FISHFINDER , BOTTOM, MACHINE ECHO SOUNDER, DEPTHSOUNDER, SOUNDER…these are all the same thing and all are of a type of active sonar known as a fathometer, a specialized type of echo-sounding system.
Each uses active sonar (like the Navy uses to find submarines) to detect fish and ‘the bottom’ and displays them on a graphical display device, generally a LCD or CRT screen. They are comprised of a screen and a transducer. The screen and the transducer operate on pulses of a set of frequencies intended to show us the bottom we’re over and the water column between us and the bottom.
We’re talking frequency today: Frequency is the number of complete cycles or vibrations that occur within a certain period of time, typically one second. Sound waves can vibrate at any one of a wide number of frequencies. The easiest way to understand frequency is to think of it in terms of sounds that are familiar. For example, a kettle drum produces a low-pitched sound (low frequency). That is, it vibrates relatively few times per second. Whereas, a flute produces a high-pitched sound (high frequency). It vibrates many more times per second than a kettle drum. The frequency of sound waves is measured in a unit called a hertz. A hertz is one cycle per second. For example: a 150 kHz transducer operates at 150,000 cycles per second. REAL FAMILIAR NUMBERS: 50/200 khz. Think of your transducer as a search light pointed straight down. (This analogy only goes so far but it’s helpful.) The more power you have the better you can see. At the same time, the higher the frequency leaving your transducer the more detail will be revealed about the stuff under the boat. Think about a big stereo system. Ever notice that you can hear the bass (low frequency energy, like a kettle drum) much farther away from the speakers than the treble (high frequency energy, like a flute)? In a way, the most (musical) detail is in the treble sounds while the broader information is in the bass. Low frequency generally is longer range but at the expense of detail. High frequency generally is shorter range with the best detail.
Transducer manufacturers recommend using 200kHz for shallow water operation in less than 300 feet. In depths greater than 300 feet, they recommend 50kHz. However, fish finder operators using a dual frequency, split screen mode when fishing in all depths will gain more data. The reason for this is the 200kHz narrow beam will show better fish & bottom resolution, while the 50kHz wider beam will mark more fish, bait, and more bottom area. Using these frequencies together brings out the best in any fishfinder.
To review, this class of device uses sound waves to look through the water. The sonar device then displays a picture of what’s below the hull of a boat via a sound pulse (high or low frequency at some energy level). It sends a pulse of sound down, via the transducer (the searchlight speaker) this sound bounces off objects in the water, like rocks, wrecks, and fish. It measures the time that sound pulse takes to return and uses that information to display on the screen. Sound waves reflect best off objects with densities different than water’s. So rocks, mud, metal, and the air-filled swim bladders of fish all show up well. (There are also side-scanning and forward-scanning sounders, normally found on commercial fishing, military, and research vessels; these are expensive units.) Whether you use your device to navigate, to determine bottom composition or to find fish, you’re watching the result of energy at some frequency traveling through the water…TWICE: it’s a round trip!)
Now we have more questions: How much resolution? How much power? What beam width? Is side scan for me ? In-hull, thru-hull or transom mount? There’ll be more to come in future articles. Thanks to Richie Gerard at Eastside Bait & Tackle in Washington for the idea for this month’s column.
In the meantime, for specific questions on marine technical issues, email JoeZinc@techmastermarine.com. or call 888-JOE ZINC.