A fish finder is only as good as the transducer it uses. So boating anglers need to put just as much thought into what type of transducer they choose as in the fish finder itself. One of the biggest decisions facing anglers revolves around transom-mount versus thru-hull transducer styles.
To help lend insight and perspective, we asked Craig Cushman, director of marketing for Airmar Technology Corp., to explain the upsides and downsides of each. Today’s leader in transducer research, development and manufacturing, Airmar builds in the US many of the transducers offered by major electronics brands, including Furuno, Garmin, Lowrance, Simrad and Raymarine.
To keep the discussion focused, we asked Cushman to compare and contrast two of Airmar’s most popular and advanced transducer series—the TM185 transom-mount series with the B175 tilted-element low-profile thru-hull series. Both are available with 1kW chirp-ready high-, medium- and low-frequency transducer elements in a range of beam angles.
Before getting to the pluses and minuses, Cushman offers an important caveat. Both of these transducer styles are best suited to outboard-powered conventional hulls under 36 feet in length overall, he says. Boats longer than 36 feet tend to create a thicker “boundary layer”—aerated water under the hull while underway—than smaller boats. Air in the boundary layer tends to interfere with the transducer’s echo-sounding signals and returns. “Both the TM185 and the B175 series are relatively low profile in nature, and that works well on smaller boats with comparatively thin boundary layers,” Cushman notes.
“But, larger boats need a thru-hull transducer that protrudes farther down into the water, beyond their inherent boundary layers,” says Cushman, who cites the Airmar B265 stem-style thru-hull with an optional fairing block as a popular choice for bigger boats.
Transom-Mount Upsides
Ease of installation represents the biggest upside of a transom-mount transducer such as the TM185. Unlike the B175, there is no need to cut a hole in the bottom of the hull, an anathema to some boaters, Cushman points out. Instead, the TM185 requires drilling four 5mm screw holes, each 22mm deep near the bottom of the transom, for the mounting screws. This makes the TM185 an easy do-it-yourself installation. Should you make a mistake, patching a 5mm hole is much easier than repairing a 3.75-inch hole needed for the B175.
Transom-mount transducers are also less expensive than comparable thru-hull models. For example, Hodges Marine retails the Airmar TM185HW (high-frequency chirp, wide beam) for a Furuno echo sounder for $921.60. On the other hand, Hodges’ price for the comparable B175HW for a Furuno starts at $1,250.86.
Adjustability stands as another advantage of a transom-mount transducer. The bracket of the Airmar TM185 series allows you to easily move the ’ducer up or down to fine tune the height. You can also adjust it horizontally, tilting the aft end of the transducer ever so slightly to better penetrate the boundary layer.
“Transom-mount models also prove easier to replace should one become damaged or you decide to change to another chirp-band,” Cushman says. “If trading for another TM185 model, you can use the same bracket and just route the new cable.”
Finally, if you trailer your fishing boat, transom-mount transducers are less likely to interfere with bunks or cross-members when launching and loading.
Thru-Hull Upsides
“The greatest advantage of the B175 tilted-element series lies in how deep these transducers are immersed in the water,” Cushman says. “Depending on the boat hull, they can be mounted very close to, if not on, the centerline on boats that have wide keel pads back by the transom.”
On my single-outboard 22-foot deep-V center console, for instance, the B175 transducers (I have two—one High Wide and one Low, both 20-degree tilted elements—on each side of the keel) sit 5 inches deeper on the hull than the transom-mount model. The B175s are 4 inches from the centerline, while the transom-mount model sits 10 inches from the centerline.
Because B175 transducers extend only about ¼-inch below the bottom of the hull, they create minimal turbulence and so do not disrupt water being fed to outboard propellers while underway.
Unlike transom-mount models, the B175 series can perform effectively on step-hulls, but need to be installed forward of the first step to avoid interference from the aeration streaming under the hull aft of the steps while underway, Cushman points out.
Read Next: Selecting the Right Transducer Frequency for Deepwater Fishing
Transom-Mount Downsides
Susceptibility to aeration ranks as the main disadvantage of a transom-mount transducer, says Cushman. This can impair fish finder performance while the boat is underway, but not when at idle speed or at rest. Excessive aeration is often the result of improper installation or poor adjustment, but the good news is that with the right adjustments, it often can be corrected or at least minimized.
Thru-Hull Downsides
As mentioned earlier, overcoming the apprehension of cutting the 3.75-inch hole required for installation ranks as the biggest downside of the B175 series. With cored hulls, installation gets even more complicated, and in this situation, Cushman recommends turning to a qualified shop to handle the work. Cored hulls or not, installation costs are going to be higher than for a transom-mount.
Finally, it is not nearly as easy—or inexpensive—to replace or upgrade a B175 transducer as it is with a TM185. “This is partly because removing one of the big thru-hull transducers can be quite an undertaking,” Cushman explains. “Then comes the installation of the new transducer.”
Ultimately, the answer to the common question—thru-hull or transom-mount transducer—boils to the type of boat and the boat owner’s mindset. Both can sometimes change as boating anglers buy new boats and/or consider new transducer technologies. I am a case in point. I was a transom-mount man for many years, but as tilted-element technology advanced, I eventually switched over to the thru-hulls and found to my delight that they perform best on my boat.
