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How A Fishfinder Works


How a fishfinder works

how a fish finder works

All fishfinders operate using Sonar. Developed during World War II, this technology uses sound waves to "view" underwater objects. A sound wave is produced by the fishfinder and sent through the water. At the source, the wave is narrow; however as it penetrates deeper, the sound wave spreads forming a cone, or what is commonly called a beam (think flashlight). When the sound wave encounters something within this beam, it bounces back to the fishfinder. By measuring the very small amount of time between when the sound wave was send out and when it bounces back your fishfinder calculates the distance and draws it on the screen.

If the signal doesn't encounter anything along the way, it reaches the bottom. Soft bottoms like mud and weeds tend to absorb the signal. Hard bottoms such as rock reflect a stronger signal back. These subtle differences in sonar reflections appear on the display screen. That's how a fishfinder "reads" the bottom and everything in between.

Cone Angles The cone angle is the angular measurement of the sonar beam in degrees. For instance, a 24 degree beam is broader and covers more area than 16 degree beam. Cone angles are measured at "-10db", which is a method to assure consistency of measurement from one transducer to another, and accurately represent the capability of the fishfinder.

That cone angle varies by fishfinder model and manufacturer. Many fishfinders use only one sonar beam; however Humminbird has advanced multi-beam sonar technology that sends out, 2, 3 or even 6 sonar beams to cover much wider area with much greater detail than ordinary fishfinders.

Sonar Beam Coverage Area



Below is a quick reference chart for the area covered by the specific cone angle listed. For example, if you are fishing in 10 feet of water and the cone angle on your transducer is 20 degrees, the area across the bottom is 3.5 feet.
  • 20 degrees - 0.35 or roughly 1/3 of depth
  • 24 degrees - 0.42 or roughly 2/5 of depth
  • 30 degrees - 0.53 or roughly 1/2 of depth
  • 40 degrees - 0.72 or roughly 3/4 of depth
  • 50 degrees - 0.93 or roughly 9/10 of depth
  • 60 degrees - 1.15 x depth
  • 70 degrees - 1.4 x depth
  • 73 degrees - 1.48 x depth
  • 80 degrees - 1.68 x depth
  • 90 degrees - 2 x depth
  • 100 degrees - 2.38 x depth
  • 110 degrees - 2.85 x depth
Fish Targets Fish targets are displayed as either the unprocessed sonar return called an "arch", or as a Fish ID symbol. You can choose either presentation.

To display Fish ID symbols, a Humminbird fishfinders use sophisticated software to analyze the "shape" of the sonar return from a suspended target to determine whether it is a fish. When a fish is detected it is displayed as graphic fish symbols on your viewing screen. If you have a multiple beam configuration, Fish ID provides the added benefit of identifying the location of the fish: either, to the left, right or directly below your boat.

To display "arches", the fishfinder relies on the characteristics of sonar. When a fish passes directly through the transducer's beam, it can form an arch on the display screen. The size of the fish arch is affected by the sensitivity setting of the unit, your boat speed, the depth of the water and the location of the fish within the sonar beam. Often only partial arches will be displayed because the fish does not travel directly through the center of the beam.

Water Clarity The water's clarity also has a great deal to do with the integrity of the signal. Strong winds or currents can create bubbles in the water that disperse the signal. Suspended mineral particles or floating algae, plankton or other microorganisms could absorb sound rather than reflect it back. Wave action can also stir things up - the greater the chop, the more air gets into the equation.

Thermoclines Some fishfinders also show thermoclines, which are the "fronts" where warmer waters intersect with cooler waters. Typically, the water is colder as you go deeper and certain fish prefer certain temperatures. The change in water temperature causes some of the sonar signal to reflect back creating a line across your screen at the depth of the thermocline. Identifying thermoclines brings you one step closer to catching the fish you're after.

The Transducer The transducer takes the electrical signal from your transmitter, turns it into sonar, and sends it out. It also captures all returning echoes and converts them back into electrical signals. Transducers are mounted in the water, either on the transom, the inside the hull, or on the trolling motor. All transducers should be mounted straight down and away as far from motors, props and hull obstructions as possible. A cable connects the transducer to the fishfinder.

To select the transducer that's best suited to your needs, you need to consider the transducer's operating frequency, cone angle and type of installation.

Frequency Most of the sonar units that we sell accessories for operate at 192 or 200 kHz (kilohertz). Some models use 50 kHz. A few are dual frequency capable, meaning they can use both 50 and 200 kHz transducers. Typically, high frequency (192 or 200 kHz) sonar units provide the best resolution and definition of structure and targets. They excel at showing minute details of the underwater world. 50 kHz units have much greater depth penetration capability, but show less definition. 50 kHz transducers also usually have a much wider cone angle than 192 or 200 kHz transducers.

You must match the transducer's frequency to the sonar unit. For example, a 192 kHz sonar unit requires a 192 kHz transducer.

Cone Angle A transducer's cone angle determines its coverage area of the underwater world. The wider the cone angle, the greater the area that's covered. We offer a variety of 192 kHz transducers with either a wide (20°) or narrow (8°) cone angle. We also offer a variety of 200 kHz transducers with either a wide (20°) or narrow (12°) cone angle. The 50 kHz transducers come with a 35° cone angle. And the dual frequency transducers come with both a narrow (12°) 200 kHz and a 50 kHz cone angles.

Generally, use a wide cone angle for fishing shallow to medium depths. The narrow cone penetrates to deeper depths, but shows less fish and structure due to its narrow beam.

The depth capability of your sonar units depends on its transmitter power, receiver, sensitivity, frequency, transducer and transducer installation. Other things that effect depth capability are: water conditions and type, (all sonars will show deeper depth readings in fresh water than salt) and bottom conditions.

Types of Transducer Installations Most of our permanent-mount transducers are designed for high-speed operations. For the best results, the transducer should be placed where a smooth, undisturbed flow of water will pass across the face of the transducer at all boat speeds. Read your transducer's owner's manual before installing the transducer!

Transom Mount The transom-mount transducer is the most popular, and it's generally the easiest to install. The Skimmer® transducer design performs best when it is slightly below the boat's hull. A plastic transducer is recommended on aluminum or steel-hulled boats to avoid potential electrolysis problems.

Mounting the transducer on the transom is recommended for outboard and stern-drive (I/O) powered boats only. Transom mounting is ideal for high-speed operation and models with the "kick-up" feature will prevent damage if the transducer strikes an object.

Make certain that the chosen location doesn't interfere with the boat's trailer. DO NOT mount the transducer directly behind the ribs, or thru-hull fittings. Typically, on aluminum boats, mounting the transducer between two ribs works best. On all hulls, mount the transducer at least one foot away from the engine's lower unit. This helps to prevent air bubbles from the transducer interfering with the propeller.

Periodically wash the bottom of the transducer with soap and water to remove any oil film or growth that may collect. Oil and dirt reduce the transducer's sensitivity and can even prevent its operation.

Shoot-Thru-Hull Mount In this installation, the transducer is bonded to the inside of the hull with epoxy. Ideally, the transducer is placed in the aft third of the hull close to the centerline. The signal "shoots through" the hull with some loss of signal strength. This installation must be made in an area of the hull that is made from solid fiberglass, with no air bubbles or separated layers. If the hull is of multi-layer or "sandwich" construction, you will have to remove the inner layer of fiberglass and the wood or foam core to expose the outer layer of the hull. This type of mount is recommended only with 192 or 200 kHz transducers.

Bolt-Thru-Hull Mount In this type of installation, a hole is cut in the hull and the transducer is mounted through the hull by means of a threaded shaft and nut. If the boat hull has a dead rise higher than 10 degrees, fairing blocks made from wood or plastic must be fabricated so that the transducer will mount in a completely vertical position. The TH-FLW P5 model does not require a fairing block.

On in-boards, the transducer must be installed ahead of the propeller, shaft(s), and engine water intake(s).

If the boat's hull is made of steel or aluminum, use a plastic transducer to prevent electrolysis problems.

Trolling Motor Mount The PD-W "pod" transducer is designed for mounting on an electric trolling motor. It has two slots for a hose clamp (which must be purchased separately). Skimmer® transducers can also be mounted on a trolling motor using the TMB-S trolling motor bracket. It's curved to fit the contour of most electric trolling motors.

Sonar Update Speed. This is how fast the sonar updates per second. Some newer units send and receive a sonar signal at up to 40 times per second at about 60 feet of depth (it deeper it gets slower, and shallower it gets faster) some older units update sonar at 20 times per second). The faster the sonar updates, the more information it puts on the display and better image you get. Keep in mind this is single beam mode. Since some units use several beams, additional time is required for each beam - this is just a problem with physics, you can't make the speed of sound in water any faster.

The LCD Display Screen The LCD displays the sonar information and is the one of the most important aspects of the fishfinder. LCD displays are measured in "pixels" - the little square blocks on the screen that make up the image. More pixels provide better resolution and a clearer picture. FSTN displays, standard on all Humminbird products, offer the best visibility from any location on the boat, and provide high contrast for easy reading. FSTN displays are easy to identify because the pixels are black, not blue. Grayscale technology offers even more detail by showing the sonar image in multiple shades of gray; this makes it easy to clearly distinguish bottom type, structure and fish.