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As the technology of sonar (an acronym for Sound Navigation and Ranging) has developed, prices have dropped dramatically at the same time that choices have widened. What once was a simple electronic device meant to determine the water's depth now has numerous capabilities to find fish, scan the bottom for obstructions, or create underwater maps.
Transducer
Whether it's a simple portable fishfinder or a commercial mapping system weighing several tons, all sonar devices operate on the same principles using two parts: a transducer and a processor/display. The transducer is both a sender and receiver. It is placed in the water, or in a medium adjacent to the water, and emits an acoustic signal with a characteristic waveform. This signal passes through the water at a known rate of speed and is reflected off any object with a density different than water. When part of the acoustic signal strikes an object, it is reflected back to the transducer, which receives it and sends it to the processor.
The processor/display is a group of electronic circuits that interprets the signals received by the transducer. It will have some mechanism to display the results, ranging from a flashing light to color LCD graphic maps. Dedicated units often have some controls for signal gain or for other features. Some scanning models may even use a laptop computer as the processor and display.
Finding the proper depth sounder or fish finder to meet your needs requires a choice between numerous brands and models. Many of us have a tendency to solely concentrate on the features or appearance of the processor/display unit, neglecting the transducer altogether. This mistake can lead you to the wrong choice.
Transducer Frequency
Transducers for recreational and light commercial boats usually operate on frequencies between about 25 and 400 KHz, with 50 and 200 KHz being the two most common. As with most radio or radar transmissions, lower frequencies generally have greater penetrating power while higher frequencies offer higher resolution or detail. A low frequency around 50 KHz is excellent for displaying a wide area of the bottom, especially if the water is deep, while a higher frequency shows more bottom detail in shallow water, a very useful trait for fishfinders and recorders. A depth sounder signal that reads up to 400' in fresh water may lose as much as half of its penetrating power in saltwater, so lower frequencies may be favored for saltwater use.
Many fishfinders have dual-frequency transducers, with 50/200 KHz combinations being the most common. Since the body of a fish is mostly water, the depth finder signals don't bounce directly off their bodies — it is the air in fishes' gills and air bladder that create the target. With dual frequencies, the low frequency can search a wide area of deep water for targets, showing schools of fish as wide patches on the screen. The higher frequency offers more detail of objects directly below the boat, showing better bottom formations, bottom-feeding fish, and schools of baitfish, especially in shallower waters.
There is wide variety in dual-frequency and multiple frequency models. Some have the ability to switch back and forth between two frequencies, while others offer two screens to view simultaneously. A few even have the ability to detect their own acoustic signals when a large number of boats fishing nearby with similar units create "crosstalk and other interference". The transducers of some high-end scanning sonars have multiple elements within their housings, allowing them to "see" in front of the boat or out to the sides.
Transducer Beam Width
A second important feature when selecting a depth finder transducer is the width of the beam (also known as cone angle) it was designed to emit. Transducers with a wide beam width of up to 50° will "see" around their location at a wide section of the bottom. As the amount of area is increased, the bottom resolution is decreased, so a wide beam is fine for searching for fish in the water column but not detecting bottom details like breaks, shallow holes or small contours.
A narrow beam of 10-15°, on the other hand, concentrates its energy on a section right below the boat. When looking for individual fish or bottom structure in shallow water (say under 100-200'), a narrow-beamed transducer should be the first choice. Note that there is a certain amount of correlation between a transducer's frequency and its beam width. As a general rule, lower frequency and a wider beam angle tend to go together while higher frequency and narrower beam width are usually companions. This makes sense, as the latter makes a great depth sounder while the former is an obvious choice as a fishfinder.
Output Power
Output power is quoted in watts just as any transmitter or audio receiver. Here, however, the ratings have little meaning and more power is not always better. Too much power in shallow water is as ineffective as too little power in deep water. Generally speaking, units in the 100-200 watt range are sufficient for shallow water use and units up to 1,000 watts and more may be necessary to probe deep canyons and drop-offs. Using a transducer from a 100-watt unit on a 1,000-watt unit is also not a good idea as the excessive energy from the more powerful fishfinder will soon destroy the crystals within the small transducer.
Transducer Materials
In the distant past, the working portion of all transducers was captured in a traditional bronze housing with a long, narrow stem and a curious aerodynamic shape. Today, many transducers are contained in space age strong plastic housings. These tough new materials have great advantages over bronze in reducing corrosion and electrolysis, and are designed to stand up to the abuse of long-term immersion in seawater.
Transducer Mounting and Style
Transducers may be installed through the hull, in water- or oil-filled boxes inside certain hulls, attached to the inside of some fiberglass boats, or mounted on the transom. The only requirement is that no air space or high-density material blocks the transducer's stream of acoustic pulses. Because of this flexibility in mounting, several styles of transducers are manufactured, assuring easy installation on all types of boats.
Long-stemmed bronze transducers are still best when mounting through the hull on a boat with a very deep "V" or high deadrise angle where fairing blocks must be used to keep the transducer close to vertical.
Low-profile plastic transducers with a mushroom head are easiest to mount through the hull on a boat with a flat surface and little "V" shape.
A few low-profile through-hull models have a hollow plastic housing and a removable inner transducer. These are ideal for trailerable boats where the transducer might be damaged in the launching process.
Flush-mount transducers are ideal for depth sounders on racing sailboats and other applications where a fair hull is important. These sometimes require a special countersinking tool for proper installation.
Transom-mount transducers are usually flat on their leading edge, with an adjustable bracket or pivoting hardware to attach to the boat's stern below the waterline. When installing a transom-mount transducer, make certain it is mounted in an area of the transom with the least amount of disturbed water. It is not uncommon to find the ability of your transom mount transducer to show clear images is drastically reduced as the speed of the boat increases. Most often a small "adjustment" of the tilt or depth of the transducer can greatly improve its performance at high speed. On some vessels however, you may need to experiment by placing the transducer at different spots on the transom to obtain the optimum results.
Some transducers are specifically made to shoot through the hull. These are generally more accurate than standard transducers used in that manner.