How Does a Fish Finder Work?

How Does a Fish Finder Work?

You are at the right place if you are wondering how a fish finder works. A fish finder is a handy device that helps identify fish and objects underneath water using sonar technology. They feature an LCD display screen that shows the signals received from the fish finder. Professional anglers can use the signals to catch and also locate fish.

You can use advanced fish finders to gather lots of other additional information such as the temperature and depth of the water, GPS coordinates and bottom consistency. This article gives a detailed overview on how a fish finder works, how to use it to catch and also find fish.

How Does a Fish Finder Work?

Fish finders have two different parts – main processor along with the display and transducer. The processor is typically a smaller computer which helps process information from the transducer then transforms it to digital signals on the LCD screen.

The transducer has piezoelectric crystals that send sonar pulses to the water through vibrations at different frequencies. When the pulses detect an object, it reflects on the transducer, which in turn, receives signals and sends them for interpretation.

Principle of sonar signaling (Source: Georg Wiora, CC BY-SA)

Depending on the time taken to travel through the water and the returning signal’s intensity, a fish finder basically calculates both the location and shape of the objects reflecting the sonar pulses. The process happens a couple of times in a second to generate live images of the happenings underneath the water and also help improve resolution.

You can interpret the details from different sonar signals depending on the frequency and power of the pulses. That’s why most fish finders usually use different frequencies simultaneously when it comes to gathering information known as CHIRP sonar. However, the most frequently asked question is how does a fish finder work out of water?

A fish finder can only work in the water. However, you can also use it to shoot sonar through the ice. This is because ice tends to propagate sonar signals readily.

How Does a Fish Finder Work?

Different Types of Fish Finders

To better understand how fish finders work, you should bear in mind that these devices use three different types of sonar technology including:

  • Side imaging sonar
  • Down imaging sonar
  • 2D Sonar

Let’s dive deeper into these categories!

Down Imaging Sonar vs 2D Sonar

Most traditional fish finders usually rely on the latter including CHIRP sonar which the advanced ones use down imaging sonar. A traditional fish finder that uses 2D sonar has round shape transducers and emit sonar signals that tend to widen to a rounder cone when travelling down through water. That’s why they gather any sonar information from the circular area under the boat.

Fish finders that use down imaging sonar feature thin rectangular transducers which send narrower sonar beam to the water. For this reason, down imaging sonar gives detailed information from the narrower cross section under the boat. All these types of sonar technologies have their pros and cons.

2D sonar is ideal for scanning under the boat and the areas around it to locate fish that are displayed as arches on the LCD screen. However, most of the time, the outlines can be indistinct and fuzzy on the screen which makes identifying the details an uphill task. For instance, on a fish finder that uses 2D sonar, bait fish tends to show up as a blob although CHIRP provides high performance.

Down Imaging Sonar works remarkably well when it comes to revealing objects under the water as some discrete structures. This includes the fish resting at the bottom since 2D sonar can barely detect them. The only drawback is that down imaging sonar is that it captures a narrow section of the things right under the boat and misses those further away.

Side Imaging vs Down Imaging

Just as in the name, down imaging sonar focuses in a very narrow section directly downwards while its counterpart sends signals directed to both the right and left side. This will help you get any information concerning the whole area around the boat. These two types of sonar technologies have their own pros and cons.

If you are looking to get an overview of the general area surrounding the boat on both sides, then a side imaging fish finder would be your best bet. You can spot different structures quickly including weed beds, sunken trees and boulders. Plus, it helps you determine the bottom consistency. The only downside is that it’s not ideal when it comes to identifying small objects like fish.

Down imaging, as earlier mentioned, is best suited for getting detailed information about the objects at the bottom of the boat. However, down imaging doesn’t give the user the general view of the surrounding.


  • Traditional 2D sonar fish finders are ideal for spotting fish closer to the boat and displays them as arches on the screen.
  • Side imaging sonar is ideal for scanning larger areas especially around the boat
  • Down imaging sonar is best suited for getting a detailed overview of everything underneath the boat


  • Traditional 2D sonar fish finders have low object cluttered screen and separation owing to wider sonar cone
  • Side imaging sonar makes spotting fish a bit difficult as it pays very little attention to detail
  • Down imaging sonar has a narrow focus as it only gives an overview of the surrounding around the boat

From the above advantages and disadvantages, you’ll realize that all these types of sonar technologies are not perfect on their own. A combination works remarkably well and that’s why most advanced fish finders come with three types of sonar technology. You can always switch between them after learning to use the sonar to get the right kind of information.

What’s CHIRP Sonar Technology?

CHIRP otherwise known as Compressed High Intensity Radar Pulse is a bit new. Traditional 2D sonar technology utilizes one frequency while CHIRP sonar uses different frequencies that send low and high frequency signals.

Fish Finder Batteries

Most fish finders usually use electrical signals then transform them to sonar pulses sent to the water depending on the sufficiency and efficiency of the powered battery.

Fish Finder Models with GPS

Most advanced fish finders on the market incorporate the GPS feature for optimal performance. The GPS functionality allows the user to navigate different areas on the map with ease. The best thing about a fish finder with GPS is that you can place the GPS marker on certain locations you plan on using as points of reference. For instance, a reef or a big structure under the water like a wreck and transition areas in bottom consistency that tend to attract fish.

High-end fish finder models usually use GPS along with a chart plotter. These two features allow you to create customized maps as you explore the waters. You can also mark hotspots and areas with lots of fish to return later. Currently, the Lakemaster and Navionics are the most common mapping software available.

How Transducers Works and the Different Types Available on the Market

Most advanced fish finders usually come with transducers. However, you can also purchase the transducer separately which typically means that you can use one fish finder with other different transducers. It’s also important to note that the transducers usually operate at very different frequencies both high and low. In this case, you’ll need to pick the right frequency for a specific application.

The frequencies usually range from about 50 to 300kHz but the most common frequency is 200kHz for sport fishing in fresh water. You should also note that CHIRP sonar is quite different from traditional 2D sonar transducer. If you are going for CHIRP, ensure you get the CHIRP transducers best suited for the frequencies you plan on using. For fresh water applications, 150kHz to 200kHz would come in handy.

How to Mount a Transducer

Just like there are different types of transducers, there are also different styles of mounting. Let’s dive right in!

In Hull – Some transducers transmit sonar through a hull since they don’t necessarily need to come in contact with water. In this case, you can glue the transducer inside the hull. The only downside is that it only works for a specific type of hull which is something you want to put into consideration before making a purchase.

Transom-Mount – Most people use this mounting style to mount transducers and rely on the adjustable bracket which is on the exterior part of the hull towards the rear end of your kayak or boat. The transducer gets in contact with water with this type of mounting style.

Trolling Motor Mount – You can attach some transducers on the side of the trolling motor or insert into a propeller hub. This means that just like the transom-mount, it also comes in contact with water.


How does a fish finder work? By now you have all the answers to this question especially since the technology behind fish finders has come a long way since it was first invented. Some high-end models usually combine CHIRP sonar down and side imaging along with GPS functionality. All these features are quite handy especially for professional anglers as it not only allows them to catch more fish but also mark the hotspots for ease of navigation.

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