2008 REED’s East Coast Edition - Navigation
Tracks
of the most important, because it requires no
It’s important to have a clear understanding
information from beyond the boat. In a thick
of “heading,” “course,” and “track.” Although
fog with no electronics and no sight of any
these terms are often used interchangeably and
other aids, you can still plot a DR position.
differently by authors and sailors, there are
To plot your DR on a chart, simply extend
several essential distinctions to keep in mind.
a line for the length of the distance you’ve
The direction that your boat is headed at
traveled from your last known position in
any instant is usually somewhat different
the direction of the course steered.
from the course you’re steering. If the oscil-
To determine the distance traveled, you’ll
lation is evenly distributed on both sides of
need to know your speed and the time you’ve
your course, your average heading equals
been under way. The formula to convert back
your course. Under some conditions, though,
and forth between speed and distance is:
the helmsman tends to head more to one
60D = S x T or D = (S x T)/60
side of course than the other. Steep seas on
when dealing with time in minutes, dis-
your starboard quarter, for example, tend to
tance in nautical miles and speed in knots.
move you to the right of your course as the
To remember this formula, think of the ad-
waves lift your stern and your boat yaws to
dress 60 D Street. If, for example, your speed
starboard. If you don’t deliberately compen-
is seven knots and you run for 40 minutes,
sate for this tendency, the difference between
then your distance run, D, = (S x T)/60 or (7
your average heading and your intended
x 40)/60 or 280/60 or 4.6 nautical miles.
course can be significant. Estimated Position and Fixes
Such tendencies usually produce a differ-
When you take account of known current,
ence between your intended course and the
leeway and error at the helm in advancing
course you actually make good through the
your position, you’re establishing an estimat-
water. Your intended course usually differs
ed position (EP). When you establish your
from your course, or track, made good over
position with reference to directions and dis-
the ground, as well, for other reasons. One
tances to surrounding or far-off objects, you
factor is current that moves the entire body
get a fix. As the distinctions between these
of water in which you’re traveling—as well
types of positions is very important, they are
as your boat with it—in a direction (set) and
conventionally plotted with different sym-
at a speed (drift) that you can’t control. An-
bols: a semicircle for a DR position; a square
other factor is leeway, the sideways slippage
for an EP; and a full circle for a fix—each
especially noticeable in sailboats beating to
with a dot in the center to mark its exact lo-
weather. To gauge leeway, look astern and ob-
cation. An electronic fix obtained using GPS
serve your wake. If it’s tracking directly astern
or radar is plotted with a triangle.
in line with your boat’s centerline, then you
have no leeway. If it angles off to windward
Compensating for Current and
of that line, the angle it forms with the exten-
Leeway
sion of your centerline is the amount of your
When you’re reasonably sure what the cur-
leeway. To measure this angle accurately, you
rent is doing, you can project an EP based on
can tie a light line to a point on your boat’s
it. You do this by drawing a vector diagram
centerline and trail it astern. If you’re making
on your chart using your course and speed
leeway, the line will trail off slightly to wind-
through the water as one entering vector,
ward and you can measure the extent that it
the set and drift of the current as another,
does, which is your leeway.
and the estimated course and speed over the
Dead Reckoning:
ground as the resulting vector.
Time-Distance-Speed Calculations
You can also modify this process to deter-
mine an unknown current by comparing the
Among the three types of positions you can
location of a fix (based on a course and speed
plot, the one based on dead reckoning—your
over the ground) with a corresponding DR
DR position—is likely to be the least accu-
position (based on a course and speed through
rate because it’s based only on a projection
the water). The direction and distance of the
of the course you’re steering and your speed
resulting vector, from the DR position to the
through the water. However, it remains one
fix, is the set and drift of the current.
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