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How to choose a fish finder

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A Little Bit of History on Fish Finders

It was the US Navy that first developed and used the technology known today as the fish finder. This method of search was adopted by the Navy some 75 years ago, and is known as Sonar. Sonar is an abbreviation for ‘SOund, NAvigation, and Ranging’.

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How a Fish Finder Works

Sonar works by sending an electrical impulse from the transmitter, converting it into a sound wave by the transducer and then sending that sound wave (in the shape of a cone) into the water. When the sound wave strikes an object it rebounds off the object sending this echo back to the transducer. The transducer than converts the echo back into an electrical signal. The receiver amplifies this signal and sends it to the display for the viewer to see.

Few fishermen realise that although everything appears to be directly under the boat, but it isn’t! The figure below shows the way it really is in our underwater cone of sound and the way we think it is, based on watching a flashing dial or a two dimensional graph. We always have assumed the fish were right underneath the boat. Not so!

How a fishfinder shows fish

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Black & White Fish Finders vs. Colour Fish Finders

Monochrome, or grey scale displays show images in black and white or in shades of grey. The low priced fish finders may have no grey scale at all or a 4 level grey scale. This means that there will not be too much contrast on the display for different objects. The higher priced models may have up to a 12 level grey scale per pixel. This higher level will allow for more target contrast and definition.

Color displays are generally more pricey. You will also pay more as you increase the resolution of the display. In general color displays actually have fewer pixels than monochrome fish finders, but each pixel will have up to 256 color choices. Using a good colour fish finder provides clarity in direct sunlight, helps you distinguish between fish species, and helps separate fish images from bottom images such as foul, weed, rock, sand etc.

One thing that is really important with a fish finder is to be able to see the screen in direct sunlight. Most of the quality brand name LCD finders work really well in direct sunlight.

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About Transducers

Overall, there are a lot of choices in fish finders. The transducer is one of the key critical components of fish finders. Selecting a quality transducer and paying attention to its mounting is critical to the function of your fish finder.

What the Transducer Does

The Transducer is attached to your fishfinder with a small cable. It takes the electrical signal from the transmitter on your fishfinder, turns it into sonar or a sound wave and beams it out in the shape of a cone. The cone angle is simply the wideness of coverage in the water. The wider the cone angle - the greater the coverage. Fishfinder cone angles vary from model to model so be sure to have a good look at all the features on the model you are considering.

Types of Installations

The way your unit is installed in your boat is something you’ll want to pay close attention to as well. If mounted incorrectly the fishfinder will not perform due to airation, noise and cavitation.

Transom Mount or Through Hull Mount

Transom Mount transducer are generally easier to install and usually less expensive. You can purchase a transom mount transducer in single or dual frequency models. They generally work best in water that is somewhat calmer and are not terribly effective at much more than 15 knots.

Through Hull transducers are the best for clarity and performance, but are harder to install and cost more. A need for high speed bottom reading make these a worthwhile investment.

Transducer Quality

The material used in the transducer elements also affects the performance of the fishfinder. A poor quality transducer is inefficient at transferring the fishfinders (transmission) power in the water. A weak signal means poor fish detection, low definition and poor depth capabilities.

There are two basic types of transducers - magnetostrictive and ceramic. Ceramic transducers are much more efficient and provide a vastly better quality picture.

Transducer Frequency

Transducers come in low and high frequencies. Low frequencies generally range from 50 to 100 KHz. High frequency models generally range from 180 to 200 KHz. Lower frequency sound waves can travel greater distances. They penetrate further in the water.

Lower frequency = greater depth.

IMPORTANT: If you purchase a transducer separately, or you already have one, make sure your model will work with the frequency guidelines of your fish finder. This applies for portable fish finders as well as mounted models.

A transducer with dual frequency is probably the best bet for accurate fish readings. Dual frequency (or beam) models, as the name implies, have two different beams that project at different frequencies. One is set to a low frequency and the other to a high frequency. Many fish finders will allow the fisherman to switch between the two beams on the display. There are also models that have a split screen display that allow the angler to see images from both beams separately at the same time. Some manufacturers even have displays that combine the imagines from the two beams into one image. The display essentially combines the high-frequency detail with the wider beam and deeper search capabilities of low frequency.

For deep water fishing, dual frequency transducers are a great option. They let you find the general area of the fish over a large volume of water using a low frequency-wide beam setting, and then, for greater accuracy, pinpoint the depth and location using a high beam-narrow cone setting for greater clarity.

For shallow water fishing, a dual frequency transducer may be overkill. There is little need to see fish at a depth greater than a hundred feet so a single beam, high frequency model may a better option.

Cone Angle

A transducer works by concentrating a sound wave into a beam and emitting this beam into the water. This beam is emitted in a small arc (measured in degrees) from the transducer. As the beam travels deeper into the water, the arc covers a wider area. If you plot the area the beam covers on graph paper it will look like a cone, thus the term “cone angle”. If you draw a line from the very tip of the cone down to the cone bottom you have identified the centre line of the cone. The emitted sound wave is strongest closer to this centre line and diminishes as the distance from centre line increases.

Cone Angle becomes important because the greater the angle the bigger the coverage area and therefore the bigger the area that the fish finder can “see”. As you move further away from the centre line, the clarity of the image decreases.

A large cone angle ranges from 40 to 60 degrees, has a larger coverage area but provides less clarity at greater depths.

A narrow cone ranges from 15 to 30 degrees, has a smaller coverage area but provides more clarity at greater depths.

Frequency and Cone Angle Working Together

High Frequency transducers come with either a narrow or wide cone angle. For shallow water fishing, a high frequency (180 - 200 KHz), narrow cone angle (20 – 30 degrees) model may be a good choice. You will not be able to see great depths, but you’re in shallow water. The narrow cone angle will allow you to see things more crisply than a wide cone and will allow you to be able to distinguish the objects you see on the display easier. If you really want to see a large area, then think about a wide cone angle instead.

For deep water fishing you will most likely want to search the highest volume of water possible in the shortest amount of time. Consider fish finders that have a low frequency (50 - 100 kHz) for greater depths, and a wide cone angle (40 – 60 degrees) for greater horizontal distances from the cone centre line. This combination will allow you to see at greater depths, but the definition (how crisp the image is on the fish finder display) will not be as good. You can increase the definition by buying a fish finder with a higher sensitivity display.

Low frequency, wide cone angle setups do have some limitations. For example, you can cover a wide area and a greater depth which gets you into the ‘general’ area of where the fish are, but we don’t want to fish in the general area, we want to put our lines directly in front of the fish.

Output Wattage

The depth your fishfinder will reach will be affected by water temperature, salinity, frequency, and power output which is measured in Watts. The following general guidelines will give you the maximum depth that can be reached:

100W = 400ft Max Depth
200W = 500ft Max Depth
300W = 600ft Max Depth
600W = 800 - 1200ft Max Depth
1KW = 1800 - 2500ft Max Depth
2KW = 1500 - 4000ft Max Depth

Most recreational fishfinders come with a transducer with a wattage rating that matches the output wattage of the fish finder. However, when purchasing a transducer separately, make sure the power output of the fish finder unit is less than or equal to the wattage of the transducer, otherwise the transducer may be damaged.

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Major Brands of Fish Finders

Major brands of fishfinders include Lowrance, Eagle, Humminbird, Furuno, Simrad, JRC, Raymarine, and Garmin.

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