How does a Fish Finder Work? [Guide for Dummies]

How does a Fish Finder Work

In this modern day and age, anglers can use technology to make their fishing experience better.

To find fish, you do not have to walk around a lake, or try to navigate your boat to a location where you think fish could be.

You can get a fish finder and rely on it to show you where to present your bait.

Fish finders use SONAR technology to scan objects, including fish, underneath the water surface. These devices send pulses which reflect from underwater objects. Those reflection pulses travel back to the device, and display what is there. Depending on an object hardness, fish finders are able to distinguish fish from inanimate objects like rocks.

Here I am going to tell you more about how does a fish finder work, and all the information you should know about it, in the following chapters:



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What is a Fish Finder?

Before going deeper into how does a fish finder work, you have to know what exactly a fish finder is?

In simple words, fish finder is a device that shows you where the fish are. These devices have digital screens that display different information (depending on a fish finder type), and the most important information is the fish location. More importantly, it will show you the depth, so that you can adjust your fishing technique and present the bait at the right location.

Fish finders consist of the main unit and transducer.

The main unit is usually positioned at a convenient place on a boat, where anglers can see it and use it easily. The main unit has a display, and a few buttons which are used to control the device and set up different options.

Some models can be connected to your smartphone, and you can use it instead of the main unit. This is convenient for small boats and kayaks.

Modern fish finders can have GPS, electronic compass, and similar systems that can help anglers navigate, not just to fish.

Fish finders have been on the market for a long time, but in the beginning, they were used mostly by professional fishermen. Today, these devices are small, affordable, easy to install and use, and recreational anglers can enjoy using them.

What type of Fish Finders do we Know?

You can find different types of fish finders on the market, and although they serve the same purpose, these devices are quite different.

Depending on your needs, you can choose the best one for you ...


This is the “basic” type. The 2D fish finders usually have simple screens where the image moves from let to right.

It can show you where the fish are, display very rough textures, and the bottom. Soil hardness can also be displayed. Images shown on the screen are not very realistic, and anglers usually need a short adjustment period to learn how to read the signs, but the covered area is decent. These devices will recognize fish and display them as dots or arches.


Downscan fish finders will show you very realistic images between your boat and the bottom.

These devices are among the most popular ones on the market. They are a bit more modern, and display better pictures than 2D, but come at a slightly higher price.

They scan a relatively small area under your boat, and are very popular for smaller lakes. You can recognize fish species on the screen if the water is calm, but in case of too much movement, fish species can become unrecognizable.

The biggest disadvantage is that you can’t get a wide picture.


Sidescan fish finders are the most expensive type. These devices will scan on the sides and give you a wide image of the surroundings. Sidescan fish finders are an excellent choice for larger lakes and open waters, but they do not provide as much detail of an area underneath you, which can be a problem if you are fishing in a very deep water.

The best thing about these is that they cover a lot of area.

Except for these three types of fish finders, you can find models that combine these technologies; however, they work on the same principle as those mentioned here.

How does a Fish Finder Work - for Boat

How does a Fish Finder Work?

Understanding how does a fish finder work will help you with its usage while fishing.

SONAR is actually an abbreviation from Sound Navigation Ranging. A sonar device, which is in this case, a transducer mounted on the bottom of your boat, emits sound wave pulses.

These pulses travel through the water, and when they hit an obstacle, like a rock, vegetation, fish shoal or a bottom, they reflect and travel back towards the surface.

Devices that sent the pulses detect these reflections, and based on time that passed from emitting to receiving, can calculate the distance (in this case the depth) of an object.

The strength of a returning pulse depends on how hard the object that reflected it is. Rocks will cause harder reflection than a fish. Devices detect it and based on the information received can distinguish the objects.

Transducer is connected with the main unit that gets all of this information from it, and it shows this information in a comprehensible way in a form of images.

By observing the screen, you will get information about object shape, depth, and hardness.

On the screen, you will be able to see different symbols that represent fish or other objects. Sometimes, fish are displayed as arches. These shapes are unique, and angler monitoring the screen will not confuse a fish form something else.

How does a Fish Finder Transducer Work?

I already mentioned transducer, and its purpose, but how does a fish finder transducer work exactly?

Transducer it the most important part of the system. The display is there only to show you all the information that your transducer collected. As previously explained, it sends pulses and receives their reflection in order to identify the object which is then shown on the screen.

Depending on a type, it sends these pulses in a different way.

The 2D fish finders have round transducers that emit round signals which spread as they travel. They cover greater area, but with less detail.

Downscan transducers that come with downscan fish finders are rectangular and send pulses straight to the bottom. The result is a narrower, but highly detailed picture.

Sidescan fish finders emit pulses to the sides. The downside is that you have to move slower to get a detailed image. Covered area is large but not as detailed as with the downscan transducer.

All transducers have piezoelectric elements (e.g., ceramics) within them. These parts vibrate at and this vibration produces waves, i.e. pulses, that travel through water. Vibration occurs at a certain frequency, but I will tell you more about that later on.

When the signal is returned after it has been reflected, it gets converted into an electrical signal that travels into the main unit. The main unit software “reads” and “convers” these signals into pictures.

What is CHIRP on a Fish Finder? And How CHIRP Works?

If you are in search of a fish finder, you are going to notice that CHIRP is one of the features mentioned.

So, what is chirp on a fish finder?

CHIRP is an abbreviation from Compressed High Intensity Radar Pulse. In simple words, this technology works in a way that it emits more than one frequency pulses. It emits pulses ranging from low to high frequencies continuously.

Upon the signal return, the technology can interpret those signals. The result is a better, more detailed, and accurate picture on your display. I already mentioned that every underwater object reflects pulses differently, due to its hardness. By sending different frequencies, those differences in reflections are minimized, and the result is a better picture.

This range of frequencies enables more accurate information gathered at larger depths.

But it is not only for deep sea fishing. Anyone who is presenting their bait in depths greater than 10ft will notice a difference. Of course, this precision comes at a price.

I will not mention a lot of technical details, because you can find them on different manufacturers websites, but the thing I have to mention is reading the CHIRP sonar, or any other sonar you have.

Fish can be displayed as arches, and those arches can vary when the fish are on the move, and travel to through the pulses your transducer emits. These arches can have different shapes and sizes, and you will have to use your fish finder in various situations to get to know these signs.

Some other fish finders interpret these arches for you, and display fish icons. This feature is great for beginners who are still learning how to fish with the help of modern technology.

How does a Fish Finder Work - 2D and 3D

Do I Need GPS & Chartplotters on My Fish Finder?

GPS and Chartplotters are a great addition to any fish finder, however, you have to know that these come at a certain price.

But, before we get into that, I have to mention the difference between GPS and chartplotter.

GPS will inform you about your location, in terms of latitude and longitude. Chartplotters use a GPS location to show it on a map. This means that chartplotters can’t work without GPS, but GPS can work without chartplotters.

For example, when you are driving and using one of the popular fishing navigation apps, those apps are actually chatplotters. GPS is a technology within your navigation device that calculates your position. And this location is then transferred to a map.

Having these options on your fish finder can be beneficial but it is not something you can’t live without.

Knowing your exact position, and being able to mark it on a map will be useful if you want to “save” your favorite fishing spots for the next time. Some fish finders have the option of making your own charts of previously uncharted lakes. But, no one can guarantee that you will find your fish on the same spot next time.

Today, we all have smartphones that can be used as GPS and chartplotters, so if you are fishing on smaller lakes, you do not need it. GPS is a good safety feature in case of an emergency. You will now your exact position.

Offshore anglers can benefit from this.

Here you can check my comparison of high quality hydrographic maps: Navionics vs Lakemaster.

How to Install New Fish Finder on a Boat?

The accuracy of a fish finder depends greatly on the installation.

Main unit can be installed anywhere you like, but the transducer has to be in a specific place. If installed incorrectly, it will show you low quality and inaccurate images.

So, how to install fish finder?

You will have to drill a few holes, so if you think you are not able to do so, call a professional.

The first step is to find a proper location under your boat, a flat surface. And before drilling, make sure there are no cables or anything similar under the outside panel. It greatly depends on your boat. Mounting it on a yacht is quite different than mounting it on a kayak. Boats are also made from different materials.

Make sure there are no obstacles around the place where the transducer will be (like sensors or a motor).

All fish finders come with mounting instructions, and you can find detailed instructions for almost any boat type online.

It would be too long to make a detailed guide for all of them, but here are some general rules:

Invest in a transducer shield to keep it safe in case you hit something with your boat

Reputable manufacturers give you the opportunity to register your device so that you can get access to software updates. Update your fish finder.

Make sure you have a good power source, and a fuse between a power source and the unit. This is mostly applicable to larger boats.

Get a transducer mounting board. Attach a board to your boat and transducer to the board. It will give you enough clearance from any obstacle on a boat and it is a flat and secure surface.

Mount the transducer as close the boat centerline as possible.

Make sure that the location is submerged at all times.

Try to mount it as leveled as possible. Keep in mind that a boat full of people will change the angle a bit and you may have to adjust the transducer angle too.

What Kind of Battery do I need for a Fish Finder? [Best]

Before you buy a battery, read the manufacturers instructions about required current draw. Also, you probably want to invest in a good, high capacity, battery that will last a long time.

If you are using it on a small kayak, you probably can get away with a light and small model, but for larger boats where fish finders are used as navigation devices and chartplotters, on multi-day trips, you will need a more powerful power source.

There are two options to choose from, and those are:

- lithium batteries and
- sealed lead acid batteries.

Lithium ones are lightweight, small and dependable, but a bit pricier. Sealed lead ones are heavier, cheaper, and work excellent in different temperatures.

The best fish finder battery for a smaller boat is a lithium one.

Is a Fish Finder Worth it?

Well, this is a complicated question. High-end models come at a price, and not every angler is willing to spend a lot.

You can fish perfectly fine without one, but having it to make things easier for you is great. To be honest, nobody wants to go home empty-handed, and a good fish finder will help you find fish when you can’t do so by yourself.

Those who are fishing in open waters, can benefit from navigation systems incorporated into some fish finder models.

In my opinion, fish finder is worth the price if you are frequently fishing from boats in large lakes or in saltwater.

But, is a fish finder worth it for you?

That is a question that you have to answer in accordance with your fishing habits, budget, and wishes.

If you decide to get one, make sure it is made by a reputable manufacturer. Cheap ones are usually not very effective. Fish finders need precise and well-made hardware, together with a good software to be able to function properly. Also, make sure it is compatible with the charts you want to use, if you want it to have a chartplotter function.

And even if you have a great piece of equipment, you have to learn how to use it correctly.

If all of this sounds like fish finder is not the best option for your needs, there is an alternative. You can find some great, dependable and affordable underwater cameras for fishing, that are easy to use, show you real-life images, and have certain benefits.


To be able to use it correctly, you must know how does a fish finder work.

That will help you to choose the right type for your needs, install it correctly, and obtain all the necessary information about it before you head out to your favorite fishing spot.

Yes, you can fish without it, but this little piece of equipment can make it much easier and more successful even on days when it seems that fish are nowhere to be found.

Just make sure to choose models from good manufacturers, because those few dollars more you invest in the beginning, will pay off later on.

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I am Siniša Pintar (friends call me Sina), the guy behind and eBook writer. This site is base camp for fishing enthusiasts from all over the world. I love fishing and want to share all my stories, knowledge and my experience with any and all potential anglers. Read more ...

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