The transducer must be able to withstand high transmitter power impulses, converting as much of the impulse into sound energy as possible. At the same time, it must be sensitive enough to receive the smallest of echoes.
The transducer is the sonar unit’s "antenna". It converts the electric energy from the transmitter to high frequency sound. The sound wave from the transducer travels through the water and bounces back from any object in the water. When the returning echo strikes the transducer, it converts the sound back into electrical energy which is sent to the sonar unit’s receiver. The frequency of the transducer must match the sonar unit’s frequency. In other words, you can’t use a 50 kHz transducer or even a 200 kHz transducer on a sonar unit designed for 192 kHz. The transducer must be able to withstand high transmitter power impulses, converting as much of the impulse into sound energy as possible. At the same time, it must be sensitive enough to receive the smallest of echoes. All of this has to take place at the proper frequency and reject echoes at other frequencies. In other words, the transducer must be very efficient.
The active element in a transducer is a man-made crystal (lead zirconate or barium titanate). The chemicals are mixed, then poured into molds. These molds are then placed into an oven which "fires" the chemicals into the hardened crystals. Once they’ve cooled, a conductive coating is applied to two sides of the crystal. Wires are soldered to these coatings so the crystal can be attached to the transducer cable. The shape of the crystal determines both its frequency and cone angle. For round crystals (used by most sonar units), the thickness determines its frequency and the diameter determines the cone angle or angle of coverage (see Cone Angles section). For example at 192 kHz, a 20 degree cone angle crystal is approximately one inch in diameter, whereas an eight degree cone requires a crystal that is about two inches in diameter. That’s right. The larger the crystal’s diameter - the smaller the cone angle. This is the reason why a twenty degree cone transducer is much smaller than an eight degree one - at the same frequency.
Transducers come in all shapes and sizes. Most transducers are made from plastic, but some thru-hull transducers are made from bronze. As shown in the previous section, frequency and cone angle determine the crystal’s size. Therefore, the transducer’s housing is determined by the size of the crystal inside.
There are four major housing styles in use today. Thru-hull, shoot-thru-hull, portable, and transom mount.
are inserted through a hole drilled in the hull. These typically have a long stem that slides through the hull and is 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 "vee" hull, then a fairing block must be made from wood or plastic that allows the transducer to be mounted vertically. Thru-hull transducers are typically used on inboard powered hulls so the transducer can be mounted in front of the rudders, propellers, and shafts.
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 cost of some loss of sonar performance. (You won’t be able to "see" as deep with a shoot-thru-hull transducer as 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. Another disadvantage of the shoot-thru-hull transducer is it can’t be adjusted for the best fish arches. Although there are disadvantages to a shoot-thru-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 laminar flow of water passes over the hull. Three, it can’t be fouled by marine growth.
Portable mount transducers
, as their name implies, temporarily mount to a boat. These transducers typically use one or more suction cups to hold them to the hull. Some portable transducers can also be adapted to electric trolling motors.
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 (like the Lowrance HS-WS Skimmer®) will work on almost any hull (except inboard powered boats) and at high speed.
Speed and the transducer
Years ago, when the sportfishing sonar unit was in its infancy, most fishing boats were small, outboard powered affairs. A really large outboard had 50 horsepower. At the same time, most sonar units were portable, with the ability to move from boat to boat easily. This was considered more important than high speed operation. However, as boat capabilities increased, more and more people wanted a permanently mounted sonar unit that would work as fast as the boat. Thus, the search began for a transducer that would work at all speeds.f
Cavitation is a major obstacle to achieving high speed operation. If the flow of water around the transducer is smooth, then the transducer sends and receives signals normally. However, if the flow of water is interrupted by a rough surface or sharp edges, then the water flow becomes turbulent. So much so that air becomes separated from the water in the form of bubbles. This is called "cavitation". If these air bubbles pass over the face of the transducer (the part of the housing that holds the crystal), then "noise" is shown on the sonar unit’s display. You see, a transducer is meant to work in water - not air. If air bubbles pass over the transducer’s face, then the signal from the transducer is reflected by the air bubbles right back into it. Since the air is so close to the transducer, these reflections are very strong. They will interfere with the weaker bottom, structure, and fish signals, making them difficult or impossible to see.
The solution to this problem is to make a transducer housing that will allow the water to flow past it without causing turbulence. However, this is difficult due to the many constraints placed upon the modern transducer. It must be small, so that it doesn’t interfere with the outboard motor or its water flow. It must be easy to install on the transom so that a minimum of holes needs to be drilled. It must also "kick-up" without damage if struck by another object. Again, the patented design of the HS-WS transducer is Lowrance’s latest improvement in high-speed transducer technology. It combines high speed operation with easy installation and will "kick-up" if struck by an object at high speed.
The cavitation problem is not limited to the shape of the transducer housing. Many boat hulls create air bubbles that pass over the face of a transom mounted transducer. Many aluminum boats have this problem due to the hundreds of rivet heads that protrude into the water. Each rivet streams a river of air bubbles behind it when the boat is moving, especially at high speed. To fix this problem, mount the face of the transducer below the air bubbles streaming from the hull. This typically means you have to mount the transducer’s bracket as far down as possible on the transom.
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