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.

In-hull, or puck-style, transducers are a hybrid, designed to be mounted inside the hull, up against the bottom of the boat, where they shoot their beam through the fiberglass. A well designed in-hull transducer loses very little sensitivity. They can perform better than a transom mount and be easier to install than a thru-hull. But speed and temperature sensors, which require direct contact with the water, are not an option.

SELECTING A TRANSDUCER

The brand and capabilities of the transducer you select may largely be determined by the rest of your gear. While most transducers are made by one company, Airmar, you will need to select a transducer that plays well with the display unit of your system. The choices in beam width, frequency, dual-beam, dual-frequency and even the cable connectors will play into your selection. It makes sense to buy the transducer recommended by the manufacturer so that you know the gear will play well together.

Often, the question will be whether to stick to a baseline model or whether to upgrade. Again, look to the rest of your equipment first. If you are buying high-end gear, you may need a more advanced transducer to take advantage of all the features. You should also give a thought to future upgrades, and whether your transducer will support the gear you plan to buy.

Overall, like so many boating decisions, your choice should be guided by how and where you use the boat. Do you fish, or are you simply trying to avoid shoals? Do you cruise in shallow river waters, or are you out on the ocean? By answering simple questions like these, and taking a look at the capabilities of your display and sounder, you can begin to develop a profile of your needs.PAUL ESTERLE

From there, it's a matter of research. Read the reviews, both on-line and in print, and look at the materials published by the manufacturers. Most companies have the manuals for their units posted on their web site, where they can be downloaded for free. Reading the manuals will give you a better feel for what units can do, what the options are, how they compare and how they are installed.

You can also seek out knowledgeable people. Talk to your mechanic, an electronics dealer and walk the docks and find out what the folks in your marina are using. When you have a model or two in mind, you can post questions to online marine forums and hear from other owners what their experience has been.

Unlike many other choices you make about equipment, this one should narrow itself down quickly, based on your needs, your existing gear and your budget. But the research is worth the time, because you don't want to buy a transducer that limits the rest of your gear or compromises.

THE ART OF INSTALLATION

Installing a transducer is a common do-it-yourself job, and we'll cover how to do all three types. Transom mounted transducers are probably the simplest. A plastic bracket is fastened to the transom, holing the transducer at the proper level in the water. Some transducers also carry a paddle wheel for speed, as well as a temperature sensor.

Location is critical for accurate readings. It must be installed so that it is parallel to the water line and slightly underwater water at full speed, with the boat on plane. The transducer should be located at least fifteen inches from the prop, in an area free of turbulence and bubbles. These bubbles can be caused by rivets, strakes and the like. Look over the transom when the boat is at speed and locate an area where water is coming off smoothly and without bubbles. That is your target.

Be sure to properly seal the fastener holes used to hold the bracket to the transom. Even a small mounting screw can allow enough water to leak into the transom and eventually cause rot. Ideally, you should drill the holes oversized and fill then with epoxy filler, allowing you to screw into solid epoxy rather than the core of the transom. Barring that, take care to seal the holes extremely well using a sealant like 3M's 4200. Do not use silicone; there are better sealants.

Being exposed on the transom also means that the transducer is also subject to damage from debris, grounding or, on a trailerable boat, during launching and retrieving. Luckily, these types of transducers are also among the cheapest, so a replacement won't bankrupt you. If you decide to install your transducer on your trolling motor, make sure you have the appropriate transducer (motor-mounted units are often separate models) and follow the instructions carefully.

Transom-mount transducers can also be removed from the bracket and installed as a thru-hull mount in some cases, but be sure to follow the instructions for proper mounting and orientation. You may experience a slight loss of sensitivity installing a transom mount transducer inside the hull.

THRU-HULL INSTALLATIONS

Perhaps the most common type of transducer is the thru-hull type. As the name implies, a hole is drilled through the hull and the transducer is fastened in place. These transducers have a mushroom head and a threaded shank. The transducer is installed from the bottom and a nut threaded onto the shank inside the hull to hold the unit in place.PAUL ESTERLE

As in most jobs, the details are important. If the hull is fiberglass, it is important to seal the edges of the fiberglass with epoxy. If the hull is cored, remove the core around the hole and replace it with epoxy filler. This will prevent any water intrusion into the laminate or core and keep the transducer nut from crushing the core. Take careful note of the sealant recommended by the manufacturer's instructions, because some plastic transducers are incompatible with certain sealants.

One problem installing a thru-hull transducer is that the hull may be angled, as it is in a V-bottom boat. This shoots the transducer beam at an angle, not directly downward. If the angle (deadrise) is slight (less than 10 degrees), it may not pose a problem. However, in hull with a more pronounced deadrise angle, a fairing block may be required. A properly designed fairing block can also reduce turbulence around the transducer, improving performance.

A fairing block has the top surface angled to match the hull while the bottom surface is horizontal. To make installation easier, a matching block on the inside of the hull allows the mounting nut to tighten evenly. The block itself can be made of wood, plastic or bronze, though wood is not recommended. Some transducers are available with a fairing block attached, or as an option.

To make the installation as trouble free as possible, do your homework before hauling the boat. Have the right hole saw, which will be noted in the manufacturer's instructions; make sure there are no obstructions inside the hull, where you intend to mount the transducer; make sure the transducer cable is long enough to reach the display; and have all the necessary supplies on hand, such as sealant and tools. When you install the transducer, don't over-tighten the nut. You'll squeeze out most of the sealant, providing a spot that could leak.

IN-HULL INSTALLATION

With all the potential problems caused by drilling big holes in the bottom of a boat, it is not surprising that in-hull mountings are popular. They work well in fiberglass hulls that are made of solid glass laminate and are not cored. Any core, or even bubbles in the glass, will produce erroneous readings. Some hulls already have a pocket molded in the bilge for such a mounting. If you can't find it, ask the dealer or manufacturer if the boat has one.PAUL ESTERLE

Some owners just go ahead and mount the transducer, then hope for the best. A more successful approach is to systematically test the accuracy of the transducer location. You will need to have the head unit installed and powered up and the transducer connected.

With the boat motionless over a level bottom, hold the transducer in the water and note the depth. Then, test the trial location in the bilge. If the bilge area can be dammed off, fill it with water and place the transducer in the pool. If the location is okay, the reading on the head unit should closely match the reading taken with the transducer held over the side. If the location is okay, mark it and proceed to install the unit.

If you can't provide the requisite pool of water, there is another solution. Take a short length of PVC tubing and angle the bottom to match the deadrise of the hull. The PVC tubing should be vertical. Temporarily seal the tubing to the hull with a strip of silicon sealant. Fill the tube with water and place the transducer in the tube. Weight it down so it is at the bottom and take a reading, if the reading matches the outside reading, you're good to go.

The reason for the water is simple: transducers do not work shooting through air. A transducer spots fish in part by bouncing the acoustic signal off their air-filled swim bladders. The bigger the bladder, the bigger the fish, the bigger the blip. Even air bubbles in fiberglass or in a boat's wake can throw off readings. By submerging the unit in water, or light oil in some cases, it provides a contiguous signal path, interrupted only by the fiberglass in the hull. Check the transducer instructions for the right liquid.PAUL ESTERLE

It sounds like a lot of work but once you've done it, you know the location is correct. Remove the tubing or pool of water and dry out the mounting area. You can epoxy the tube in place, cap it and run the transdiucer cable through the cap, if you choose. Either way, the transducer must be placed in the location and permanently mounted. Here is where practical experience differs from the manufacturers instructions. Most manufacturers suggest using a thick, slow setting epoxy to mount the transducer in place. Coat both the hull and the transducer and push the transducer in place twisting the transducer to ensure that there are no air bubbles underneath.

A more common mounting method is to place a large blob of sealant in the mounting area and push the transducer into the sealant, making sure there are no bubbles. This makes for a much quicker installation. Just be sure to have the boat in the water so you can test the accuracy of the unit before the sealant cures. Also, make sure you use the appropriate sealant suggested by the manufacturer, as some can attack the plastics used in the transducer. Avoid 3M's 5200 because it will be difficult to remove the unit later if you want to replace it.

There are some special considerations in mounting transducers in metal or wood hulls, which should be covered very well in the manufacturer's installation directions (another good reason to preview the manuals online before you buy).

Like most of the projects aboard our boats, it isn't rocket science. But it does require that you pay attention to the details and follow the instructions. Do that and you will be tracking down those big fish pretty quickly.

Paul Esterle is Technical Editor for Small Craft Advisor Magazine, and his freelance work has appeared in Sail, BoatWorks, Voyaging and Good Old Boat magazines. He has produced a series of boating videos and lectures widely. He also works at West Marine and has written product reviews for the company.