Your chart, and a quick calculation, tells you that if you
maintain your course and speed, you will reach another
fathom curve and the water will shoal to about 20-24 feet
in about 10 minutes, and soon thereafter become
dangerously shallow. You proceed cautiously, set your
depth alarms, watching your depthfinder closely, and as
soon as you see your depth changing from around 30 ft.
to 23 ft., you alter course at once. The harbor you want
is up the coast and as long as you follow this fathom
curve, i.e., steer to stay within 20 to 24 ft. of water, you'll
reach your harbor entrance safely.
Simply adjust the shallow alarm to sound at a depth
between two to four ft. shallower than the bottom
contour you wish to follow. (see "Use the Shallow
Alarm" section from the Operating Instructions in this
manual). Then set the deep alarm to sound at a set
point which is two to four ft. deeper than that contour
line. Naturally, the actual set points of the alarms will
vary with the sea conditions and wave heights. Then,
steer as close a compass course as you are able; if the
alarm sounds, note the depth from the LCD readout and
steer shallower or deeper as required to follow the
contour and quiet the alarm.
Two precautions
Before going on to position finding, two important factors
must be emphasized again. Your Aqua-Probe digital,
like all depthfinders, measures depth from the bottom of
your transducer-not from the surface of the water, and
not from the bottom of your keel. This difference, which
is often a couple of feet or more; must be remembered
and factored into your readings.
The second factor is seasonal change on lakes and
rivers, or tides in coastal areas. Since your digital
depthfinder makes no allowance for the season of the
year (thaws, rains, etc.), or tides that change water
depths every hour, these must be factored in as well.
Your chart gives the reference plane-or "datum"-of its
charted soundings.
Any basic book on piloting explains how to use tables to
calculate the height of tide at any given hour at any
given location. It is then a simple matter to modify the
"depth of water" readings given on your Aqua-Prove
digital so that they correspond to the "depths of
soundings" given on your chart.
Position by line of soundings
Suppose visibility is bad and you're navigating by "dead
reckoning." You know your compass course, estimated
speed over the bottom, and are making allowances for
current and/or wind. How can your digital depthfinder
tell you almost precisely where you are? The procedure
is to establish a line of position by soundings. You study
your chart to find promising bottom terrain ahead-that is,
you look for fathom curves that lie ahead of your
estimated course or "track." You then maintain course
and speed and make a series of soundings (corrected
for tide) at regularly timed intervals-say 5 minutes apart.
You record six or more of these soundings and plot them
on a transparent sheet (clear plastic or tracing paper)
with the distance between each sounding matching boat
speed and scale of chart. You then move the sheet
around on your chart until your plots are aligned with the
chart soundings. Assuming they closely agree, your last
sounding along the line of position can be considered
your "fix".
It goes without saying that soundings on a flat bottom
are worthless in determining position, and that the more
pronounced the unevenness of the bottom, the more
reliable your "fix." It is also obvious that you have to be
reasonably sure of your speed over the ground, and that
this position finding method should be planned a bit in
advance and practiced in non-emergency situations-i.e.,
with knowledge of exactly where you are.
Negative information
Your digital depthfinder can tell you where you are; and,
it can tell you where you are not. Suppose, again, that
you're navigating by "dead reckoning" and you have an
"estimated position." If your digital depthfinder agrees
with the charted depth at this position you may or may
not be right where you think you are (depending on
whether or not there are other areas of equal depth in
the vicinity). If, on the other hand,
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