2008 REED’s East Coast Edition - Navigation
compass. This is a job for a professional com- cally determine where to add or subtract east
pass adjuster. The procedure is complicated or west values by noting whether magnetic
because the deviation changes as your boat’s north is left or right of true north on your
heading changes (with changes in heading chart’s compass rose; by changing the plus
come changes in the directional relationships or minus signs associated with east or west
between your compass, metal objects on your deviation when you translate the values from
boat and the north-south axis). errors to corrections; and by reversing the
Even after careful adjustment, some devia- steps you use to convert from true to com-
tion will likely remain. You need to deter- pass when you convert from compass to true.
mine the residual deviation on various head- It’s no surprise that many novice mariners at
ings so you can apply corrections to your first find the concept challenging.
magnetic compass readings. These values of A simple mnemonic, one that’s been passed
deviation are normally determined as part of along by generations of sailors, will greatly
a professional compass adjustment and are simplify the conversion process: Can Dead
recorded on a deviation card—a simple table Men Vote Twice At Elections? When you
with compass headings, say, every 15° in one match this phrase with its compass-correct-
column and the corresponding deviation in ing counterpart, the pairing looks like this:
the other. Can Compass
Digital Compasses
Dead Deviation
Men Magnetic
A fluxgate compass uses an electromag-
Vote Variation
netic device to sense the direction of mag-
Twice True
netic north. Installing a digital magnetic
At Add
compass that compensates continuously and
Elections East
automatically to within an accuracy of half
Let’s put it into practice. Suppose you want
a degree is one way to avoid dealing with the
to know the true course that corresponds
deviation problem. If you mount the unit at
to your compass course of 060°. You look at
a dry place well away from large metal parts,
your deviation card to find that the deviation
you’ll get stable readouts at your steering
on that heading is 6° east. The compass rose
station in the roughest weather. A digital
on the chart shows a variation of 8° west.
compass interfaces readily with electronic
Keeping our mnemonic in mind, you’ll
navigation systems, charts and autopilots.
remember that you add the 6° easterly devia-
However, there are drawbacks to weigh
tion to the compass course to get a magnetic
against the considerable advantage that digital
course of 066° and then subtract the 8° west-
compasses provide. Digital compasses rely on
erly variation to get a true course of 058°.
steady electrical power. Also, their physical
Going the other way, from true to compass,
makeup can limit their use in taking bearings.
you reverse the order and the signs. When
Compass Correction
you know any three values, you can find the
You’ve got to take account of both devia-
other two. The single memory aid prompts
tion and variation. When converting back
you to correctly handle all the variables go-
and forth between compass and true di-
ing in either direction.
rections, you must take into account both
Even so, some mariners like to use a second
deviation and variation, values which have
mnemonic when correcting from true to
both magnitude and direction: east or west.
compass. At the risk of overtaxing your capac-
In making these conversions, you can logi-
ity for mnemonics, it’s included here: True
Virgins Make Dull Company: Add Whisky
Aids To Navigation System
Understanding the various buoys, marks tional Association of Lighthouse Authorities
and lights that make up the U.S. Aids to Nav- (IALA). In addition, there are numerous
igation system is critical to safe navigation. short-range marks and privately maintained
Most of the major aids in the U.S. conform aids.
to the guidelines established by the Interna-
840