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Transducer Selection


By Capt Ed Wiser for Coastal Angler Magazine, July and August 2013

The relationship between the transducer and the fishfinder is similar to that between the VHF radio and its antenna. They are very valuable together and worthless without each other. The transducer is basically an underwater antenna transmitting a signal into the water. When it bounces off an object or the bottom, the signal is reflected back to the “ducer.” The time it takes for this to occur is measured in milliseconds and the fishfinder computer uses this time lapse to calculate depth of object. Radar functions the same way in air.

The clarity and overall accuracy of the fishfinder is critically dependent upon the type, frequency, and location of the transducer. Transducers may be fixed to the transom, glued inside the hull, thru-hull mushroom type with internal element, and thru-hull type with high speed fairing block.

The transom mount is the least expensive, the most susceptible to damage, most likely to be fouled by weed or debris, and the least accurate. There is a common misconception they are also easiest to install but that is not necessarily correct. They are often mounted on the wrong side of the boat, placed too deep or too shallow, or use inadequate fasteners or sealant. It is not unusual to remove an old transducer and have water seep or even flow out of the transom. This water intrusion may be due to an opening elsewhere in the transom but it usually is due to poor installation and use of silicon as a sealant or no sealant at all.

Transom mount transducers are often broken while retrieving the boat on a trailer or by forklifts in dry storage facilities. They can interfere with trim tabs and trailer tie downs. They can snag lots of weed and need frequent clearing – not a very safe activity when underway. The biggest problem for serious fishermen is that no matter how well installed, there is no avoiding the constant turbulence boiling off the transom and degrading the signal. This can be a big problem or negligible but it is always present to some degree and degrades the image. Finding the optimal position to mount one is complicated by all these factors plus any obstruction in the hull like thru-hulls, molded in strakes, or steps. Using a transom-mount transducer with a sophisticated bottom machine is like getting a top-of-the-line stereo and using a coat hangar for an antenna.

There are a number of different in-hull transducers. They range from a transom mount transducer glued inside the hull, or a two-part “puck type, to the highly capable 1000 watt units. These are glued or otherwise fixed in place within the hull and do not have direct contact with the water. The common element is that they cannot be used in hulls with a core material like foam, honeycomb, and often not even with a laminated coring material. When in doubt contact the manufacturer. Many boatbuilders embed an area of solid laminate designed specifically for transducers. Call customer service if in doubt. If the boatbuilder is out of business (a common occurrence these days) and you are in doubt, it is smart to consider another type of transducer. The in-hull transducer is sometimes difficult to install, especially the 1000 watt variety. It may involve construction of a box for the unit or cutting a manufacturer supplied box to conform to the contour of the hull. Be forewarned, these boxes are not cheap and if you get the angle wrong you may have to buy a second one. As with many transducer installations, simply getting access to the desired point can be a real challenge.

In-hull mounts can be problematic too. I recently installed thru-hull transducers in two boats that had voids in the laminate. One small one was in a 10-year old Grady-White 24. The other was a huge void in a 19′ Key West. If you install an in-hull transducer and it does not read the bottom, you may have the same problem.

The third common type is the thru-hull transducer. There are two basic varieties, the nearly flush mount flanged head and the tapered lozenge mounted in a high speed fairing block. The latter is the most difficult to install since it requires a precise measurement of the hull deadrise at the mounting point and precision cutting with a band saw. Remember, deadrise is not necessarily constant and is usually different at various points on the hull. The fairing block is a hard polymer with a recess to insert the transducer. It is one of the biggest and bulkiest transducers, usually the most expensive of the types available for a given frequency and application, but also the most turbulence free and reliable at high speeds. It requires skilled application and usually is a two man job.

The modern flanged head transducer is a boon to fishermen using trailerable boats. It is made of bronze and will withstand a pressure of 2,000 psi or more. That does not mean you can pull it across a steel post without damage but it does mean it can go across a bunk or roller. Plastic thru-hull transducers like the type used in sailboats cannot stand up to this punishment and should be avoided. Some bronze Airmar transducers as used by Garmin, Lowrance, and Raymarine have internally tilted elements that are angled to accommodate the deadrise of the hull. These are mounted flush with the hull and do not require a fairing block to make it shoot straight down. The days of having to fashion and install a synthetic or wood fairing block are over. The flanged head measures about three inches in diameter and only protrudes 3/16″ beyond the hull surface. I very strongly recommend this type for trailerable boats though the cost is about three times that of the transom mount models. The results are far superior at all speeds. They do require cutting a 2 3/8″ hole in the bottom of the boat though. Some owners are skeptical about cutting a hole in the bottom of the boat but in thirty years I have never had a transducer leak.

There are other types, too. Some are built into a well or recess in the keel and some inside a well built into the hull. These are more common in large yachts and fishing vessels and are rare in recreational fishing boats. There are also specialty transducers for metal hulls.

Transducers and fishfinders use different frequencies and power ratings with a wide variety of capabilities. These are selected based upon most commonly encountered depths, low frequencies generally being best for deep water and high frequencies for shallow water. Most transducers are capable of using two or more frequencies. The signal frequency and depth of water determine the size of the “cone” the signal projects to the bottom.

The power of the fishfinder is determined as watts. More power means greater ability to penetrate sediment in the water column, reach deeper depths, and achieve better target resolution. While 500 watts is generally adequate for our shallow Gulf waters, serious deep water anglers will want to think about the 1000 watt units. Lake boaters are fine with 200 watt models. Generally, the more power the better the results. Whichever power rating you choose, it must match well with the transducer.

Frequency is another important consideration. Transducers may have one, two, three frequencies or a myriad number  using the broadband CHIRP type. Use lower frequencies for deeper water. Higher frequencies give better resolution with less background scatter and are more suitable to higher boat speeds. If you fish in 200′ or less use 200kHz and save the 50kHz and 80kHz for deeper waters

The tradeoff is clear. For deeper offshore waters tend toward a low frequency ducer with high power.  Do the opposite in shallow waters where bottom detail and wide beamwidth are more important.

Transducer placement is as critical as transducer type. Avoid installing a transducer aft of a step on any hull. It is always surprising that some builders of high end “fishing” boats make optimal transducer positioning impossible. Think Donzi, Yellowfin, and Contender as three offenders among many. They will either include steps that force the ducer to be so far forward it occasionally comes out of the water, or mold in a tank over the only good mounting location in the boat. This type of negligence in design and construction will continue until the boating public forces a change. Never mount a transducer aft of a step.

Reduction of turbulence and creation of smooth water flow is critical to transducer performance.

Most transducers transmit depth, temperature, and a few include a paddle to give speed. This latter feature comes at addition cost and complexity. In an era when most boats have a chartplotter it is redundant and a needless expense.

The critical factors for satisfactory results are transducer power, frequency, location, and installation. Even the finest and most powerful transducers will give mediocre performance if not properly located and installed.