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Understanding How to Tune Your Fish Finder

Welcome to part 2 of the series on sonar and fish finders. In part 1, we learned about what sonar is and how to interpret what we are looking at on the sonar display. In this segment on sonars and fish finders, I going to share with you common settings with sonar displays, and how to optimize them for finding more fish. Lets begin with the basics first about how your fish finder actually works.

How it works?

The transducer(s) mounted on the bottom of your boat as close to the center as possible. The transducer admits pulses of sound. That sound travels until it either is blocked by an object or reflects off of an object and travels back to the transducer mounted on the vessel. This is called a return. The time it takes for a single return to travel to the object and back represents a single data point. If there is no return data from a certain area, a shadow will appear in your imagery. So, the more pulses of sound put out in the environment gives you more data points which determines the quality of side scan imagery. This is why the detail on a 1800kHz sonar is much more detailed than a 50kHz sonar. These pulses or data points make up an image which is then displayed on the sonar monitor. The monitor itself can be adjusted to your desired settings.

Those settings can include:





Frequency is the number of waves or cycles of sound that your transducer emits per second also called kiloHertz (kHz). Transducers put out a set frequency of sound which can be anywhere from 50kHz to 1800kHz or more. Most of the older models frequencies are 50kHz and 200kHz. These frequencies effect the range that the sonar can reach as well as the resolution of the image itself. You are constantly trying to balance frequency, resolution, and range to provide the best sonar results for the environment you are fishing.

So what is chirp sonar? Basically, chirp sonar sends a variety of frequencies at once rather than one frequency at a time. This gives a much more complete sonar picture over traditional sonars. There's a really good detailed article on chirp sonar here.

Range is how far the sonar waves will reach or sometimes referred to as coverage. With side scan sonar, the lower the frequency the further the range. This sounds great but the problem is what you gain in range, you will lose in detail. For example, an 800 kHz side scan sonar may get range out to 100 meters, but quality or solution of the image will be drastically reduced towards the edge of that range. In contrast, if you have a 1800kHz sonar, you will have amazing image resolution out to 20 meters. In simple terms, the range will be significantly reduced because the ping rate or pings per second are reduced the further out the range. This concept also applies to downward looking sonar. Depending on what frequency your depth sounder is using determines how accurate and what depth it can reach.

Gain is basically the sensitivity of the sonar operational environment. The more gain, the more detail you will see in your sonar display. And the less gain, the decrease in returns in the water column and on the sea floor. You can adjust the gain on your display in order to make your objects in your sonar picture more pronounced or discrete. In simple terms, the gain controls the intensity of the display colors based on the strength on the sonar return.

Depth refers to the depth of the water or how far off the bottom the transducer is sending and receiving data. For fishermen like us, it mainly refers to the distance between the boat and the bottom. Depth will determine the size of the space directly under the boat not covered by the sonar. This space grows and shrinks depending on the depth of the water because the sonar sends out a “ping” at a certain angle. The deeper the water or the greater the distance between the seafloor and the transducer, the more of a gap that the sonar is going to have when scanning. The gap is the area directly underneath the boat that the sonar is not covering. The depth of the water or height off the bottom that the transducer is determines the width of that gap in the sonar picture. In the picture below the gap is about 5m on each side for a total of 10m not covered in the sonar picture.

The black data directly in the middle of your side scan display represents the water column above the sonar gap. You may be able to see objects in this area as you are scanning with side scan sonar. However, the best way to cover this area is with your traditional downward looking sonar. Depth setting on your downward looking sonar allows you to adjust the sonar to what depth you want to be looking for fish. If you are looking for fish and able to meter fish in a 1000’ of water, you may want to adjust your sounder to 1000’ and utilize the low frequency sonar such as 50kHz. Now, if you are searching for a wreck in 40’ of water you can use the high frequency sonar such as 1200kHz which gives you much more detail with less range. I typically set my downward looking sonar to a depth that I can find the fish and bring them up in the water column in order to dive down and meet them.

Having said all of this in regards to depth, range, frequency, and gain, most fishing sonars change their angle of ping depending on what frequency is being used automatically. This is why duel frequency transducers provide a huge benefit to fishermen than a single frequency transducer. Most modern fish finders have multiple frequency capability than older versions. If you have an older version, I would recommend updating your electronics on your boat. It can be expensive, but it will allow you to take full advantage of your time on the water and greatly increase your chances at catching or spearing more fish.

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