We've all seen those World War II submarine movies with the ping-ping of sonar ringing in the background. Though we no longer hear the pings, today's depth sounders and fish finders operate in much the same way as their military cousins.
The unit producing the sound is a transducer, a hull-mounted sensor that collects vital information and sends it to the monitor at the helm for display. Transducers remain one of the least-understood components in a modern electronics system, yet choosing the right marine transducer and installing it properly can make a major difference – good or bad – in the quality of the data you receive behind the wheel.
The principles behind sonar (Sound Navigation And Ranging) are really pretty simple. A sound wave is sent from the bottom of the hull down into the water; the wave bounces off structures, fish or the bottom; and the returning echo is interpreted as data and pictures. Think of it as under-water radar.
It is the transducer that generates the sound and interprets the echo, making it an essential component of the system. But it stays under the boat and out of sight, and many owners put far more time into selecting the head unit at the helm. Choose the wrong transducer or install it poorly, however, and even the most capable head unit will be crippled.
BEAM WIDTH, FREQUENCE AND POWER
There are two main transducer characteristics that define their utility, the first of which is beam width (also called cone angle). Typical beam widths range from wide beams of 45 to 50 degrees to narrow beams of 10 to 15 degrees. A wide beam width obviously covers more area in the water beneath the boat, but this also generates more information to process and results in a slower display. A narrow beam gives a quicker response, especially in deeper water, but an obstruction or school of fish might be missed if it falls outside of the beam.
PAUL ESTERLE
A second characteristic is frequency of the acoustic pulse or ping. The two most common frequencies are 50 KHz and 200 KHz. Low frequencies penetrate further, giving better readings at greater depths. They also give a better look at bottom composition. High frequencies give sharper focus but tend to reflect better, making them less capable in deep water. They also give less bottom definition.
Many modern fish finders can utilize dual frequency transducers, providing the best of both frequencies. Some high-end fish finders are also designed to use multiple transducers, providing greatly expanded bottom coverage at no loss in definition.
Of lesser importance, despite what manufacturers may say, is the unit's power output. Unless you are fishing at great depths, high powered transducers–those of more than 500 to 600 watts–are usually unnecessary. Most low-end units start at about 200 watts, which is good for fishing shallow waters and lakes. Look for a power rating of 300 to 500 watts for deeper waters.
TRANSOM MOUNT, IN-HULL AND THRU HULL
Most transducers are made of plastic, which holds up well when properly installed. Stronger models are made of bronze, but they are more expensive. One manufacturer, Airmar, makes most of the transducers on the market and there are a wide range of choices.
Transducer designs can be reduced to three basic categories: transom-mount, thru-hull and in-hull. Transom-mount transducers are installed, as their name implies, on a bracket attached to the transom (some also mount on the lower unit of an electric trolling motor). They are often made of plastic and sometimes come with sensors to read speed and water temperature.
Normally associated with smaller boats, transom mounts sometimes offer readings of lesser quality, because they are subject to turbulence from the propeller. However, they are also easier to install that other designs and they make a relatively simple weekend project.
Thru-hull transducers require drilling a hole through the hull and bolting the transducer in place utilizing a big nut on the threaded stem of the unit. These models, which also can be equipped with speed and temperature sensors, may come with a fairing block to align the transducer beam downwards on a deep-vee hull.
Often installed on larger boats, these units offer superlative performance, but they do place another hole in the hull that can leak and cause problems. Installation is also more complicated and expensive, requiring the boat to be hauled from the water.
