How Fish Finders Work

By Andrei Loskoutov

A Fish Finder is a subset of a group of instruments called sonars.

A Sonar consists of a transmitter, transducer, receiver and display.

In the simplest terms, an electrical impulse from a transmitter is converted into a sound wave by the transducer and sent into the water. When this wave strikes an object, it rebounds. This echo strikes the transducer, which converts it back into an electric signal, which is amplified by the receiver and sent to the display. Since the speed of sound in water is constant (approximately 4800 feet per second), the time lapse between the transmitted signal and the received echo can be measured and the distance to the object determined. This process repeats itself many times per second.

DISPLAY: The display shows a history of the received echoes. The user can make a number of adjustments to tailor the display to his or her preference, such as senitivity, the depth range and chart speed. Displays use a variety of technologies, provide different resolutions and number of shades of gray or color. Each display is made up of a number of pixels, which are little square blocks that make up the images. The more pixels and shades of gray or color the better resolution and image clarity.

Fish Targets: Echoes from fish within the beam will be shown on the display by illuminated pixels. What image appears on the display depends on a number of factors: the sensitivity setting on the fishfinder, the cone angle of the transducer, the speed of the boat, and the size, depth, speed and direction of the fish. A fish that is swimming directly beneath the boat, it will create a consistent echo that will cause a continuous line to appear on the display. A stationary fish caught in a narrow beam transducer appears as a single point on the screen as the boat passes above it, whereas under the same conditions the fish appears as an arch if a wide beam transducer is used.