and strength of your current to change from
location to location as you move toward that
bridge. So over-reliance on the chartplotter for
an ETA can lead to trouble.
Even without inlet interventions, chang-
ONE AID TO NAVIGATION
ing currents with the tidal cycle, as hap-
pens in many rivers and sounds, can render
meaningless an ETA based on your speed
over ground. The operator has to factor in
information on currents, when they occur,
and their force. Some chartplotters display
arrows showing the direction and anticipated
velocity of the current, but this can only be a
very rough guesstimate because of changes
made by moon phase, wind, and other fac-
tors. Also, we’ve seen chartplotters displaying
information that doesn’t reflect the fact that
tidal current doesn’t necessarily change direc-
tion at the very moment of high or low tide.
To further complicate it, the change of tide
moves up and downstream. It doesn’t start
running 10 miles up the river at the same
time it starts running at the river mouth.
Perhaps the most serious deficiency of the
chartplotter is its inability to do what you
alone can and must do: Look around. Once
I was a passenger on a very fine yacht. Its
owners were taking it to the Bahamas for
the first time. They had excellent navigation
equipment. We entered via the North Rock
Channel, north of Bimini. There is reef on
all sides as you follow (hopefully) the deep
water. The owner’s eyes were glued to the
chartplotter. At one point the screen blinked
for some reason. There was great consterna-
tion because we were in a narrow part of the
passage and shallow reef was on both sides.
Actually, there was no problem. All they
had to do was look around. You could see the
reef perfectly well because the light was good
and the water calm. The chartplotter was
helpful, but not really necessary.
Chartplotters can make boating easier,
more fun, and safer. They are a remarkable
aid to navigation, but that’s all they are.
Remember, never rely on one aid to navigation. Don’t let your reliance on the chartplotter make you lose familiarity with your other
Tom Neale, a technical and lifestyle writer and
liveaboard cruiser, leads our distinguished “Ask
The Experts” team.
CURREN T COURSE 90º
DES TINATION WAYPOIN T
Many chartplotters can display cross track
error (XTE), which tells you the perpendicular
distance from your intended course. A large
XTE may indicate you’re drifting off course.
If you’re being set to the side in a narrow
passage, look at landmarks or aids
to navigation both ahead and
astern to keep yourself in
line with the channel.
For more on how to relay your GPS
coordinates accurately, see this story
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