Transducer Install Tips

Garbage in, garbage out. The phrase was coined in the early days of computer programming to convey the idea that, since computers couldn’t think for themselves, no machine could produce good output based on bad input.

Arguably, that’s changing with the advent of artificial intelligence and machine learning, but it’s still true of our fishfinders. No matter how slick and intuitive their interfaces, how fast their processors or how big and bright their displays, the quality of their output is entirely dependent on the quality of their input — the raw data they receive from the transducer.

And that depends on both the performance characteristics of the transducer and the quality of the installation.

There’s little debate that, for most fiberglass boats, through-hull transducers deliver the best results, especially at speed. They are located in clean water, well forward of the engines, and their location makes it easier to run cables separately from other cables and electronics that can cause interference.

That being said, there are several drawbacks to through-hull transducers. The transducers themselves are generally more expensive; installation requires a 1” to 3” hole in the hull bottom and is not a job for most DIYers; and models that extend below the hull bottom can be damaged by hitting floating debris, the bottom and even trailer bunks.

All things considered, a transom-mounted transducer is the best overall choice for most trailerable boats. They’re (relatively) affordable, easy to install, don’t require cutouts, and perform comparably to a through-hull transducer with equivalent specs at less-than-planing speeds. They’re also easy to remove and replace when you upgrade to a new unit.

The main shortcoming of transom-mounted transducers is spotty performance when on plane. Even the best transom-mount installations won’t hold bottom at speed as well as a through-hull. But with a careful installation and some experimentation, they can come very close.

Here are a few things to keep in mind beyond what your installation instructions will tell you:

This really goes for every situation in which you’re tightening self-tapping screws into fiberglass, but it’s even more important below the waterline. Resist the urge to use your favorite electric drill or driver and instead tighten your screws by hand to avoid stripping.

Somewhat more difficult — at least on V-bottom boats — is getting the transducer level with the waterline when viewed from the back. A few degrees one way or the other doesn’t make a big difference with down-looking sonar, but it can significantly affect performance with side imaging. The challenge is that you can’t use a level since you can’t assume your boat is sitting level on its trailer.

If your transom has a straight, horizontal line across it — a bottom paint line, a boot stripe, an aluminum transom cap, the bottom of a jackplate etc. — you can use that and a framing square or something similar to mark a vertical line and then line your transducer bracket up with that line. If there’s no obvious horizontal mark, you can use a string to mark a line across the transom from one chine to the other.

Most installation manuals recommend positioning the transducer so that its bottom surface is parallel with the hull bottom and either flush with the jull bottom or 1/8” below. 

Why not just mount it lower in the first place? Because the lower your transducer is mounted, the more drag it produces and the more exposed it is to strikes from floating debris. When mounted too low, the “nose” of a transducer can even create turbulence of its own, hurting performance. If you’re getting a roostertail of white water from the nose of the transducer, it’s too low.