SONAR Transducers

This article is a comprehensive explanation of SONAR transducer operation and installation with special emphasis on their use with Boston Whaler boats.


SONAR, an acronym for sound navigation ranging, refers to the use of sound waves to locate objects and measure their depth underwater. The technique of emitting sound wave and timing their reflection off the bottom has been used to measure water depth since the early 1900's. With the advent of solid-state electronics, sonars became commonplace on recreational boats, particularly popularized by the LOWRANCE company who augmented the sensitivity of their device so that it could also indicate echoes from fish. Thus was born the Fish-Finder/Depth Sounder.

Essential to any SONAR is a transducer, a device which can convert electrical energy into water-borne sound waves and vice-versa. Electrical impulses from a transmitter are carried to the transducer by cable, where they are converted to sound waves that are directed under the boat. Sound travels at a fairly constant velocity in water (although there may be some change with temperature and density so that calibration for warm salt water may not be accurate for cold fresh water). Echoes return from the bottom (or from other underwater objects like fish) and are converted back to electrical energy, carried by cable to a receiver, where they are measured and displayed in various forms. By timing the interval between the sending of a sound and its return, the depth to the bottom (or other reflector) can be deduced.

Typically the same transducer is used for both sending and receiving, particularly in recreational type devices. The receiver may simply time the interval and convert this to a depth display, or it may rasterize the signals and display them for human interpretation on a CRT or LCD display. Modern units add sophisticated digital signal processing, color coded displays, and other enhancements.


Depth sounders or fish finders require a transducer which must be able to direct sound downward beneath the boat. There are three basic styles of transducer mounting available: in-bilge, thru-hull, and on-transom.

In-Bilge Transducers

In some situations it is possible to mount the transducer inside the hull of the boat in the bilge area, and allow it to shoot through the hull structure into the water below. In order for this to be possible, the hull material must be a good sound conductor. Solid fiberglass resin and laminates are adequete sound conductors and permit this type of installation. Some loss of sensitivity occurs, but the advantages include lack of any underwater appendages, preservation of hull integrity, and serviceability of the transducer.

Thru-Hull Transducers

More common are transducer installations in which a hole is bored in the hull and the transducer installed on the boat's bottom. This provides an excellent location for the operation of the transducer, but also puts a hole in the boat hull. Service or replacement can be problems. Also, there is an underwater appendage which must be protected when the boat is placed on a trailer or in lifting slings.

On-Transom Transducers

The most common transducer location on smaller recreational boats is on the boat transom. Only small mounting screw holes need to be made into the transom for installation. There are seldom problems with protecting the transducer from damage when loading or trailering. The water flow at the transom is somewhat more turbulent, but good performance is obtainable.


Hull Construction

The hull construction of a Boston Whaler is somewhat unique among recreational boats and places some contraints on the choice of mounting location for a SONAR transducer. Because of the foam cored hull and basic lack of any interior bilge space, the use of in-bilge transducers is seldom seen on a Whaler. Because of the typical thickness of the hull section in areas appropriate for transducer installation, thru-hull transducers are also not generally used. On some larger boats the hull molds have made provision for a small area where a thru-hull transducer could be installed, but even on these models many owners opt to not take that choice for the reasons mentioned above (hull integrity and damage when trailer loading).

This leaves the transom mount transducer location as the site of choice for most Boston Whaler boats.

Transom Mounting Location

The first critera in choosing a location on the transom is to select a spot that remains in the water when the boat is on plane. Next, the transducer location must not interfere with the operation of the outboard motor or outdrive propulsion. Finally, the transducer must be located in an area which will have clean, non-turbulent or aerated water. Underwater hull appendages upstream and in line with the transducer, like strakes, can cause disturbed water and affect the performance.

Single Engine

On boats with a single engine, the generally preferred location for a SONAR transducer (if using a right-hand propeller) is on the PORT side of the transom. Propeller torque will tend to keep this side of the transom slightly more immersed in the water than the starboard side. Allow at least 12-inches from centerline to provide clearance for the engine. The SONAR transducer must not create aerated water flow into the engine lower unit. The cooling water for the engine is obtained from the engine lower unit, and the pickup must be running in clean water. Turbulent water from a transom mounted transducer could cause problems for the engine water flow. Typically on a Boston Whaler there will be several inches of transom available for mounting of a transducer before one encounters any underwater strakes on the hull or sharp chine lines. Any strake on the hull upstream of the transducer will tend to create turbulence which will affect performance.

Dual Engines

On boats with twin engines, the SONAR transducer may be mounted on or near the keel centerline. While it is often seen that the transducer is mounted precisely on centerline, some suggest offsetting it several inches to move away from any disturbed water coming from the keel itself.

Mounting Details

When installing the transducer, be sure to mount it so that the sound beam will be directed orthogonal to the sea bottom. This will often mean the transducer will make some angle with respect to the boat bottom, particularly on deep-Vee hulls. Most all hulls have deadrise at the transom, so the transducer must be mounted out of alignment with the hull itself and oriented directly downward to the sea bottom.

When drilling holes for mounting, be sure to use a good sealant to prevent any water from entering the interior of the hull. Use stainless steel screws, and bed the screws with bedding compound like Bostik 920, Boat-Life Caulk, 3M-4200, or a similar product. Be certain to make the mounting screws water-tight. They will be immersed whenever your boat is in the water. The last thing you want is water leaking into the transom to rot any wood embedded there.

When drilling the mounting hole, use an oversized drill to remove the gelcoat layer to larger diameter, equal to the shank of the mounting screw. If you don't do this you will likely experience radial cracks in the gelcoat coming from your screw holes. There is excellent laminate and wood beyond the gelcoat layer and they will provide good purchase for your screws.

Most manufacturers provide good instructions for determining how much immersion a particular transducer will require for good performance. When gauging the mounting height, keep in mind that water coming off the underside of the boat will rise slightly as it comes out from under the transom; allow for this in locating the bracket. Excessive immersion creates problems with additional drag and with generation of spray. If a transducer throws a roostertail of spray which constantly lands on the cowling of your outboard engine you will be generating future problems with corrosion or paint fading on the engine.

Typically the mounting bracket will allow for some adjustment of the immersion without having to make new holes. Often a change of as little as a quarter of an inch can make a difference. A well-installed transducer can retain echo sounding capabilities at high speeds. It may not be able to spot fish at 40 MPH, but it certainly should be able to track the bottom echo very reliably. It will also not throw much spray.

Cable Path

Although it is frequently done, I do not recommend routing the cables from the tranducer through the drain holes of the motor well transom drains.

I prefer to route the cables up and over the transom. Installing them this way has these advantages:

The cables should be well secured to the transom so that in the event the transducer is struck by an underwater object it cannot fly completely free and swing on a long length of cable. It could be projected back into the boat and strike someone or become entangle with the propeller.

Cable Path to Receiver

The cable connecting the transducer to the sender/receiver unit is carrying broadband radio signals but it may not be as well shielded as one might like. For this reason I recommend keeping the transducer cable away from long parallel and tightly-bundled runs with other cables that carry electrical or radio signals. This includes especially the VHF marine radio antenna lead, but also the engine's electrical cables. There is often the potential of radio interference from the engine sparking, and cable separation may help to suppress this somewhat. If in doubt, experiment with the cable bundling and path.


[Photo: Transducer mounting on transom
Transom Transducer Mounting
The transducer orientation must be aligned with the sea bottom, not the hull bottom. The unit shown is a LOWRANCE Skimmer transducer. Slots in the bracket permit slight adjustment of the height. This installation works at speeds of 35-MPH.
Photo Credit: JWH.


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Additional Reading

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Copyright © 2002 by James W. Hebert. Unauthorized reproduction prohibited!

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Author: James W. Hebert
This article first appeared June 2, 2002.