
Note: The author, Marvin L. Schenker is a retired U.S. Navy Captain.
He presently is a professional compass adjuster in South Florida and can
be reached for advice or assistance at compass@schenker.net.