Sounders are commonly available in
a single frequency configuration of
either 200kHz or 50kHz and a 50/200kHz
dual frequency configuration with the
ability to choose between the two.
200kHz Frequency
There are advantages to each frequency,
but for almost all freshwater applications
and most saltwater applications, 200
kHz is the best choice. It gives the
best detail, works great in shallow
water and at speed, and typically shows
less “noise” and undesired
echoes. Target definition is also better
with these higher frequencies. This
is the ability to display two fish
as two separate echoes instead of one
blob on the screen.
50kHz Frequency
There are some applications where a 50 kHz frequency is best. Typically, a 50 kHz sonar (under the same conditions and power) can penetrate water to deeper depths than higher frequencies. This is due to water's natural ability to absorb sound waves. The rate of absorption is greater for higher frequency sound than it is for lower frequencies. Therefore, you'll generally find 50 kHz used in deeper saltwater applications. Also, 50 kHz transducers typically have wider coverage angles than 200 kHz transducers. This characteristic makes them useful in tracking multiple downriggers. Thus, even when these downriggers are in relatively shallow depths, 50 kHz is preferred by many fishermen. |
Transducers are available in three
types: Thru-Hull, In-Hull, and Transom
Mount.
Thru-Hull Transducers
Thru-hull transducers are inserted
through a hole drilled in the hull
of your boat. These typically have
a long stem that slides through the
hull and is then held in place with
an equally large nut. If the hull is
flat, this is the extent of the installation.
However, if the transducer is to be
mounted on one side of a v-hull, then
a fairing block must be made from wood
or plastic that allows the transducer
to be mounted vertically. (For more
information on fairing blocks, see
the section in the next column.) Thru-hull
transducers are typically used on inboard
powered hulls so the transducer can
be mounted in front of the rudders,
propellers, and shafts.
Transom Mount Transducers
Transom mount transducers are installed
on the boat's transom, directly
in the water, and typically sticking
a little below the hull. Of the
four housing types, the transom
mount is by far the most popular.
A well designed transom mount transducer
will work on almost any hull (except
inboard powered boats) and at high
speed.
In-Hull Transducers
In-Hull transducers are epoxied directly
to the inside of fiberglass boat
hulls. The sound is transmitted
and received through the hull of
the boat – but at the loss
of sonar performance. (You won't
be able to see as deep with an
in-hull transducer as with one
that’s mounted on the transom.)
The hull has to be made of solid
fiberglass. Don’t attempt
to shoot through aluminum, wood,
or steel hulls. Sound can’t
pass through air, so if there’s
any wood, metal, or foam reinforcement,
it must be removed from the inside
of the hull before installing the
transducer. Although there are
disadvantages to an in-hull transducer,
the advantages are considerable.
One, it can't be knocked off by
a stump or rock since it’s
protected inside the hull. Two,
since there is nothing protruding
into the water flow, it generally
works quite well at high speed
if it is mounted where a clean
flow of water passes over the hull.
Three, it can't be fouled by marine
growth. Four, The boat does not
need to be hauled for installation. |
 |
Thru Hull Mounts – On large
non-trailered boats, thru-hull mounting
is preferred. Thru-hull transducers
can be made from a variety of materials,
including bronze, aluminum, and plastic.
The thru-hull transducer is mounted
forward of the propellers to position
it in front of the disturbed water.
The acoustic signal is transmitted
directly from the transducer to the
bottom without the nominal signal loss
that occurs with a bilge mounted transducer.
Like the transom mounted transducers,
the thru-hull makes contact directly
with the water, and you can use the
specialty multiducers that provide
boat speed and water temperature information
to the depth sounder. |
| Fairing Blocks – Most
boats have a certain amount of angle
(deadrise) to their hull. If a thru-hull
transducer is mounted flush to the
hull, placing it at the same angle
as the deadrise, the transducer will
not function to its best capability.
For accurate readings, a thru-hull
transducer needs to “look down” as
straight as possible. The previous
thru-hull illustration shows a fairing
block that, when cut to the angle of
the hull, produces a “sandwich” effect
to allow the transducer to look straight
down into the water. Fairing blocks
are available for most of the transducers
we sell. |
