piloting tips and techniques 73
give you is speed through the water, which will diff er from speed over
ground to the extent that your boat’s progress is aided or impeded by
current. Again, we’ll look at these eff ects in Chapter 5.
Distance: Once you determine an appropriate time interval and your DR
speed, calculate the estimated distance traveled (remember the formula,
Distance ÷ Speed = Time; see Chapter 3) and plot your DRs ahead of
your last known position.If you point your boat at a charted
object—a buoy, tank, or lighthouse—
and look across your steering
compass, you are sighting a bearing
to the object. For instance, let’s
say you sight a tank onshore at
350 degrees magnetic. This is the
bearing from your boat to the tank,
so draw a line on the chart from the
water to the tank in the direction of
350 degrees magnetic. You have
taken a bearing on the tank, and you
are somewhere along that line.
If you were to wrap that line
completely around the earth, it
would eventually come back and
intersect the tank again. In effect,
this proves that bearings are actually
circles, but navigators call them lines
of position, or LOPs, as we saw in
Chapter 3. For practical purposes,
you only plot a short segment of
the circle onto the chart, and that
segment looks like a straight line—
hence the name line of position.
Getting back to our tank, we need
to determine where we are along that
bearing line. So we look for a second
charted object whose bearing will
cross the tank bearing at an angle
approaching 90 degrees. Ah—there’s
a spire over there, and it shows on
the chart. We point the boat at thespire, take a bearing to the spire
from the steering compass, plot
the bearing on the chart, and note
where it intersects our fi rst LOP. This
fi xes our position (thus the word fi x).
The closer the intersection is to 90
degrees, the stronger the reliability
of the fi x. Your GPS receiver provides
reliable fixes in part because the
LOPs—a line of latitude and a line of
longitude—intersect at 90 degrees.
A fi x must be derived from at least
two LOPs, and a third LOP provides
increased reliability. Seldom will three
LOPs intersect at a single spot; more
commonly their intersections will
form a small triangle that navigators
call a cocked hat. Your actual position
is somewhere in that triangle.
A good handbearing compass
makes it easy to sight a bearing. You
don’t need to point your boat at the
sighted object and read the bearing
from your steering compass. Instead,
just point the handbearing compass
at the object and read the bearing
through the sighting vane. (Find a
spot on deck where the handbearing
compass is unaffected by deviation,
as discussed in Chapter 3.) When
bearings are this easy to get, you’ll
get more of them, and that will
improve your DR navigation.Skipper’s Quick Guide to Bearings
and Lines of Position (LOP)