Fish Finders and Depth Sounders

Whether for recreation or work, fishermen and divers readily invest in high tech sonar equipment to seek out fish and undersea structure. Why? In simple terms- a good fishfinder sees through the murky depths. Modern fishfinders evolved from sonar technology, a science pioneered for undersea warfare and surveying sunken assets. Sonar is an acronym for SOund NAvigation and Ranging. The need developed during World War II to track enemy submarines. With technology advances, sophisticated fish finders can now show on a monitor a 3 dimensional view of seabed, structure and any fish swimming through the foggy soup within the transducer scope. With a monitor beside the helm, a Captain can spot the wreck before any diver wets a fin. A fisherman will see a clearly defined bait ball 60 feet underwater and pay out lines to the appropriate depth.

Depth Sounders vs. Fish Finders - What's the difference?

In broad terms, depth sounders and fish finders are the same - both are forms of active sonar. Sonar uses four basic components, a display unit, transducer, transmitter and receiver. Sonar begins with a transmitter which emits an electrical impulse. This goes to a transducer. The transducer, like an antenna, converts this impulse into focused sound waves shot into the water. The sound frequency is inaudible to humans and fish. As the sound wave bounces off objects it reflects back and is heard by the same transducer. The transducer converts the sound back to an electrical signal and sends it on to the receiver. The receiver amplifies the return signal or "echo", and passes it along to the display unit. A microprocessor within the display calculates the time lapse between the transmitted signal and echo return to determine distance to the object. The result is the readout on display. The whole process repeats itself several times each second.The principle of operation is the same in both depth sounders and fish finders.

So what's the difference? A depth sounder displays a number for depth whereas a fishfinder has a scrolling timeline graphic. The transducer is basically the same for depth sounder and fishfinder. Add a specialized microprocessor to a depth sounder (which can calculate in a time line display) and it becomes a crude fishfinder. This is true in principle, but in reality there is much more complexity involved in modern fishfinders. One major difference, the transducer in a fishfinder is engineered to reflect fish. This is tricky because fish are mostly water based. Most sonar frequencies pass through a fish without registering. The fishfinder emits sonar frequencies tuned specifically to detect the air within fish swim bladders. Many incorporate dual frequency transducers for various depth ranges. The two frequencies are optimized, 192 or 200 KHz for shallow depths and 50 KHz for deeper depths up to 1000 ft. Add multiple sonars reporting back to one display, and the picture gains dimension. These are "Multi-beam fishfinders," which means they use two or more transducers. With multiple sonar readings, the fishfinder has a parallax to create dimensional images. Thus multi-beam transducers are capable of creating a 3 dimensional readout. Rather than a spike in depth, the display shows bottom structure such as fallen trees and shipwrecks. Taken another step, newer multibeam fishfinders will scan sideways. This function is like an undersea radar. It scans 360 degrees around the boat from straight down to horizontal in search of fish and structure. Such fishfinders paint a virtual picture of the bottom.

Not All Monitors Are Alike

This wonderful technology is all for not if you cannot read the display. Some displays have better resolution than others. A basic monochrome display is least expensive to manufacture. Poor resolution and difficult viewing in bright daylight are it's major drawbacks. The benefit is typically less power consumption. These displays are best suited for shaded areas such as pilot house or night time fishing, but not in open cockpits. Color LCD displays perform better in direct sunlight. Some will use a trans-reflective LCD display: "A transflective liquid crystal display is a liquid crystal display (LCD) that reflects and transmits light (transflective = transmissive + reflective). Under bright illumination (e.g. when exposed to daylight) the display acts mainly as a reflective display with the contrast being constant with illuminance. Only in dim and dark ambient situations an auxiliary transmissive backlight should be provided. When an illuminance sensor is added for control of the backlight, such a transflective LCD can be read over a wide range of illuminance levels. This is why that technique is often found in automotive instrumentation. In portable electronic devices the transflective mode of operation helps to save battery charge, since in bright environments no backlighting is required.
The essential component for a transflective LCD is the transflector, a polymer sheet that is reflecting and transmitting at the same time."
(Transflective liquid crystal display) Displays with color LCD's are effective in all light conditions. They tend to cost significantly more. If the color LCD relies on backlighting to be visible in bright daylight it will use more power. In extreme tropical heat some monitors relying on backlighting for daytime viewing have overheated. Newer model fishfinders promote HD technology. This typically means flat screen LCD technology as used in computer monitors. The proper name is TFT LCD, or Thin film transistor liquid crystal display. It is an excellent display with high resolution for LCD, not to be confused with another HD flat screen technology - Plasma Displays (PDP). Plasma makes claims of highest resolution of any flat screen. The highest resolution monitor technology is Cathode Ray Tube (CRT) displays. The drawback to CRT displays is the large boxy size and higher voltage requirements. For this reason and production costs, most manufacturers have favored flat screen LCD type displays.

With the growing high cost of monitors, both manufacturers and consumers are favoring multi-function electronics. High end fishfinder monitors incorporate navigation electronics: GPS, chartplotter and radar. This saves on real estate at the helm, and the single unit makes installation easier. Best of all it lessens overall cost. In such units, a larger display screen is an advantage for legible split screen viewing. Screen sizes are typically available from 2.5 x 3 inch up to 12 inch diagonal. Pixel measurement provides an accurate value of larger displays - increased screen size without increased pixels equates to reduced resolution. Quality resolution is needed not only for clear defined bottom structure, but also in reading navigation charts in the chartplotting function. For large vessels which require simultaneous monitoring of electronics, a second display monitor may be added. This enables constant monitoring of more devices, and provides a redundancy should one monitor fail.

Fishfinder Features

There are several common fishfinder features useful for analyzing echoes. Basic functions include bottom lock, zoom, grayline and sensitivity. Zoom helps focus in on targets such as a fish arch. The zoom may be clearly marked, or found within the Depth Range function - "Upper and Lower Limits." Grayline lets you distinguish between strong and weak echo strength, useful to differentiate hard vs. soft, muddy bottoms. Sensitivity by default is made automatically in most models. It adjusts based on temperature, noise and depth to reduce clutter from various factors - rough seas, thermoclines, acoustic noise, dirty water, etc. Radar like manual sensitivity control, called Time varying gain, gives manual sensitivity control to suppress clutter during periods of high acoustic noise, or to tune in sensitivity to see fish swimming close to the bottom. Additional features include a depth cursor - a horizontal line across the screen to display exact depth of targets. Some fishfinders accept an input feed for boat speed. Speed data is used to pair automatically the picture advance rate so a fish reflection looks the same size at all boat speed ranges. Speed and temperature equipped models display the sea surface temp and speed. In the realm of high sophistication, some fishfinders add digital depth displays with "A" scope images to show the progress of an acoustic pulse in radar fashion. Another impressive feature is transducers capable of scanning to the side or forward. The forward looking feature is great for shallow water navigation, and side view, sometimes called "side imaging," is excellent for complete bathymetric views. Side view sonar sees bait balls, caverns or fallen trees. The ultimate system may include a virtual vision by using cameras tethered over the side of a stationary boat. The limitation to this technology is still range of visibility underwater.

Fishfinder & Depth Sounder Installation

Installation of fishfinders is a relatively easy process requiring only some basic tech skills. Pay particular attention to transducer location when mounting, as this is critical to both fishfinder function and hull integrity. Follow the instructions provided by the manufacturer. Select a fishfinder with compatible voltage to your boat system. Most all units operate on common 12V DC systems, newer ones often accept a voltage range between 10V - 35V DC. Wiring is simplified It is advisable to separate sonar wiring from audio equipment, as the pulsing could potentially cause interference on VHF and moreso stereo units. If limited space makes this impractical, noise filters are available to solve interference problems. For more on fishfinders, check out the Lowrance Sonar Tutorial and some of the Fishfinder videos hosted by Jamestown Distributors.