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Fish Finder and Sounder Transducers Explained

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So you have your eye on the latest and greatest fish finder thinking it was just what you needed to get that fish of a lifetime on the line. But have you thought about the transducer? If it is not the right one for your particular application the fish finder could be the best in the world but will just not perform as expected. Knowing the difference between transducers and learning which is best for your application will determine just how well your new fish finder will work.

An illustration of a correctly installed Airmar p32 transom transducer.
An illustration of a correctly installed Airmar p32 transom transducer.


To first understand which transducer is best for you it is important to know what the transducer is and how it works. Put simply the transducer is the part of the fish finder that emits a signal at certain pre-determined frequencies or frequency and is sent through the water.  The signal is then reflected back off the bottom, structures and fish. The reflected signal is received by the transducer and sent to the fish finder to be processed. The signal that is sent out can vary in frequency, with the most common being 50 and 200Khz. New high definition image transducers such as those from Lowrance can operate on multiple frequencies from around 400Khz to upwards of 800Khz.

A simple way to remember which would be best for your current fishing is that lower frequencies travel better over longer distances and would be better for deeper water, while higher frequencies will give a better return in shallower waters. Higher frequencies tend to operate in a narrower cone than lower frequencies and despite covering less of the bottom tend to be the better choice in shallow water. If you are operating in shallow water and want a greater view of the bottom, some manufacturers offer a high frequency with a wider cone than other manufacturers of similar frequency. When selecting a transducer you must ensure that the frequencies it operates on are appropriate for the depth of waters you will be fishing in and also match the capabilities of the fish finder. A good resource for determining the right transducer for your application can be found on Airmar’s  web site ( a major manufacturer of transducers for many marine electronics companies).

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There are different physical types of transducers depending on where on the vessel they are going to be mounted, with the transom mount transducer being the most common. Many fish finders come pre-packaged with transom transducers and these will work well in many applications. They are a good option for outboard and outdrive boats as they can mount easily on the transom and give good results even while running at higher speeds. It is important to follow provided installation instructions, however. If not followed completely you may find that the fish finder only gives readings while idle or running at slow speeds, while it fails to read at higher speeds.


Airmar's p32 transom transducer with speed and temperature.
Airmar’s p32 transom transducer with speed and temperature.

The next option is the In-Hull transducer. These are proving to be very popular with all kinds of vessels as they are relatively easy to install and leave nothing on the outside of the hull to be struck or damaged. They are made up of a small outer housing that is epoxied into the hull at a location where the non-metal hull material depth is not more than an inch thick. The tank is then filled with mineral oil or other viscous liquid and the transducer element is bolted into the housing. The transducer element then sends the signal through the viscous liquid, through the hull and into the water. Because it is sending the signal through all these elements there is a slight loss of power, resolution and effectiveness. If use is going to be restricted to shallow water then it could be argued that the interference is negligible. This type of transducer should be mounted towards the rear of the vessel and in a place where cavitation and water bubbles will not travel under it so as not to lose depth signal while underway. This type of transducer is not an option currently with high definition fish finders as those signals need little to no interference to work effectively.


The p79 in hull transducer from Airmar is an effective transducer that is simple to install and requires no drilling into the hull.
The p79 in hull transducer from Airmar is an effective transducer that is simple to install and requires no drilling into the hull.

The third type of transducer is the Thru-Hull transducer, a popular choice among bigger craft. This transducer requires a hole (usually 2 inches around) be drilled in the hull for the transducer to pass through and be bolted in place. Some of these transducers have a speed wheel element in the center and if the hull has a dead rise, require a fairing block that mounts the transducer at an angle that allows the signal to travel straight down. Airmar now offers bronze and polymer thru hull transducers with their internal elements pre-angled to the dead rise of the hull in either 12 degrees or 20 degrees. There is usually a 5 to10 degree leeway before needing a fairing block or tilted element transducer. It is important to remember when these transducers are being installed that they be mounted away from intakes, chines, hull steps or anything that can create cavitation upstream from the transducer.


The P19 tilted element transducer allows for the good signal even when there is a dead rise on the hull it is mounted to.
The P19 tilted element transducer allows for a good signal even when there is a dead rise on the hull it is mounted to.

No matter what transducer you end up with it is important to take the time to determine which type is best for your application, vessel and sounder/fish finder, and that you follow installation instructions completely, then there isn’t a fish out there that can hide from you and you will end up with tight lines.

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Glenn Hayes
Glenn Hayeshttp://www.HayesStudios.com
Glenn Hayes is a writer and photographer based out of west central Florida and has marine industry background spanning almost a quarter century. He can be reached through his web site www.HayesStudios.

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