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.