With the Low-
rance Down-
Scan system,
fi sh can look
just like fi sh.but transmit a wide band of frequencies at
once versus a single frequency with a tradi-
tional system. Then digital signal process-
ing analyzes the chirp returns to paint color
images of the underwater world that are
very well-defi ned. There’s also less screen
clutter than with fi sh fi nders of the past. A
wide range of frequency bands, measured
in kilohertz, are available for chirp sys-
tems. Generally speaking, the greater thewater depth, the lower the frequency band
is needed, though there are other factors
such as power to consider. All things being
equal, a high-frequency band of say 150 to
250 kHz, for instance, is good for depths
to about 600 feet, while a low-frequency
band of 28 to 40 kHz may be needed to
read at extreme depths of 3,000 feet or
more. Medium-frequency bands of around
85 to 165 kHz provide a compromise for
both shallow and deep waters.
Fish fi nders with transducers that are
installed properly can mark the bottom at
relatively high speeds — up to 40 mph in
some cases, though the slow-
er the boat speed, the better
the on-screen images. While
sounders are better than ever
thanks to chirp, the returns
still require a certain levelof interpretation to recognize fi sh, bait
schools and bottom composition.IMAGE IS EVERYTHING
T
here’s another way to look down-
ward. Imaging technology such
as Down Imaging (Humminbird),
DownScan (Lowrance/Simrad), Down-
Vision (Raymarine) and DownVu (Garmin)
uses transducers that electronically scan
downward in side-to-side sweeps (think
pizza slice). Digital signal processing then
assembles these slices into a coherent im-
age as the screen scrolls from right to left.
Frequencies are higher for imaging
systems than for fi sh fi nders. Raymarine’s
DownVision chirps across a high-frequency
band of 320 to 380 kHz. Lowrance’s Down-
Scan lets you choose between two frequen-
cies — 455 or 800 kHz. Humminbird’s Mega
Down Imaging is the highest frequency of
all, transmitting at 1,200 kHz.
Returns from these imaging systems
rival those of medical imaging. Fish images
can look just like fi sh. You can discern the
branches of submerged trees. Shipwreck im-
ages can take on the ghostly profi le of a ship.
Yet because of the high frequencies,
down-imaging sonar doesn’t possess the
depth capabilities of conventional fi sh fi nd-
ers. Maximum depth for Humminbird’s
Mega Down Imaging, for instance, is 125
feet. Raymarine’s DownVision can reach to
depths of 600 feet.
Because down imaging samples in rela-
tively thin slices, the systems are not prac-
tical to use at high speeds. Keeping speeds
under 5 mph helps ensure the best possible
target defi nition. Also, the display images
for these systems are monochrome, though
the chromatic shade is selectable. Think in
terms of the monochrome images you see
in medical ultrasounds.ILLUSTRATION: TIM BOWER; PHOTOS: (FROM TOP) COURTESY RAYMARINE, COURTESY LOWRANCECHIRP SYSTEMS OPERATE IN MUCH THE SAME WAY
AS OLD-SCHOOL FISH FINDERS BUT TRANSMIT A
WIDE BAND OF FREQUENCIES AT ONCE VERSUS A
SINGLE FREQUENCY WITH A TRADITIONAL SYSTEM.
Chirp fi sh-fi nder systems such as the
one in Raymarine’s Axiom Pro produce
crisp returns with little interference.86 | BOATINGMAG.COM | APRIL 2018