Pennsylvania Angler & Boater - January/February 2015 - 38

The

Magnetic
p
ComHowass
-Why and
It Works
by Alex Zidock Jr.

photo-Alex Zidock Jr.

With all the sophisticated marine electronics available
today, a novice boater could overlook mounting a magnetic
compass near his or her helm station. The same magnetic
compass that saved the day for many sailors for thousands
of years is still the go-to guide for simple navigation, and it
could be a lifesaver when electronic navigation devices fail.
The magnetic compass is a simple tool that works
because of a sightless, tasteless, odorless, natural wonder that
envelops the earth called a magnetic field. Development of
the magnetic compass started thousands of years ago when
it was noticed that a rock rich in iron and called a loadstone
always roughly oriented itself along the same axis (north and
south) when suspended by a thin silk thread.
History notes that the Chinese in about 1040 used a
magnetic device for land navigation. By the time Christopher
Columbus made his first journey to America in 1492, simple
magnetic compasses were used for marine navigation.
Science has come a long way to understand the earth's
magnetic field and while the magnetic compass has been
greatly refined, it's simple technology is what makes it so
desirable among those who use boats.

How it works

The reason a magnetic compass is so easy to use is that
a needle magnetized and mounted on a frictionless pivot
always points to magnetic north and magnetic south. North
on the needle is usually marked by an arrow or painted with
bright or luminous paint. A person can tell which direction
he or she is traveling in relationship to the direction of north
being pointed out by the compass needle.

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Pennsylvania Angler & Boater * January/February 2015

Marine compasses

A modern marine magnetic compass is usually a liquid
filled housing, mounted in a gimbals and containing a dial
referred to a "rose" or "card." In place of the needle used in
land-based compasses, a magnetized bar is fastened to the
underside of the dial that rests on a pivot, suspended in the
liquid. The end of the bar that points north is aligned with
the N (north) or "0" position on the card. The card rotates
on the pivot until the end marked north (N) points to the
earth's magnetic north.
Some modern compasses are constructed of a liquid-filled
housing with a floating dome inside and a stationary mark
on the outside clear cover. Direction is determined simply by
lining up the stationary mark with the numbers on the inside
floating dome.

The rose

The dial is marked with 360 points beginning with
"0" for north. There are four "cardinal directions;" east
is at 90-degrees, south is at 180-degrees and west is at
270-degrees. Between these points is a variation of markings
depending on how sophisticated the compass rose is
decorated.

The lubbers line

Imbedded in the clear dome of the compass is a
stationary mark called a "lubbers line." Remember that the
rose or card is mounted on a pivot and rotates to always
point north. The compass housing is mounted to the boat,
so the lubbers line is aligned with the centerline of the boat
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Pennsylvania Angler & Boater - January/February 2015

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