Raymarine Transom Mount Transducer
Choosing a Transducer
The first step is to determine what material the transducer should be
- Plastic housings are recommended for fiberglass or
- Stainless Steel housings are recommended for steel or
- Bronze housings are recommended for fiberglass or
Bronze is preferable to plastic for wooden hulls because the expansion of
wood could damage a plastic transducer and cause a leak. Installation of
a stainless steel housing in a metal hull requires an insulating fairing.
A metal housing should NOT be installed in a vessel with a positive
How the transducer should be mounted on the boat is also important.
- Through-hull with fairing blocks offer the best
performance, especially at higher speeds.
- Through-hull flush mounts are best for trailer boats
where good performance is required and there are no protrusions from the
- In-hull transducers do not penetrate the hull, but do
sacrifice some performance.
Ensure that the transducer you select has the features that you want to
see displayed: depth, speed, temperature, or a combination.
Power refers to the strength with which the transducer sends the sonar
"ping", expressed as watts RMS. Higher power increases your chances of
getting a return echo in deep water or poor water conditions. It also lets
you see better detail, such as bait fish and structure. Generally, the more
power you have, the deeper you can reach and the easier it is to separate
echoes returning from fish and bottom structure from all the other noises
the transducer detects.
The accuracy with which your fishfinder detects bottom and other objects
is also determined by the frequency selected for the depth you are viewing.
Raymarine depth transducers can be tuned to two different frequencies: 200
kHz (high) or 50kHz (low).
200 kHz (high)
200 kHz works best in water under 200
feet/60 meters and when you need to get an accurate reading while moving at
faster speeds. High frequencies give you greater detail to detect very small
objects but over a smaller portion of water. High frequencies typically show
less noise and fewer undesired echoes while showing better target
200 kHz Echo Sounder Display in 50' (15m) of water
50 kHz (low)
For deep water, 50 kHz is preferred. This is
because water absorbs sound waves at a slower rate for low frequencies and
the signal can travel farther before becoming too weak to use. The beam
angle is wider at low frequencies, meaning the outgoing pulse is spread out
more and is better suited for viewing a larger area under the boat. However,
this also means less target definition and separation and increased
susceptibility to noise. Although low frequencies can see deeper, they may
not give you a clear picture of the bottom.
Mud, soft sand, and plant life on the bottom absorb and scatter sound
waves, resulting in a thicker bottom image. Rock, coral and hard sand
reflect the signal easily and produce a thinner bottom display. This is
easier to see using the 50 kHz setting, where the bottom returns are wider.
A rule of thumb would be to use the 200 kHz setting for a detailed view
to about 200 feet and then switch to 50 kHz when you want to look deeper.
Better yet, display both views side-by-side on a split screen for both
50 kHz Echo Sounder Display in 50' (15m) of water.
The transducer concentrates the transmitted sound into a beam. In theory,
the emitted pulse radiates out like a cone, widening as it travels deeper.
In reality, beam shapes vary with the transducer type and typically exhibit
"side lobe" patterns. The following figures give a graphic representation of
the transducer's actual transmit radiation patterns.
Low frequencies have wider beam angles than high
For the scope of this discussion, however, the idea of a cone works just
fine. The signal is strongest along the centerline of the cone and gradually
diminishes as you move away from the center. Wider angles offer a larger
view of the bottom, yet sacrifice resolution, since it spreads out the
transmitter's power. The narrower cone concentrates the transmitter's power
into a smaller viewable area. Cone angles are wider at low frequencies and
narrower at high frequencies.
To sum up, a wide cone angle can detect fish around the boat and not just
those directly under it while exhibiting less target separation. A narrow
cone concentrates the sound output enabling it to better detect small
details, such as fish or bottom structure, but only scans a small amount of
water at a time.
In reality, beam shapes vary with the transducer
type and typically exhibit "side lobe" patterns.
Transom Mount Transducers
As the name implies, transom mount transducers are installed on the
boat's transom, directly in the water and typically sticking a little below
the hull. Transom mounts are composed of plastic and tend to be less
expensive than other transducers.
Transom mount transducers are recommended for planing hulls of less than
27 feet (8 meters), such as personal watercraft and powerboats with
outboard, inboard-outboard and jet drives. They are not recommended for
large or twin screw inboard boats because aerated water from the propeller
reduces performance. They are also not recommended for operation at very
Transom mounts adjust to transom angles from 3° – 16°. For angles greater
than 16°, a tapered plastic, wood or metal shim will be needed. However, the
transducer should be adjusted so it is angled slightly forward when the boat
is in the water.
In-hull (a.k.a. shoot-through) transducers are epoxied directly to the
inside of the hull. These are only used in fiberglass hulls. In-hulls will
not work with wooden, aluminum, or steel hulls, or in foam sandwich/hulls
that have air pockets. Any wood, metal, or foam reinforcement must be
removed from the inside of the hull.
With an in-hull transducer, the signal is transmitted and received
through the hull of the boat. As a result, there is considerable loss of
In other words, you won't be able to read as deep or detect fish as well
with an in-hull transducer as with one that's transom mounted or thru-hull
Fiberglass hulls are often reinforced in places for added strength. These
cored areas contain balsa wood or structural foam, which are poor sound
conductors. The transducer will need to be located where the fiberglass is
solid and there are no air bubbles trapped in the fiberglass resin. You'll
also want to make sure that there is no coring, flotation material, or dead
air space sandwiched between the inside skin and the outer skin of the hull.
- No holes drilled in hull
- Excellent high speed performance
- No obstructions in the water
- Low maintenance
- Reduced maximuum depth reading
- Reduced fish detection
- Can only be used with fibreglass hulls
Through-hull transducers are mounted through a hole drilled in the bottom
of the boat and protrude directly into the water. This type of transducer
generally provides the best performance.
Through-hulls are recommended for displacement hulls and boats with
straight-shaft inboard engines. You'll also need a fairing block that allows
the transducer to be mounted properly. Through-hull transducers must be
installed with a fairing to ensure proper alignment and a secure fit.
Through-hull transducers must be positioned in front of the propeller,
rudder, keel or anything else that may create turbulence. They must be
mounted in a position that is always underwater and angled straight down.
Tilted Element Transducers
Tilted Element transducers are mounted through a hole drilled in the
bottom of the boat and protrude directly into the water. Tilted Element
transducers offer performance similar to through-hulls.
Tilted Element transducers are mounted flush against the hull. Unlike
traditional Through-Hull transducers, Tilted Elements do not need a fairing
block. The element inside the transducer acts as a leveling agent, working
with the deadrise (angle) of your hull to ensure the transducer's beam is
directed straight down.
These transducers will generally come in two configurations based on your
hull type, a 12º and 20º version. Select a 12º tilt when the deadrise of
your hull falls in the 8º to 15º range. Select the 20º tilt if your hull's
deadrise is in the 16º to 24º range.
When installing a Tilted Element transducer make sure to position it in
front of the propeller, rudder, keel or anything else that may create
turbulence. They also must be mounted in a position that is always
underwater and angled within the appropriate deadrise range.