Transducer Buying Guide
A transducer is the center-piece of all things happening underwater. The device sends pulses downward into the water that are then converted to sound. The sound pulse “reflects” and travels back to the boat and is then translated into usable data such as depth, structure, etc. That data is “translated” into an easy to understand format to show on a fish finder or mapping system. If you’re fishing and have the desire to see water depths, structure, or movement under your boat, a transducer is a must-have feature.
Transducers for recreational and light commercial boats usually operate on frequencies between about 25 and 400 KHz, with 50 and 200 KHz being the two most common. As with most radio or radar transmissions, lower frequencies generally have greater penetrating power while higher frequencies offer higher resolution or detail. A low frequency around 50 KHz is excellent for displaying a wide area of the bottom, especially if the water is deep, while a higher frequency shows more bottom detail in shallow water, a very useful trait for fishfinders and recorders. A depth sounder signal that reads up to 400' in fresh water may lose as much as half of its penetrating power in saltwater, so lower frequencies may be favored for saltwater use.
There is wide variety in dual-frequency and multiple frequency models. Some have the ability to switch back and forth between two frequencies, while others offer two screens to view simultaneously. A few even have the ability to detect their own acoustic signals when a large number of boats fishing nearby with similar units create "crosstalk and other interference". The transducers of some high-end scanning sonars have multiple elements within their housings, allowing them to "see" in front of the boat or out to the sides.
Mounting styles for Transducers
Flush mount thru hull: Best on fast planing hulls under 36’ long, requires a hole in the bottom of that boat. Some people shy away at the large hole. In reality when properly installed they never catastrophically fail, and they are far safer for the long term health of a boat than anything attached to a cored transom.
Fairing block thru hull: They hang below the bottom of the boat and are designed for slower, longer vessels. The fairing block on a 1 kw thru hull is very large.
In Hull: They shoot through a fiberglass bottom. The fiberglass is acoustically similar to water, usually a transducer will have it’s range limited 2 or 3% less if it shoots through fiberglass. Make sure the transducer works with your sounder before ordering, some manufacturers don’t support all shoot throughs.
Transom mount: The easiest to install, hangs off the back of the boat on the transom. They can be finicky to get installed perfect, and on some hulls they won’t work at all due to steps, or curved transoms.
Trolling Motor: Clamped to the outside or permanently installed inside the propeller hub of the trolling motor. A new product from Lowrance, the SpotlightScan™, clamps to your bow-mount, cable-steered trolling motor and lets you see picture-like images at 455/800kHz. You aim the transducer with your trolling motor’s remote pedal, and it has a horizontal range of about 150'. Garmin’s Panoptix PS31 Forward Transducer is a similar product, attaching to a trolling motor’s shaft, with a 417kHz beam that’s steerable both mechanically and electronically.
Beam Angle: also known as cone angle, this is the amount of bottom marked by the transducer at one time.
Resolution: resolution is affected by three things, transducer sensitivity, beam angle, and frequency. The higher the frequency the more resolution, the narrower the beam angle the more resolution, and the more powerful the transducer the more sensitive it is.
Max Depth: A lower frequency will penetrate deeper. A frequent misconception is that you need to be able to hit the bottom in whatever depth you are fishing at. If you are trolling offshore for pelagic species you should not need to see the bottom. Most pelagics will be in the top 200’ of the water, if not in the top 120’. These are the fish that you are looking for, not targets 1000’ down. You are much better served with transducer that can target the top 200’ with high resolution and a wide cone than a transducer that will mark the bottom in the canyon.
Chirp vs Traditional: Chirp transducers are able to shoot a range of frequencies and have a low “Q” value. This tends to produce a more sensitive transducer. In practice you are more likely to see individual arches instead of blobs, and more likely to see fish close to the bottom instead of blending into the bottom.
Transducers for bottom fishing: A bottom fishing transducer will deliver resolution and a narrow cone a the depths you need it at. For bottom fishing under 600’ a high frequency works well. For bottom fishing past 600’ and under 1200’ a medium frequency works well. Airmar makes 2 narrow cone high frequency elements for most sounders. The 600 watt B75H and 1000 watt B175H. The B175H is a little narrower and a little more powerful than the B75H. Both are only available in flush mount through hulls. Airmar makes a medium frequency narrow cone 1000 watt element, the B175M. It works well at shallower depths, but really shines past 600’ where the high frequencies start to peter off. It is available in a transom mount and is the best chirp transom mount for bottom fishing. The B260 is a traditional transducer with a very sensitive narrow cone 200 khz element and a very sensitive medium 50 khz element. It has been the go-to bottom fish transducer for a long time and still holds its own against most chirp transducers.
Transducers for fishing in the water column: A good water column transducer will have a wide cone to mark fish off to the sides when you are trolling or drifting. It will have enough resolution to help discriminate bait vs game fish, and the size of game fish under the boat. The go-to transducer element for this is the 1 kw High-Wide. It is a high frequency wide cone element. It is very sensitive and works well bottom fishing under 100’ feet even though it was designed to mark fish in the water column. It is available in a transom mount or through hull. If the high-wide is outside your budget or you need to target fish deeper in the water, a good option is the 600 watt B75M. It is a medium frequency, medium cone transducer. You can run it in a fixed low frequency for a wide cone and a fixed high frequency for a narrow cone. It is only available in a thru-hull mount.