the ship’s bottom, which will then be captured by the
transducer. By measuring the time elapsed between
transmission and reception of ultrasound (propagation
velocity), the depth of the fish school is calculated.’
The propagation velocity of ultrasound under the
water is about 1500 metres per second. However,
factors such as current, water temperature, micro-
organisms (i.e. plankton), air bubbles and radio noise
can interfere with this.FREQUENCIES AND ADVANTAGES
Frequencies commonly used by a recreational fishfinder
are 50 kHz (low frequency) and 200 kHz (high
frequency). A lower frequency has a wider search
angle and area. Generally, the searchable angle of 50
kHz beams is approximately 25 degrees, whereas the
search angle of a 200 kHz beam is about 12 degrees.
While the power of the unit will determine the depth
penetration of the ultrasonic beam, the frequency
at which it is transmitted also greatly affects the
resulting image. A narrow beam delivers more energy
on-target, resulting in stronger echoes and hence
improved target resolution. Higher frequency beams
are therefore better suited for focused, narrow-area
searches, while low frequency beams are more
suitable for wide-area searches.
We can make use of these characteristics by first
using low-frequency (i.e., 15 kHz) beams to find the
general locations of fish schools, and then narrow the
search area using high-frequency beams (200 kHz) to
detect their details and exact location. This enables us
to position the boat more accurately.BEAM PURITY
While interference can come from many sources, the
most common source of beam interruption is air bubbles
from the hull, or from another boat’s wake. When several
gameboats are working a similar patch of ground, their
sounders will often interfere with each other.
In some cases, interference can also be caused
by electronics on your own boat. To prevent this
occurring, ensure that your electronics are properly
grounded and avoid running electrical cables alongside
your transducer cable.
Multiple-element transducers are available that can
significantly enhance the performance and sensitivity
of your fishfinder. Some high-end models utilise
between seven or even fifteen 50 kHz elements, along
with a large-diameter 200 kHz element.
Acoustic noise is always present, and these sound
waves can interfere with your transducer’s operation.
While ambient (background) noise from sources such
as waves, fish and other vessels cannot be controlled,
by carefully positioning your transducers you can
minimise the effect of vessel-generated noise caused
by propellers and shafts, as well as other machinery
or fishfinders.
The lower the noise level, the higher the Gain setting
you’ll be able to use effectively on your fishfinder. The
smoother the flow of water across the face of your
transducer, the better the signal performance you’ll enjoy.HITTING ROCK BOTTOM
Rocky areas on the seabed will give a strong echo,
displayed prominently on the fishfinder display as a
thick bottom line. On the contrary, when the seabed
is soft it will be displayed as a very thin line. This is
because the soft seabed is absorbing much of the
ultrasonic wave emitted from the transducer.Above: This screenshot shows a Furuno FCV1150 running
a Furuno 82B-35R transducer. This transducer enables you
to select from 40 frequencies between 66 and 109 kHz in
either high or low frequency. It can also run low and high
frequency together on dual range, each with their own Gain
control, which is like having two sounders at once. The left-
hand side of the screen shows the drop-off in Marker Zoom
mode. The base of this cliff edge would be a great place to
start fishing for swords.A Furuno FCV1150
set in dual-range
mode, showing the
difference in image
results between low
frequency on the left
and high frequency
on the right. Both
the Furuno 1150
and 295 models can
run two transducers
from 1kW to 3kW,
together with a
broad range of
frequencies from
28 to 200 kHz. This
enables you to run
a 200 kHz 1kW
transducer on high
frequency and a 28
kHz 3kW transducer
on low frequency at
the same time.“Small-boat sonar has progressed to the extent
we now get clear images of the bottom.”
42 facebook.com/BlueWatermagazine
Sounding Out The Final Frontier