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.